structural detachments in lime-based wall paintings: causes ...

STRUCTURALDETACHMENTSINLIME-BASEDWALLPAINTINGS:CAUSES,IDENTIFICATIONMETHODSAND

DECISION-MAKING–CASESTUDYOFMARIACHURCH,NISSE

MScThesisinConservationandRestorationofCulturalHeritage|HistoricInterior

Name:ValentinaGatto|StudentNo°:11445351|Date:08/2020

ContactEmailAddress:[email protected]

Supervisor:MerelSchrojensteinLantmanMA|PDRes

SecondReader:Dr.HermandenOtter

ExternalRCEAdvisors:BerniceCrijns,RutgerMorelissen

STRUCTURAL DETACHMENTS IN WALL PAINTINGS: CAUSES, METHODS OF IDENTIFICATION

AND DECISION-MAKING – CASE STUDY OF MARIA CHURCH, NISSE.

M.Sc. Thesis

Conservation and Restoration of Cultural Heritage, Historic Interiors

Author: Valentina Gatto

Student No°: 11445351

Contact Email Address: [email protected]

Thesis Supervisor: Merel Schrojenstein Lantman MA|PDRes

Second Reader: Dr. Herman den Otter

External RCE Advisors: Bernice Crijns, Rutger Morelissen

Word Count (pp. 6-79 and 88), excluding figure captions and tables: 17997

University of Amsterdam

Rijksdienst voor het Cultureel Erfgoed | Cultural Heritage Agency of the Netherlands

Original version submitted on: 24/08/2020

Cover: Visible light photography of the wall paintings representing the coronation (centre) and the annunciation of

Mary (left) and the Tree of Jesse (right) on the triumphal arch in Maria Church. Photo Credit: The Cultural Heritage

Agency of the Netherlands (RCE) and Jorien Duivenvoorden.

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Structural Detachments in Wall Paintings: Causes, Methods of Identification and Decision-Making.

TABLE OF CONTENTS

A C K N O W L E D G M E N T S _____________________________________________________________________________ 5

A B S T R A C T ______________________________________________________________________________________________ 6

S A M E N V A T T I N G _____________________________________________________________________________________ 6

1. Introducution ________________________________________________________________________________________ 7

1.1. Research focus __________________________________________________________________________________ 7

1.2. Relevance to the field___________________________________________________________________________ 8

2. Case study: maria church and the wall paintings _____________________________________________ 10

2.1. Brief history and iconography _______________________________________________________________ 12

2.2. Composite materials and techniques ________________________________________________________ 18

2.3. Previous conservation treatments___________________________________________________________ 18

2.4. Current condition of the wall paintings _____________________________________________________ 19

2.5. Environmental parameters in maria church ________________________________________________ 24

3. Causes and description of structural detachments in wall paintings ______________________ 25

3.1. Description ___________________________________________________________________________________ 25

3.2. Intrinsic causes _______________________________________________________________________________ 28

3.2.1. Instability of materials ____________________________________________________________________ 29

3.2.2. Defective application of materials and technique________________________________________ 30

3.2.3. Geological condition and composition of the ground ____________________________________ 31

3.3. Environmental factors _______________________________________________________________________ 32

3.3.1. Frost damage ______________________________________________________________________________ 33

3.3.2. Migration and re-crystallisation of soluble salts _________________________________________ 33

3.3.3. Chemical deterioration ____________________________________________________________________ 39

3.3.4. Fungi and biodeterioration _______________________________________________________________ 41

3.4. Extrinsic causes _______________________________________________________________________________ 42

3.4.1. Adverse effects of previous conservation treatments ___________________________________ 43

3.4.2. The influence of vibrations________________________________________________________________ 44

3.5. Conclusion ____________________________________________________________________________________ 45

4. Methods of identification of structural detachments ________________________________________ 46

4.1. Visual approach and percussion method ____________________________________________________ 46

4.1.1. Discussion _________________________________________________________________________________ 47

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4.2. Infrared thermography (IRT) ________________________________________________________________ 49

4.2.1. Passive thermography ____________________________________________________________________ 49

4.2.2. Active thermography ______________________________________________________________________ 49

4.2.3. Discussion _________________________________________________________________________________ 50

4.3. Acoustic methods _____________________________________________________________________________ 51

4.3.1. Ultrasonic pulse velocity __________________________________________________________________ 53

4.3.2. Ultrasonic pulse echo______________________________________________________________________ 53

4.3.3. Ultrasonic tomography ____________________________________________________________________ 53

4.3.4. Discussion _________________________________________________________________________________ 54

4.4. Electromagnetic imaging techniques ________________________________________________________ 55

4.4.1. Ground penetrating radar _________________________________________________________________ 56

4.4.2. Therahertz imaging _______________________________________________________________________ 56

4.4.3. Discussion _________________________________________________________________________________ 56

4.5. Digital speckle pattern interferometry (DSPI) ______________________________________________ 58

4.5.1. Discussion _________________________________________________________________________________ 58

4.6. Conclusion ____________________________________________________________________________________ 58

5. Decision-making: determining treatment needs _____________________________________________ 62

5.1. Conservation methodology of structural delamination _____________________________________ 62

5.2. The establishmente of need for structural treatment _______________________________________ 62

5.2.1. Injection grouting: risks and limitations _______________________________________________ 62

5.3. Development of decision-making model_____________________________________________________ 62

5.4 Prototype of decision-making model ________________________________________________________ 62

6. Maria Church and the wall paintings: a revision ______________________________________________ 73

6.1. Potential causes of structural delamination ________________________________________________ 73

6.2. Suitable methods for the identification of structural delamination ________________________ 75

6.3. Structural delamination: How to determine treatment needs? ____________________________ 77

7. Conclusion and further research________________________________________________________________ 78

8. Reference list ______________________________________________________________________________________ 80

8.1. Books, chapters of edited books, journal articles, conference papers and dissertations _ 80

8.2. Website contents and unpublished documents _____________________________________________ 86

S U M M A R Y _____________________________________________________________________________________________ 88

Appendix I: Iconography of the wall paintings in Maria Church _______________________________________ 89

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Appendix II: Local newspaper ‘Provinciale Zeeuwse Courant’ _________________________________________ 91

Appendix III: History and restoration history of Maria Church ________________________________________ 93

Appendix IV: SEM-EDX of samples no 1,2 and 3 _______________________________________________________ 97

Appendix V: Introduction to composite materials in wall paintings __________________________________ 104

Appendix VI: Moisture passage through porous building materials __________________________________ 106

Appendix VII: Questions of the prototype of decision-making model ________________________________ 109

Appendix VIII: Glossary _________________________________________________________________________________ 113

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A C K N O W L E D G M E N T S

Desperate times call for exceptionally long acknowledgments; writing a thesis during a pandemic is a

remarkably challenging task that can only be combined with a list of thanks just as significant. This

thesis would not have been possible without the guidance and supports of many supervisors,

colleagues and friends: I would like to extend my gratitude to everyone who helped me and listened

to me. I would like to start by thanking my supervisor, Merel Schrojenstein Lantman for helping me

finalising the subject of this thesis both the first time and also post COVID-19; her suggestions on how

to re-establish a meaningful research focus for this thesis have been unique and her through guidance

during this whole process has been just priceless.

Furthermore, I would like to express my gratitude to my external RCE advisor, Bernice Crijns, for

introducing me to the complex subject of structural delamination in wall paintings and for presenting

me the suitable case study of the wall paintings in Maria Church that allowed me the opportunity to

place this challenging subject into a realistic context. To Roger Groves from TU Delft, who accepted to

collaborate with me on the future assessment of DSPI and ultrasonic techniques to document

structural delamination in Maria Church and who has always been available whenever I had technical

questions. Gratitude is also due to Rutger Morelissen for his support during the initial phase of

development of this thesis and to Adri Spruit, volunteer at Maria Church, for allowing us to visit the

church..

I would like to thank all tutors, lecturers and professors of the Conservation and Restoration

Department at the UvA, who supported me during the shifting of research focus that had to be

implemented due to the arrival of COVID-19. In particular, I would like to thank: Rene Peschar, for

providing initial feedback and improvements to the structure of this thesis, Miko Vasques Dias, for

taking the time to read this thesis, giving me an insightful list of suggestions to apply and for correcting

some of the typos on the text, Maarten van Bommel and Ella Hendriks, for their constant

encouragement during this project, and Maartje Stols-Witlox for being able to manage the general

COVID-19 chaos, organise useful supporting workshops and for being always available.

I would like to express my gratitude to all my friends, colleagues and family for being there for me

during this peculiar time. Many thanks: to Jasmijn, Edith, Francesca, Laura, Sarah-Jane, Eugenia,

Raffaele, Sara, Antonia and Marjolijn. In conclusion, I would like to thank my mum Rosmarie and my

dad Guido for always supporting me during my studies, my brother Claudio, for always being there

for me and Angelo, for tolerating me during this stressful time and for always being by my side.

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ABSTRACT

Structural detachments in wall paintings: Causes, methods of identification and decision-

making – Case study of Maria Church, Nisse.

Valentina Gatto, University of Amsterdam, August 2020.

Structural detachments in wall paintings are investigated to evaluate potential causes, methods of

identification and means of determining their treatments needs. The case study of the wall paintings

of Maria Church exemplifies the relevance of analysing this topic. Lastly, a prototype of decision-

making model to approach the conservation of this phenomenon is introduced.

SAMENVATTING

Structurele delaminatie van muurschilderingen: Oorzaken, identificatiemethodes en

besluitvorming – Casestudy van Mariakerk, Nisse.

Valentina Gatto, Universiteit van Amsterdam, Augustus 2020.

Structurele delaminatie van muurschilderingen zijn onderzocht om potentiële oorzaken,

identificatiemethodes en manieren om de benodigdheden voor hun behandeling te evalueren. De

casestudy van de muurschilderingen van de Mariakerk illustreert de relevantie van het analyseren

van dit onderwerp. Ten slotte is een prototype van het besluitvormingsmodel geïntroduceerd voor

het behoud van dit fenomeen.

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1. INTRODUCUTION

Delamination or detachments are terms implemented to indicate the presence of a decaying process

that is gradually causing the loss of adhesion and cohesion of heterogeneous layers in the structure of

wall paintings, resulting in the partial separation between plaster layers, pictorial layers and their

support (Wang 2011, 121; Fricke-Begemann, Gu, and Joost 2000, 538). Currently, common

conservation approaches are based on the identification of areas of delamination and their remedial

conservation treatment, often involving the injection of either lime-based or other synthetic grouts

(Biçer-Şimşir and Rainer 2011; Biçer-Şimşir et al. 2009; Pasian et al. 2018). This procedure is

frequently described in the literature as an obvious treatment and very little explanation and

justification of such choice are provided (Rainer et al. 2017, 10, 71; Rickerby et al. 2004, 471–77;

Tringham et al. 2013, 94-95; Asp 2001, 41-44; Suneson 2001, 45-51). Moreover, the evaluation of the

structural integrity of wall paintings is usually based on the localisation of delaminated areas by

means of subjective and empirical methods such as the percussion approach (Preusser 1991, 4;

Hinsch et al. 2009, 96; Guelker, Hinsch, and Joost 2001, 188; Tornari et al. 2013, 4; More and Philippot

1968, 170). The need for a more scientific method to identify and characterize delamination in wall

paintings has been expressed since the 1960’s (Preusser 1991, 4) and generated into an area of

research involving the use (sometimes combined) of: IRT (Infrared Thermography), DSPI (Digital

Speckle Pattern Interferometry), ultrasound and other forms of non-destructive techniques.

Therefore, interests for the overall assessment of possible causes and identification methods of

structural detachment in lime-based wall paintings spawned from the case study of the mural

paintings in Maria Church in Nisse: despite injection grouting, the structural condition of the

aforementioned wall paintings appear quite poor. This case study is considered to extensively

illustrate the current problematics encountered in conservation practice. Usually, the identification of

causes, location and progress of detachments is based on approximate methods that can generate

conflict results. Consequently, their remedial treatment is rarely justified and their long-term effects

need to be evaluated.1

1.1. RESEARCH FOCUS

The main goal of this research project is to obtain an insightful understanding of the process of

formation of structural detachments, the current methods of their identification, and how, by

exploiting case specific factors (such as; previous conservation treatments, environmental parameters

and in situ logistics) specific treatment needs can be determined. Conventionally, conservation

1 For the sake of simplicity, the term lime-based wall paintings has been shortened into wall paintings throughout

this thesis. Therefore, whilst lime-based wall paintings are the main core of this research, several general notions

can be pertinent to wall paintings of different structural composition. Consult the glossary in Appendix VIII.

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treatments of wall paintings were based on preconceived assumptions about condition and, since

some remedial treatments are believed to have caused additional long-term damages, the necessity of

their implementation, together with their long-term efficacy are brought into question (Kaszewski

2018; Wong and Rickerby 2013; Cather 2003, 412).

Therefore, three main areas of research and related aims were determined: (I) a description of

potential causes associated with structural detachments; (II) an evaluation of traditional and modern

methods of identification of delamination with an overview of associated advantages and

disadvantages; (III) the need for developing a decision-making model to establish specific

treatment needs for structural detachments including risks and limitation of present methods.

Additionally, the case study of the wall paintings in Maria Church exemplifies the need of

investigating the aforementioned areas of research; Chapter 6 elaborates the probable causes,

method of identification of structural detachments and the possibility of implementing a decision-

making model to determine their treatment needs.

Consequently, the resulting research questions were framed:

I. What are the possible causes leading to the formation of structural detachments in lime-

based wall paintings?

II. What are the methods available for the identification of structural detachments in wall

paintings? What are their advantages and disadvantages and are there any possible areas

of research/improvement?

III. How can one determine the treatment needs for structural delamination in wall paintings?

1.2. RELEVANCE TO THE FIELD

Structural delamination affecting heterogeneous layer compounds of wall paintings is a commonly

observed degradation phenomenon (Hinsch et al. 2009, 184). While the nature of their occurrence is

often described, methods for their identification are frequently based on empirical and subjective

approaches, which only provides an approximate indication of the location of detached areas (Lasyk

et al. 2012, 3–4; Hinsch et al. 2009, 96; Fricke-Begemann, Gu, and Joost 2000, 537). Hitherto,

conservation treatments of structural detachments frequently involve the injection (through either

existing or pre-drilled holes) of a variety of adhesive mixtures, with several researches focusing on

the evaluation of methods and materials (Biçer-Şimşir and Rainer 2011; Biçer-Şimşir et al. 2009;

Pasian et al. 2018; Biçer-Şimşir and Rainer 2014). Generally, conservation treatments of this

phenomenon are considered to be executed rather ‘blindly’, with a great lack of description of

decision-making, reasoning and justification of the choice for injection grouting (Mancinelli 1991, 56-

66; Rainer et al. 2017, 10, 71; Asp 2001, 41-44; Suneson 2001, 45-51).

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Since the identification and consequent consolidation of structural delamination are considered the

most urgent and difficult problems in the field of wall paintings conservation, further understanding

of the formation process of this phenomenon and an evaluation of its specific treatment needs are at

the core of this research.

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2. CASE STUDY: MARIA CHURCH AND THE WALL PAINTINGS

Maria Church is a protestant church, built during the first half of the fifteenth century (St.Mary’s

church Nisse 2020). The church, a brick building constructed in the shape of a cross, has been

registered as a national monument since 1967 (see Table 2.1) (RCE National Monument Register

2020). Maria Church is located in the picturesque village of Nisse, in Zeeland, part of the municipality

of Borsele (Fig.2.1,2.2). The church adorns and dominates, with its majestic presence, the square of

the small village of Nisse, Dorpsplein conveying an atmosphere of grandeur to its landscape.

Nowadays, the church has retained its original religious function; weekly church services occur in

collaboration with two other protestant churches (in 's-Heer Abtskerke and in Hoedekenskerke, both

in Zeeland) and joint services are held occasionally.

In addition to the already established historical, national and sentimental significance attributed to

Maria Church, the discovery of a cycle of wall paintings during a restoration campaign in the 1920s,

conveyed further artistic, educational and research values to the church. The study of the iconography

and themes depicted in the wall paintings together with their dating, identification of material

composition, degradation phenomena and conservation treatments were central subjects of two other

conservation campaigns (executed in the 1980s and in 2017 respectively) and a more recent

diagnostic investigation, carried out by the RCE in 2019. Therefore, these mural paintings represented

an ideal case study for the purpose of this research project; the presence of detachments appeared to

be a phenomena affecting the structure of the wall paintings since their uncovering in the 1920s

(Crijns, Morelissen, and Duivenvoorden 2020).

Fig.2.1, 2.2: Location of Maria Church in Nisse, Zeeland and its rear elevation. Photo credit:

https://commons.wikimedia.org/wiki/File:Mariakerk_(Nisse)6.JPG

https://commons.wikimedia.org/wiki/File:Mariakerk_(Nisse)6.JPG

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Table 2.1. Building and Monument Identification

Name of Building Maria Church, Dorpsplein 49, Nisse

Address Dorpsplein 49, 4443 AG Nisse

Status of the

Building National monument

Monument

Number 10008

Cadastral

Part/Number 1578/77

Description

Gothic cross church; single-aisle nave approx. 1425, choir second half of the

15th century, south transept approx. 1500, northern transept a few decades

later. Arched vestry somewhat later than the choir. Interior: between the

nave and the choir a low narrow triumphal arch of the older choir can be

observed together with carved apostle figures under the ribs of the choir

vault. Both ribs and wall studs are painted. Mural on the triumphal

arch: The Holy Trinity, Tree of Jesse and Annunciation. Wall painting on the

north side of the ship: approx. 1430 St. Christopher. Late gothic carved gate

on the north transept approx. 1525. Bench with gothic panel work and

shield-bearing lions approx. 1500. Fences in the ship and in the south arm,

1st half 17th century. Richly carved pulpit 1679.

Function and

Current Use Religious building, Protestant church, Dutch Reformed church

Owner of the

Building Municipality of Borsele

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2.1. BRIEF HISTORY AND ICONOGRAPHY Maria Church was built in the fifteenth century, as a replacement of a much older church, which was

demolished in 1425 (St.Mary’s church Nisse 2020). Consequently, the construction of Maria Church is

known to have started right after 1425, and is believed to have lasted almost one hundred years. The

present church, which is the result of several expansions and reconstructions, consists of: (I) a nave,

original architectural element dating from a time lapse between 1425 and 1525; (II) a choir, a fifteenth

century expansion most likely to have occurred after the completion of the church in 1525,

presumably commissioned by Hendrik II Van Borselen, who requested the addition with the aim of

creating a more sophisticated church, that would have reflected his high status; (III) a transept, an

early sixteenth century addition; (IV) a tower, which underwent invasive modifications during a

restoration in 1805 (such as the removal of three pinnacles).

The attribution of a date to the wall painting in Maria Church appears to be somewhat problematic as

no historical records of their creation have been found: their origin is assumed to be dated around the

second half of the fifteenth century, approximately during the extension of the church commissioned

by Van Borselen. Therefore, a speculated connection between the lord and the creation of the wall

paintings in Maria Church can be expressed. Moreover, the whitewashing of the paintings presumably

occurred during the second half of the sixteenth century, during the Protestant Reformation (Crijns,

Morelissen, and Duivenvoorden 2020). The cycle of wall paintings in Maria Church consists of two

wall paintings, depicting a total of four religious themes: three scenes portraying the Coronation of

Mary, the Tree of Jesse and the Annunciation of Mary are respectively located in the centre, on the

lower right and on the lower left side of the triumphal arch (Fig.2.3,2.5, 2.6, 2.7, 2.8, 2.9), whilst the

fourth theme represented, St. Christopher, is positioned on the north wall (Fig.2.3,2.4, 2.10) (Crijns,

Morelissen, and Duivenvoorden 2020). A more detailed iconography description of these wall

paintings can be found in Appendix I.


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Fig.2.3,2.4, 2.5: Floor plan of Maria Church, showing the location of the wall paintings in the arch (red)

and the wall painting on the north wall (blue). Photo credit: Rob Crevecouer and Jorien Duivenvoorden.

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Fig.2.6, 2.7: Wall paintings on the triumphal arch in Maria Church in Nisse in 2019 (colour photo) and in 1987 (black and

white). Photo credit: Bernice Crijns and Rutger Morelissen, edited by Valentina Gatto.

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Fig.2.8 (A-D): Wall paintings on the triumphal arch, located on the lower right depicting the Annunciation of Mary. A and B are

details of the stained glass window and chequered floor, while C and D are details of Mary and the Archangel Gabriel with remnants

of letters on a nameplate next to his shoulder. Photo credit: Jorien Duivenvoorden, edited by Valentina Gatto.

edited by Valentina Gatto.

B

A

D C

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Fig.2.9 (A-D): Wall paintings on the triumphal arch, located on the lower left depicting the Tree of Jesse, each coloured area

correspond to the respective detail picture with matching coloured letter. D shows the representation of King David. B and C

portrays two other ancestors within the Tree of Jesse, together with a burning candle (highlighted in B). A depicts Mary within a

pointed oval, at the top of the tree. Photo credit: Jorien Duivenvoorden, edited by Valentina Gatto.

A

B

C

D

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Fig.2.10 (A-D): Wall paintings on the north wall representing St. Christopher, more clearly visible in picture A. The palm trees on

the background and the hermit exiting from a dome building, are visible in B and C. D depicts a detail of a pictorial element at the

bottom of the wall painting. Photo credit: Jorien Duivenvoorden, edited by Valentina Gatto.

A

B C

D

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2.2. COMPOSITE MATERIALS AND TECHNIQUES

The exact material composition and painting technique of the wall paintings in Maria Church are still

somewhat unclear; further research and investigation are necessary to obtain a more in-depth

understanding of the material composition and properties of the structural support of the murals and

to allow the full comprehension of the formation process of structural delamination.

The initial investigation carried out by the RCE in 2019 determined the presence of lime for the

structural support of the wall paintings, although the number of layers, differences in composition and

their respective thickness is still unknown (Crijns, Morelissen, and Duivenvoorden 2020). Analysis of

the samples collected during the first phase of investigation revealed the presence of several layers of

lime plaster with different aggregates, such as sand and, possibly, plant fibres; the last two layers of

plaster appeared to be relatively thinner than the underlying ones, therefore suggesting the presence

of a conventional layered structure of arriccio and intonachino, typical of lime-based wall paintings

(see Chapter 3 and Appendix V)(Mora, Mora and Philippot 1999, 12). The painting technique involved

for the creation of these wall paintings is also unknown. Generally, the presence of frescoes in the

Northern countries is somewhat limited and it can be assumed that the wall paintings in Maria Church

were executed in secco technique. Furthermore, the presence of common soluble salts was detected

on all samples analysed.

2.3. PREVIOUS CONSERVATION TREATMENTS

The wall paintings in Maria Church were discovered by restorer Jacob Por in 1920 and have since

endured two further restoration campaigns, in 1984 and in 2017, and a recent diagnostic investigation

in 2019.

The uncovering of the mural paintings executed by Por presumably entailed the mechanical removal

of concealing whitewash together with the filling of losses and their subsequent pictorial

reintegration. The documentation of this conservation campaign is considered too general and lacking

in useful descriptive details of the treatment executed. However, according to Por, the wall paintings

were ‘treated and preserved’ during restoration. Moreover, water infiltrations on the north wall, on

the roof above the wall painting of St. Christopher and on both sides of the triumphal arch, deemed to

presumably cause damages to both the structure and the pictorial layers of the wall paintings, were

documented and repaired (Crevecoeur 2017).

The second main conservation phase occurred between 1984 and 1986 (Crijns, Morelissen, and

Duivenvoorden 2020). This conservation campaign was executed by conservator John Post and a

team of long-term unemployed, untrained people and is considered the most invasive one.

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Conservation treatments involved extensive reintegration of losses by means of tratteggio with the

casein/oil emulsion paint ‘ETA’ from Royal Talens, filling and cleaning with acetic acid (80% in

deionised water). Furthermore, structural detachments were treated by drilling small holes in the

painted surface and the successive injection with a two-component synthetic adhesive. The final

application, over the entire surface of both wall paintings, of a polyvinyl acetate resin named

DupaGrund was carried out with the aim of securing both the original paint and the pictorial

reintegration. This PVA resin is currently sold as an ideal primer for outdoor porous paint systems,

plasters, cement and mortars (Technical data sheet Dupa-grund, n.d.). The overall restoration project

lasted approximately 6 months, and as described on the local newspaper ‘Provinciale Zeeuwse

Courant’ (see Appendix III), a vast number of holes was drilled to allow the injection of the two-

component synthetic adhesive. The identification of such holes results rather difficult, potentially, due

to the current poor state of preservation of the pictorial layers of the murals.

The poor condition of the wall paintings spawned the need for further investigation of the causes of

decay and tests for conservation treatments in 2017. These were executed by Rob Crevecoeur who

documented the presence of structural detachments on both wall paintings by means of percussion

approach. This was executed by implementing a self-made equipment consisting of a flexible rod and

a small steel ball, attached to the rod with a silicone tubing (Rob Crevecoeur, email to author, June 3,

2020). The approach relied on the documentation of the different sounds generated to identify

delaminated areas, after carefully striking the surface of the wall painting with the steel ball (see 4.3).

The additional test of a remedial treatment, which involved the injection of an acrylic adhesive

(thickened Plextol B500 in xylene) through pre-drilled holes was also implemented (Crevecoeur

2017).

Because of the presence of paint flakes and plaster debris found nearby both wall paintings, the RCE

begun an in investigation into the current condition of the murals, the environmental parameters of

the church, the possible causes of decay and whether an effective and sustainable treatment plan could

be developed (Crijns, Morelissen, and Duivenvoorden 2020). A more exhaustive description of the

history and restoration history of Maria Church and its cycle of wall paintings can be found in

Appendix III.

2.4. CURRENT CONDITION OF THE WALL PAINTINGS

The stability of the wall paintings in Maria Church has been the subject of the previous restoration

and diagnostic campaigns carried out since the 1920’s. According to the most recent investigation

performed by the RCE, the structure of the wall paintings is considered rather stable (Crijns,

Morelissen, and Duivenvoorden 2020), whereas opposite opinions have been expressed in the

condition report executed by Crevecoeur (2017). Whilst both documentations agree on the presence

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of active deterioration affecting the pictorial layer of the paintings, these seem to differ on the degree

of structural delamination present. It is striking to observe that the study executed by Crevecoeur

(2017) determined the presence of ninety-six locations of structurally delaminated areas on the wall

paintings on the triumphal arch, whereas only forty-four were detected during the RCE research in

2019 (Crijns, Morelissen, and Duivenvoorden 2020). This remarkable difference was caused by the

implementation of two different percussion methods to identify delaminated areas; the first one

involved the use of a small steel ball whilst the second one was executed by gently tapping the painted

surface with the finger tips. In both cases the acoustic response to the tapping was evaluated as

indicative of the presence or lack of structural detachments. Therefore, due to the presence of

conflicting results, it is currently difficult to determine the level of stability of the wall paintings. This

dilemma is extremely representative of the present problematics encountered in wall painting

conservation practice: the identification of structural detachments is relied on empirical methods,

their conservation approach is hardly ever justified and it usually aims at consolidating damages

without relating these to their possible causes or evaluating the long-term effects on the preservation

of the wall paintings.

The surface appearance of the wall paintings in Maria Church appears heavily affected by previous

restorations: extensive retouching executed in the form of tratteggio are visible, often overlapping

with the original paint (Fig.2.11, 2.12) (Crijns, Morelissen, and Duivenvoorden 2020). Moreover,

numerous infills can be observed, possibly applied to cover previous losses and pre-drilled holes

carried out during the injection grouting of the 1980s. The material composition of this type of infills

was analysed during the RCE investigation and resulted to be gypsum based (Crijns, Morelissen, and

Duivenvoorden 2020).2 The hardness and brittleness of this filling material did not match with the

softness usually associated with gypsum; the addition of an organic adhesive must have been

implemented. The presence of these fillings is considered somewhat problematic: their hardness is

found to have an adverse effect on the original plaster, creating cracks and the loss of original material.

The different reactions of the two materials to fluctuations of relative humidity and moisture passage,

due to differences in porosity and density, might be the associated cause of this phenomenon (see

3.4.1.).

Generally, it could be argued that the current condition of the wall paintings has been severely affected

by both environmental conditions and by previous restorations treatments. Table 2.2 summarises the

different damages and associated triggering cause of the wall paintings on the triumphal arch.

2 See Appendix IV.

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Table 2.2. Damages and Decay of the Wall Paintings on the Triumphal Arch

Damage/Decay Area Description Cause

Cracks

Over the entire arch

The number of large cracks (larger

than 3 mm) is limited, most major

cracks were repaired in the 1920s

Environmental

conditions

Centre (Coronation

of Mary)

Vertical crack following historical

filling from 1920 on the right,

causing instability in the plaster at

the bottom of the arch, probably

once caused by sinking in the

foundations

Environmental

conditions

Cracks/Loss of

plaster, paint

Low right side

(Annunciation of

Mary)

Cracks between filling and original

surface, a recent loss of original

plaster is visible in this area, the

filling material is more brittle and

stiffer than the original plaster,

leading to the formation of cracks

and losses

Environmental

conditions/

Previous

restorations

Delamination Over the entire arch

Unstable around the tip of the arch,

cracks and wooden end of the

vaulted ribs

Environmental

conditions/

Previous

restorations/

Inherent faults

Blistering and loss

of plaster, paint

Left side and centre

(Coronation of

Mary, Tree of Jesse)

Causes severe deformation of the

surface topography and losses, only

visible in the original plaster, not on

fillings, probably caused by salt

activity

Environmental

conditions/

Previous

restorations/

Inherent faults

Surface

deformation Over the entire arch

The entire surface appears three-

dimensional and very rough,

possibly connected to salt activity

and structural detachments

Environmental

conditions/

Previous

restorations/

Inherent faults

Salt efflorescence

Left side and centre

(Coronation of

Mary, Tree of Jesse)

Not widespread, visible on original

and filling material

Environmental

conditions/

Previous

restorations/

Inherent faults

Surface Gloss Over the entire arch Widespread, not on fillings, caused

by fixative applied in the 1980’s

Previous

restorations

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Fig.2.11 (A-H): Wall paintings on the triumphal

arch, showing some degradation phenomena such

as: deformed surface topography (A, C and F normal

light, B, D and E raking light) and visible extensive

retouching in form of tratteggio (G, H). Photo credit:

Jorien Duivenvoorden, edited by Valentina Gatto.

A B

C

D

E

F

G H

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Fig.2.12 (A-F): Wall paintings on the north wall,

showing some degradation phenomena such as:

widespread surface gloss (A), large areas of loss and

retouching (B, C) extensive retouching and

delamination (D, E black area and F). Photo credit:

Jorien Duivenvoorden, edited by Valentina Gatto.

A

B

C F

E

D

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2.5. ENVIRONMENTAL PARAMETERS IN MARIA CHURCH

Because of the strong vulnerability to environmental conditions of both wall paintings and structure

of the church, an initial assessment of the hygrothermal conditions of Maria Church was executed by

the RCE in November 2019 (Crijns, Morelissen, and Duivenvoorden 2020). The air temperature (T, in

C) and relative humidity (RH) were measured before and after the instalment of a portable heather

usually implemented during religious functions in the church. The main purpose was to obtain an

indication of the prevailing indoor climate conditions and its potential changes induced by the heating

system. Moreover, an infrared thermal camera was used to record the thermal distribution over the

surface of the wall paintings. This diagnostic technique is particularly useful to obtain a starting

indication of the potential weak areas of the fabric of the church and to determine the likelihood of

condensation forming over a specific surface (see 4.2.1).3

The initial climate measurement revealed the presence of a fairly constant RH at 70%, despite an

increase in temperature from 8 C to 12 C (Crijns, Morelissen, and Duivenvoorden 2020). The

occurrence of an unchanged RH can be explained by observing the mechanism of heating involved: a

gas burning heather which releases steam, therefore increasing the overall absolute humidity (AH).

Eventually, the AH increased due to the heating system but, since also the T increased, the RH

remained unchanged. Therefore, condensation would have been prevented by the presence of

temperatures higher than the dew points, which were allocated between 2.9 C and 6.7 C (TIS

Climate/Humidity Table, n.d.). According to measurements recorded by the infrared camera, it was

possible to document the following surface temperatures: on the triumphal arch initial temperatures

of 6-7 C were observed with an increase up to 11 C after the heather was started and similarly,

temperatures on the wall painting on the north wall were recorded approximately at 6 C with an

increase up to 8 C after the use of the heather. This initial climate monitoring demonstrated that the

overall internal air and surface temperatures in Maria Church are just above the dew point

temperatures, although the presence of condensation is a documented phenomenon: the relationship

of the outdoor and indoor climate is in need of further monitoring to understand both the yearly

fluctuations of T and RH and the buffering function of the church.

3By recording the temperature of the surface and the relative humidity of the environment the dew point

temperature, temperatures below which water will start to condensate, can be calculated.

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3. CAUSES AND DESCRIPTION OF STRUCTURAL DETACHMENTS IN WALL PAINTINGS

Generally, the causes of deterioration affecting mural paintings are multiple and their occurrence is

commonly simultaneous and interdependent: the presence of some usually triggers the manifestation

of others (Mora 1974, 11). A classification and a connection between fundamental (main triggering

factors) and secondary causes (those triggered by the fundamental causes) can be established.

Consequently, the successful treatment of any damage greatly relies on the correct classification and

identification of its cause; since wall paintings are integral part of the structure of their building, any

form of decay affecting the latter is also influencing both aesthetic and stability of the other (Mora,

Mora and Philippot 1999, 3). Thus, during the elaboration of any conservation plan this correlation

should be retained, especially during the documentation of causes of decay.

Structural detachments are physical forms of damage induced by either external or internal

mechanical stresses. Their main causes are the following: (I) intrinsic causes related to faults of

structural materials, defective techniques and the geological condition and composition of the ground

of the building; (II) environmental factors, primarily associated with the presence of moisture and

fluctuations of relative humidity; (III) extrinsic causes related to human factors, mainly connected to

the adverse effects of previous conservations and the influence of external vibrations (D’Ossat 1982,

8-24). The secondary causes, decaying reactions spawned by the single or combined action of the main

causes, are: frost damage, migration and re-crystallisation of soluble salts and fungi.

3.1. DESCRIPTION

Structural detachments are complex phenomena, considered among the most frequent types of

damage found in mural paintings. The most commonly observed types of delamination can affect both

structure and pictorial layer of a wall painting: discussions regarding the second type are not included

in this research, despite their occurrence being often generated by the same factors responsible for

the first type.

Structural delamination, detachment, defect or even disintegration are terminologies used to define

damages involving: the loss of cohesion within the same structural layer, the loss of adhesion between

two different layers and the loss of adhesion between different applications of the same layer (Fig. 3.1,

3.2, 3.3) (Adams et al. 2005, 525). A categorisation of the different types of detachments that can affect

both pictorial layers and structure of lime-based wall paintings with a brief description of their causes

is retrieved from the literature (Mora, Mora and Philippot 1999, 254; Calicchia and Cannelli 2005,

116). Table 3.1 summarises these two concepts, providing a linear representation of the different

types of delamination and their possible causes (see 3.2., 3.3., and 3.4., for an elaboration of the three

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types of causes described). Furthermore, for clarity reasons, a brief description of the different

materials historically used for the structural realisation of wall paintings can be found in Appendix V.

Table 3.1. Types of delamination, description and causes in lime-base wall paintings

Type of delamination Description Causes

Within the same

structural layer such

as: within the rinzaffo,

arriccio, intonaco

Material is friable and almost

disintegrated. Delamination

occurs in form or relatively

small blisters/gaps

Intrinsic/ Environmental

Lack of cohesion between binding agent

(lime) and aggregates (sand), different

rates of carbonisation between different

layers leading to the hardening of the

pictorial surface and its consequent

lifting, the exposure and direct contact

of these structural layers with water/

air/pollution is facilitated

Between different

structural layers such

as: between the

intonaco and arriccio

or rinzaffo and the

brick wall

Relatively big pockets/gaps

Extrinsic/ Environmental

Exposure to repeated cycles of relative

humidity, crystallisation of salts, re-

deposition of calcite and vibrations

Between different

applications of the

same layer: between

different application

of intonaco or arriccio

Friability of the aggregate

material and delamination in

form of relatively big

pockets/gaps

Intrinsic/ Extrinsic / Environmental

Lack of adhesion and cohesion between

layers of the same material, exposure to

repeated cycles of relative humidity,

crystallisation of salts, re-deposition of

calcite and vibrations

Between pictorial

layer and intonaco

Both blisters and larger

areas of delamination

Intrinsic/ Extrinsic / Environmental

Lack of cohesion within the paint layer

(between pigments and binder) and

adhesion to intonaco, exposure to

repeated cycles of relative humidity,

crystallisation of salts, re-deposition of

calcite and vibrations

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Fig.3.1, 3.2, 3.3: Simplified representation of three main types of structural delamination within the layer

archaeology of wall paintings. Sizes, shapes of delamination and their effect on the surface topography are only

indicative; the aim of this illustrations is to portray locations and highlight differences in space of delamination. Photo

credit: Valentina Gatto

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3.2. INTRINSIC CAUSES

Intrinsic causes generate damages strictly connected to the origin and nature of the wall painting. In

particular, faulty materials, defective techniques and the geological composition of the ground of the

building are elements considered to be directly responsible for the formation of structural

delamination (D’Ossat 1982, 11–12).

Mortars and building supports can have inherent defects that might lead to the formation of structural

delamination. However, the pictorial layer can also suffer forms of decay that might facilitate the

development of detachments. For instance, any loss of the pictorial surface enables the exposure of

the intonaco to moisture, air, dust and pollution (see 3.3). Finally, it is important to note that, due the

great variation in their composition, each type of lime mortar can exhibit different hydraulic and

mechanical properties and that their durability depends on the grain size distribution of their

admixtures (Stambolov and van Asperen de Boer 1976, 14).

Fig.3.4: Reiterative diagram of the intrinsic causes connected to the formation of structural detachments. Photo credit:

Valentina Gatto

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3.2.1. INSTABILITY OF MATERIALS

A failure of the mechanical properties of both mortar and building support might result in the

manifestation of structural delamination. This process might be instigated by two factor: (I) mortars

with equally sized small grains; (II) the mechanical failure caused by excessive external stresses (load

and thermal expansion) (Torraca 2005, 19–29).

The presence of well-rounded, well-sorted small pores in lime-based mortars can have detrimental

effects on the setting process of lime. This occurs by the reaction of lime (calcium hydroxide) with

carbon dioxide in the air, leading to the formation of calcium carbonate (Stambolov and van Asperen

de Boer 1976, 14) (see Appendix V).

Mortars composed of the admixture of small grains possess large specific surface areas; the amount

of binder needed to create structurally stable mortars is larger than that suggested in the literature

and risks of creating mortar with weaker mechanical properties might occur (see 3.2.2.). Moreover,

an excess of fine grains in mortars might lead to densely carbonised layers at the surface, which could

hinder the passage of carbon dioxide within the material and prevent the success of the setting

reaction throughout the layered structured of the wall painting. This can result in the formation of

weakened areas and, consequently, delamination.

Structural detachments can form due to mechanical behaviours instigated by external stresses

imposed by architectural elements like lintels or beams (Torraca 2005, 19–29). Bricks and lime-

mortars are defined as brittle, rigid and fragile, however, they partially exhibit plastic behaviours in

the form of irreversible deformations. For example, the external load exerted by a lintel can induce

tensile stress that would result in the permanent deformation of the structure of the underlying wall

painting (Fig.3.6.,3.7.). The heterogeneity of lime-based mortars is responsible for this behaviour:

these aggregates are composed of several different crystals and glasses held together by bonds with

variable strength. Consequently, these bonds would inevitably break unevenly causing localised

fractures (i.e. the rupture of bonds between binder and aggregates), leading to the formation of weak

areas prone to be irreversibly deformed. Therefore, tensile stress can eventually generate a lack of

cohesion properties of lime-based mortars that would unavoidably result in the formation of

structural detachments.

Fig.3.5: Carbonation reaction representing the setting of lime-based mortar.

Ca(OH)2 + CO2

Calcium hydroxide + carbon dioxide

CaCO3 + H2O Calcium carbonate + water

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Moreover, stress within a building might not be equally distributed between its components, leading

to different areas being differently affected by stress related deformations and detachments.

According to Torraca (2005, 19-29), brittle materials which exhibits areas of permanent deformation

are likely to have developed micro-cracks. As a consequence, the access of moisture is facilitated

triggering further deterioration processes (see 3.3). Thus, due to the heterogeneous nature of lime-

based mortars, it is understood how each composite material might endure different deformations.

This way, structural delamination both within and between layers can be explained: if two structural

layers suffer different deformation, a lack of adhesion between them might occur, whilst the rupture

of bonds between lime and sand can generate a lack of their cohesion leading to both detachment and

disaggregation within the same structural layer. This phenomenon can be further enhanced by the

action of moisture (see 3.3).

3.2.2. DEFECTIVE APPLICATION OF MATERIALS AND TECHNIQUE

Structural detachments might form due to mistakes occurred during the preparation of both building

support and rendering (Mora 1974, 13). The classical literature describes the cleaning and subsequent

application of abundant water on the building support as a crucial step. Dust and other debris might

hinder the adhesion of mortar to the building support whilst a dry support, especially a highly porous

one, might eventually absorb water from the mortar and inhibit the carbonation of lime. This way, the

adhesion of superimposed layers of mortars is also prevented. The presence of un-set mortar (i.e.

calcium hydroxide) within renderings is a somewhat common phenomenon that can be also caused

by the presence of small pores within the aggregates.

Fig.3.6, 3.7: Simplified representation of how the external load from two lintels can lead to the deformation of the

building support and the consequent detachment to the first structural layer (rinzaffo) of the wall painting. Sizes and

shapes of delamination are not realistically representative. Photo credit: Valentina Gatto

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Furthermore, the correct composition of mortar is essential for the long-term preservation of the wall

painting (Mora 1974, 13). Both an excess and lack of binder can form structural instabilities: an excess

in lime might result in the formation of cracks and detachment between layers whilst a lack of lime

might cause a reduced cohesion with its aggregates, leading to powdery disintegrations within the

same layer. The choice of inadequate aggregates (such as sand containing salts), the incorrect

proportions of the mix and the excessive thickness of layers can contribute to the failure of the

adhesive properties of mortars and, hence, delamination (D’Ossat 1982, 16).

3.2.3. GEOLOGICAL CONDITION AND COMPOSITION OF THE GROUND

The stability of a building relies on the condition and nature of the ground on which it is erected

(D’Ossat 1982, 15). The presence of geo-topographical elements (such as streams, aquifers or the

vicinity of the sea) can influence the composition of the ground and its ability to withstand the load of

the building. Arguably, the formation of structural detachments can be connected to three causes

related to the ground of a building: (I) the inability of the ground to sustain load transmitted to the

foundations of the building resulting in the partial sinking of these with consequent deformation of

building supports and renderings; (II) the rising of moisture and water from either a nearby source of

water or from a highly hygroscopic ground (see 3.4); (III) the presence of friable or weakened

materials (such as a deteriorated clay-based ground) unsuitable to resist the load of the building with

subsequent collapse of parts of its foundations.

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3.3. ENVIRONMENTAL FACTORS

The formation of structural delamination in wall paintings can be caused by three secondary reactions

induced by the synergetic action of environmental factors: (I) frost action; (II) the migration and re-

crystallisation of soluble salts; (III) the chemical deterioration of calcareous materials. Therefore, both

physical and chemical mechanisms can be responsible for the development of structural detachment;

the first process exposes the porous structure of building materials to large internal stresses, whilst

the second one involves the acid corrosion of the calcium carbonate, its dissolution and subsequent

re-deposition, creating weak areas more susceptible to external stresses (Torraca 2009, 83-88).

The main environmental factors directly responsible for the abovementioned phenomena are the

presence of moisture4 and atmospheric pollutants (Mora 1974, 16–22). However, secondary reactions

like fungi and biological accretions, might play a fundamental role both on the formation of structural

delamination and also on the process of re-crystallisation of salts (Garg, Jain, and Mishra 1995, 263).

Therefore, the combined action of several environmental factors can lead to the development of

structural detachments, however none of them would occur without the presence of moisture (Mora

1974, 16-22). The mechanism of passage of moisture thorough porous building materials is rather

complex and a simplified description is found in Appendix VI.

4 For simplicity reasons, this term is used to define the passage of water in its gaseous and liquid phase through

building materials and cycles of relative humidity.

Fig.3.8: Reiterative diagram of the environmental factors connected to the formation of structural detachments. Photo

credit: Valentina Gatto

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3.3.1. FROST DAMAGE

The formation of ice crystals within porous building materials is facilitated by low temperatures

(below 0C). The mechanical compression exercised by the growing crystals and the subsequent

resistance of the materials surrounding them can lead to large internal stresses, resulting in both

adhesive and cohesive failures (Torraca 2005, 31).

The development of ice crystals can follow two mechanism: frost heave and water entrapment

(Torraca 2005, 31). Generally, the formation of ice crystals occur easily in large pores, whilst their

growth in capillaries occurs only in presence of strong pressure. Water molecules inside small pores

(capillaries) are strongly attracted to the surrounding hygroscopic building material and, due to their

restrained movement, cannot move into the crystallised structure typical of ice. Oppositely, in large

pores, the water molecules have more freedom to re-arrange themselves and form ice crystals. During

this process, water from the capillaries moves to the growing crystals by means of diffusion: if the

crystal has formed and there is still water available in the capillaries, enough pressure is developed to

allow the growth of ice crystals also in the small pores. Frost heave damage is endured by materials

with a high percentage of small pores: the amount of pressure generated can lead to large internal

stresses, resulting in mechanical failures.

The development of ice crystals by means of water entrapment is slightly different, although,

eventually, similar internal stresses are formed. Liquid water can remain confined between already

frozen areas generating enough stresses to overcome the material tensile strength leading to both

adhesive and cohesive failures (Torraca 2005, 31). Interestingly, the increase in volume of water

molecules upon freezing is not regarded as the major cause of development of internal stresses:

liquids that do not exhibit an increase in volume after freezing resulted in imposing similar stresses

as water (Torraca 2009, 84). Therefore, the primary reasons behind the development of structural

delamination is the pressure exerted by the crystals to the surrounding material.

3.3.2. MIGRATION AND RE-CRYSTALLISATION OF SOLUBLE SALTS

The evaporation of water from porous building materials can cause both structural and pictorial

damages due to the re-crystallisation of its dissolved salts (Torraca 2009, 85). The mechanism of salt

and ice crystals development is similar: the growth of crystals occurs first within large pores, whilst

capillaries diffuse water to the growing crystals, eventually leading to large internal stresses caused

by an increase in pressure. Moreover, internal stresses can be caused by an increase in volume of a

salt crystal due to either thermal expansions or to the re-dissolution and re-crystallisation into a

different hydration state (Doehne and Price 2010, 15). The latter can follow two mechanisms: (I) the

hydrated salt re-crystallises into a less hydrated form (anhydrous), thus releasing water molecules

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and increasing its volume (triggered by an increase in temperatures); (II) the anhydrous salt crystal

draws water from the surrounding capillaries, becoming a more voluminous hydrated form (caused

by an increase in relative humidity) (Torraca 2005, 33). However, the majority of internal stresses are

caused by repeated cycles of evaporation, re-dissolution and re-crystallisation of salts and their

accumulation at the point of evaporation (Doehne and Price 2010, 16).

Generally, the process of migration and re-crystallisation of salts is quite complex and its

destructiveness depends on the location of the evaporation zone (Mora 1974, 18). Two main types of

crystalline formations can be classified: (I) efflorescence, where salts re-crystallise over the surface of

the mural; (II) sub-efflorescence, where salts crystals develop within the structure of the mural. The

latter is considered the most destructive mechanism and also the one associated with the formation

of structural detachment. Both phenomena are determined by the mechanism of moisture

evaporation from building materials, which is directly influenced by the environmental conditions and

the porosity of the materials. During the formation of sub-florescence, the evaporation zone of

Fig.3.9, 3.10: Schematic representation of the increase in volume a salt crystal can undergo, inside a pore: by re-

crystallising into a less hydrated form (dehydration) or by re-crystallising into a more hydrated form (hydration) .

Photo credit: Valentina Gatto

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moisture is located below the surface of the wall painting. Therefore, the water vapour generated has

to travel through the remaining porous material, where further chemical reactions can be triggered

(see 3.3.4) before it can reach the surface and evaporate. Generally, a fast moisture evaporation is

necessary for the formation of sub-florescence, a circumstance which appears to be favoured by the

presence of strong wind, relatively warm temperatures and low relative humidity (Torraca 2009, 86).

However, the location of the evaporating front below the surface of the wall painting can also be

formed by a drop in moisture supply which can be caused by an abrupt increase of high temperature

due to seasonal cycles (Mora 1974, 17). When the source of moisture is scarce, its speed of movement

decreases leading to a premature evaporation. In this instance, both efflorescence and sub-florescence

might occur simultaneously due a shift of the evaporation zone from the surface of the wall painting

to its rendering.

Furthermore, the different composition of soluble salts can have diverse destructive effects on lime-

base mortars, according to their increase in volume after re-crystallisation and their ability to

deliquesce and react with other ions to form other salts of different hygroscopicity (Piqué, Ferroni,

and Dei 1992, 217). Table 3.2 summarises the salts most commonly found in wall paintings, their form

of crystallisation, provenance and associated damage.

It is important to note that, both crystalline formations of salts can lead to the development of

structural detachments: efflorescence can cause pictorial losses which can lead to further weakening

of the mortar structure due to its facilitate exposure to the atmosphere, whilst sub-florescence can

generate internal stresses, due to the volume expansion of salt crystals and an overall increase in

pressure, which can lead to the adhesive and cohesive failure of the mortar (Mora 1974, 21). Finally,

the enhancement of the chemical degradation of lime-based mortar exerted by salt activity is

discussed in the next paragraph.

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Table 3.2. Composition of soluble salts and common damages affecting lime-mortar

I. Sulphates

Salt usual

hydration

state

Chemical

Name

Mineral

Name Origin

Form of

crystallisation Damage

Na2SO4 . 10

H2O

Sodium

sulphate

hydrate

Thenardite

Derivate of

construction

materials or their

decomposition,

from bats/bird

excreta

Efflorescence

Sub-florescence

Physical damage,

pictorial losses

and structural

detachments

Na2SO4 Sodium

sulphate Mirabilite

K2MgSO4 .

6H2O

Potassium

magnesium

sulphate

hexahydrate

Picromerite

K2MgSO4 .

4H2O

Potassium

magnesium

sulphate

tetrahydrate

Leonite

K3Na(SO4)2

Potassium

sodium

sulphate

Aphthitalite

K2SO4 Potassium

sulphate Arcanite

MgSO4 .

7H2O

Magnesium

sulphate

heptahydrate

Epsomite

MgSO4 .

6H2O

Magnesium

sulphate

hexahydrate

Hexahydrate

CaSO4. 2

H2O

Calcium

sulphate

hydrate

Gypsum

Derivate of

construction

materials, from

bats/bird excreta,

previous

conservation

treatments,

atmospheric

pollutants

Formed by

chemical

reaction with

CaCO3, can cause

pictorial losses

and structural

detachments

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II. Carbonates

III. Silicates

IV. Chlorides

Salt usual

hydration

state

Chemical

Name

Mineral

Name Origin

Form of


CaCO3 Calcium

carbonate

Calcite/

Aragonite

Derivate of

construction

materials or their

decomposition,

from bats/bird

excreta (surface re-

deposition of

calcite)

Surface

incrustations

Chemical

reaction with

CO2 leading to

the re-deposition

of calcite, which

can cause

pictorial losses

and structural

weakening

Salt usual

hydration

state

Chemical

Name

Mineral

Name Origin

Form of


CaSiO3 Calcium

metasilicate Wollastonite

Derivate of

construction

materials or their

decomposition,

previous

conservation

treatments

Surface

incrustations

Formed by

chemical

reaction with

CaCO3, can cause

structural

weakening and

pictorial losses

Salt usual

hydration

state

Chemical

Name

Mineral

Name Origin

Form of


NaCL Sodium

chloride Halite

From the

atmosphere

(vicinity of the

sea), from

bats/bird excreta

Efflorescence

(more common),

Sub-florescence

No disintegrating

effect, can react

with other ions

to form more

damaging salts

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V. Nitrates

VI. Oxalates

Salt usual

hydration

state

Chemical

Name

Mineral

Name Origin

Form of


NaNO3 Sodium

nitrate Soda nitre

From the soil, from

bats/bird excreta,

atmospheric

pollutants

Efflorescence

Disintegrating

action inferior to

that of the

sulphates,

pictorial losses

which can lead to

structural

detachments

KNO3 Potassium

nitrate

Nitre or

Saltpetre

Ca(NO3)2 .

4H2O

Calcium

nitrate

tetrahydrate

Nitrocalcite

Salt usual

hydration

state

Chemical

Name

Mineral

Name Origin

Form of


CaC2O4 .H2O

Hydrated

calcium

oxalate

Whewellite

From fungal

metabolic activity

Efflorescence

Formed by

chemical

reaction with

CaCO3, can lead

to pictorial

losses and

structural

detachments

CaC2O4

.2H2O

Calcium

oxalate

dihydrate

Weddellite

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3.3.3. CHEMICAL DETERIORATION

The cementing properties of mortars can be severely damaged by the chemical reaction of lime with

acids found in water (rain, dew or groundwater) or in the atmosphere (pollutants) (Torraca 2009,

87). Furthermore, soluble salts can enhance the dissolution of calcite, depending on the pH of the

aqueous solution formed (Doehne and Price 2010, 19). It is essential to remind that, since chemical

reactions can only occur in the presence of water, the chemical deterioration of mural paintings is

possible only if the latter is exposed to moisture.

Rainwater and groundwater are usually slightly acidic because of the presence of carbon dioxide

(Mora, Mora and Philippot 1999, 198). The latter originates either from the atmosphere or from the

respiration of living organism in the soil and, when dissolved in water, forms a very weak acid,

carbonic acid. Calcium carbonate can be dissolved by this acid, giving rise to the soluble calcium

bicarbonate. This, decomposes during drying depositing a layer of calcium carbonate at the

evaporation zone. (Fig.3.11). This process, also known as the re-deposition of calcite, often accounts

for the formation of white hard surface crusts affecting both mortar and pictorial layer. Repeated

cycles of this chemical reaction can severely affect the structural stability of wall paintings, leading to

two decay processes: (I) the loss of pictorial material, and the subsequent exposure of the rendering

to atmospheric degradation; (II) the structural delamination between the pictorial layer and the

intonaco.

Two strong acids can be formed by the reaction of pollutants with water: sulphuric acid and nitric

acid, both derived from the reaction of their respective anhydrides and oxides with water (Fig. 3.12,

3.13) (Torraca 2009, 88). These can affect the structure of building materials and wall paintings by

means of acid rains. Therefore, calcium carbonate can be transformed into calcium sulphate and

calcium nitrate by reacting with the respective acids (Fig.3.12/3.13). Consequently, structural

degradation can occur as a result of both cycles of dissolution and re-deposition of the salts at the

evaporation front (both below and above the surface of the wall painting) and also due to expansion

in volume and increase in pressure after re-crystallisation of the calcium carbonate.

CO2 + H2O

carbon dioxide + water

H2CO3 carbonic acid

CaCO3 + H2CO3

calcium carbonate + carbonic acid

Ca(HCO3)2 calcium bicarbonate evaporation

CaCO3 calcium carbonate

Fig.3.11: Re-deposition of calcite, caused by the dissolution of calcium carbonite by carbonic acid formed by carbon

dioxide and water.

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The presence of salts can enhance the dissolution of calcium carbonate depending on the composition

and ionic concentration once in solution (Ruiz-Agudo et al. 2010, 1256). Acidic aqueous solutions are

formed by the action of sulphuric and nitric acids in the atmosphere and can lead, not only to the

dissolution calcium carbonate (Fig. 3.12/3.13) but also to the dissolution of other salts found in the

structure of the wall painting, through a double-replacement reaction. During this chemical process,

the salt rearranges into its associated acid and into a new salt (Fig.3.14). It is understood that the

presence of specific cations and anions such as Mg2+ and SO42- can enhance the solubility of calcite;

therefore any reaction that can lead to the formation of such ions, will increase the dissolution of

calcium carbonate (Fig.3.15) (Dabb, 1971, 5).

2SO3 + 2H2O

sulphur dioxide + oxygen+ water

2H2SO4 sulphuric acid

CaCO3 + H2SO4 + H2O

calcium carbonate + sulphuric acid + water

CaSO4 . 2H2O + CO2

calcium sulphate dihydrate (gypsum)

Fig.3.12: Chemical reaction of calcium carbonate with sulphuric acid and subsequent formation of

calcium sulphate.

4NO2 + O2 + 2H2O

nitrogen dioxide + oxygen +water

4HNO3 nitric acid

CaCO3 + 2HNO3

calcium carbonate + nitric acid

Ca(NO3)2 + H2O + CO2

calcium nitrate

Fig.3.13: Chemical reaction of calcium carbonate with nitric acid and subsequent formation of calcium

nitrate.

2NaNO3 + H2SO4

sodium nitrate + sulphuric acid

2HNO3 + NaSO4

nitric acid + sodium sulphate

Fig.3.13: Example of a double-replacement reaction between a salt commonly found in wall paintings and

sulphuric acid. This reaction produce a new salt and nitric acid, which can further dissolve calcite.

MgSO4 + H2NO3

magnesium sulphate + nitric acid

Mg(NO3)2 + H2SO4

magnesium nitrate + sulphuric acid

Fig.3.14: Example of a double-replacement reaction between a salt commonly found in wall paintings and nitric acid. This reaction produce a new salt and sulphuric acid, which can increase the solubility of calcite.

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3.3.4. FUNGI AND BIODETERIORATION

The development of fungal mycelia and fruiting bodies within the structure of lime-based wall

paintings can be favoured by three main factors: (I) high relative humidity, above 65 % to initiate

growth and between 75% and 95% to germinate into a mycelium; (II) temperatures, generally

between 20 C and 35 C, to support the presence of moisture in the air; (III) organic nutrients, found

in the accumulation of dust on the surface(Garg, Jain, and Mishra 1995, 257). The formation of

structural delamination can be connected to the presence of fungi due to their physical and chemical

decays.

The growth of fungal mycelia and fruiting bodies can occur both within and over the surface of mural

paintings (Garg, Jain, and Mishra 1995, 257). When growing within the structure, layers dislodgments

can occur due to the physical growth of the fungi and by expansion and contraction of their thallus

caused by fluctuations of relative humidity (Doehne and Price 2010, 22). Consequently, structural

weakening and increased susceptibilities to external stresses are caused, leading to the development

of structural detachments. Oppositely, the formation of fungi over the surface generates long

filaments, responsible for the chemical decay of the pictorial layer.

The assimilation and dissimilation of nutrients of fungi can generate the chemical degradation of the

structural materials of wall paintings by producing enzymes and acids (Garg, Jain, and Mishra 1995,

263). During the assimilation of carbon based nutrients, fungi can produce a variety of enzymes that

can transform complex proteins into water soluble substances. Therefore, organic paint binders,

mortar aggregates (such as animal hair and plant fibres) and restoration materials of organic origin

might suffer disintegration resulting in their pulverisation and the subsequent weakening of the

structure of the wall painting . The excretion of waste products and metabolic activity of the fungi can

release organic acids like gluconic, citric and oxalic acids. The chemical reaction of such acids with

calcium carbonate can severely impair mortar; not only this can suffer decalcification by the action of

citric acid (this would act as a chelator of calcium ions) but the formation of salts, like calcium oxalate,

is strongly favoured (see 3.3.3).

The role of autotrophic and heterotrophic bacteria can be linked to chemical reactions that can cause

the formation of structural detachments (Doehne and Price 2010, 23). Autotrophic bacteria,

microorganisms that derive their carbon based nutriment from carbon dioxide in the air, have the

ability of oxidising sulphur and nitrogen oxides and produce sulphuric and nitric acids. The damaging

effect of such acids on lime-based mortars is described in paragraph 3.3.4. Oppositely, heterotrophic

bacteria assume their nutriments from organic materials composing the wall paintings, resulting in

the possible degradation of organic binders and mortar aggregates of organic origin.

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3.4. EXTRINSIC CAUSES

Extrinsic causes consist of all forces connected to natural and human activities (D’Ossat 1982, 13).

Whilst the main natural phenomena associated with structural delamination occurring in wall

paintings were covered in paragraph 3.3, this section primarily describes the influence of vibrations

and previous restorations on the development of structural damages. The term ‘previous restorations’

is here included as an umbrella term to categorise all deliberate modifications to the original structure

and function of both buildings and wall paintings as well as variations in the surrounding environment

and soil. In other words, it gathers all consequences of previous interventions that could have altered

the structural stability of the wall painting.

Fig.3.15: Reiterative diagram of the extrinsic causes connected to the formation of structural detachments. Photo credit:

Valentina Gatto

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3.4.1. ADVERSE EFFECTS OF PREVIOUS CONSERVATION TREATMENTS

Physical alterations of a building can disturb the load balance between its architectural elements

(D’Ossat 1982, 23). The presence of excessive loads can generate permanent deformations, resulting

in the adhesive failure of composite materials in wall paintings and, hence, structural detachments.

Similar damages can be obtained due to modification of the ground, leading to distresses of the

delicate balance with the foundations of the building (see 3.2).

More specifically, structural problems could be caused by the use of incompatible materials involved

in previous conservation treatments (Torraca 2009, 69). This concept refers to the action of materials

considered to damage the original artefact whilst contributing to its conservation. Therefore,

materials deemed to influence the development of structural detachments can be classified according

to their physical or chemical incompatibility with the original ones.

Physical incompatibilities between materials entail the mechanical alteration of the original ones by

the action of the conservation materials whilst chemical incompatibilities are associated with induced

chemical reactions which would not otherwise occur (Torraca 2009, 69). Three main alterations are

related to the physical and chemical action of incompatible conservation materials and are hereby

discussed.

1. The formation of salts can be facilitated by the introduction of salt containing materials like

cement or gypsum or by the use of acids or alkaline solutions as cleaning agents (Torraca 2005,

118). These materials, once introduced within the structure of the wall paintings, will react with

moisture: salts present in cements and gypsum based materials will be solubilised whilst acids

and alkaline materials will react with both water and calcium carbonate, forming soluble salts.

The fundamental role of salts into the formation of structural detachments is discussed in 3.3.3.

2. The deflection or interruption of moisture passage can lead to severe structural and pictorial

damages (Torraca 2005, 119). This is correlated to the previous use of hydrophobic materials

(such as epoxy or acrylic based synthetic mortars and most types of varnishes) and materials

exhibiting different permeability properties due to different pore size and pore size distribution

(cement or dense filling materials with low porosity). The use of hydrophobic materials can cause

the accumulation of moisture behind the surface and the consequent development of internal

stresses due to the re-crystallisation of salts. Generally, the diversion of moisture flow can lead to

a change of the evaporation zone resulting in localised internal stresses to areas nearby the

incompatible materials (Torraca 2009, 69–86).

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3. Mechanical failure of composite materials is among the main causes of structural delamination.

This can be triggered by differences in thermal expansion and elasticity between original and

conservation materials. Cement or synthetic materials (epoxy or acrylics) possess larger thermal

expansion coefficients than lime-based mortars. These would inevitably endure mechanical

damage, eventually resulting in deformations (Torraca 2009, 69,86; Torraca 2005, 119).

Due to the intertwined relationship of the physical and chemical incompatibilities between original

and conservation materials, a summary of these and the alterations they may cause is provided in

Table 3.3.

Table. 3.3. Alterations caused by incompatible conservation materials

Alteration Type of

incompatibility Conservation material

Causes of structural

detachment

Formation of

soluble salts

Chemical – due to salt

contamination or

presence of salt in the

conservation material

or by chemical reaction

Cement used for grouting or

filling , inorganic

consolidants containing

sands or contaminated by

salts or gypsum based, acids

(acetic acid) or alkali

(caustic soda) as cleaning

agents

Dissolution of calcite,

and physical damage

due to internal

stresses

Diversion or

interruption of

moisture passage

through the wall

painting

Chemical/ Physical –

due to the different

morphological

properties of the


Cement, synthetic mortars

(epoxy/ acrylic based),

material with different pore

size and pore size

distribution or material with

different permeability used

for grouting , filling or

varnishing

Deflection of moisture

flow and change of

evaporation zone,

consequent

accumulation of

moisture and salts in

localised areas and

damage of the original

material

Mechanical failure

Physical – due to

different thermal

expansion and low

elasticity of the


Cement, synthetic mortars

(epoxy/acrylic based) and

non-porous stiff material

used for grouting or filling

Different reaction to

thermal, external or

internal stresses, the

weaker original

material is damaged

3.4.2. THE INFLUENCE OF VIBRATIONS

External or nearby vibrations, caused by traffic, machinery or sound, can induce mechanical stresses

to building materials (Torraca, 2005, 53). Whilst their individual effect is considered insufficient to

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generate damage to buildings, vibration induced stress, if superimposed on other types of stresses

acting on buildings (such as mechanical and thermal stresses), can generate an increase in their

deterioration rate. A common source of vibration is originated by traffic and the impact of vehicles

over irregularities of the road. The intensity of the stress generated is directly proportional to the

weight and speed of the vehicle, the height of the irregularities of the road and its distance to the

building (Torraca 2009, 77). Generally, the damage that a building might endure due to any source of

vibration depends on the phenomenon of resonance: this indicates the event in which the exciting

frequency (the one generated by the vibration) is close to the natural frequency (the one of the

building) causing an amplification of the oscillation of the building and an overall increase in

mechanical stress. Consequently, it is argued that structural elements, such as plaster and mortar, are

extremely susceptible to vibration damage, especially if their state of repair is already compromised

(Torraca, 2005, 62).

3.5. CONCLUSION

Causes of structural delamination in lime-based wall paintings are complex and their occurrence is

generally concurrent and interdependent. As a result, their precise identification can be a rather

intricate task. The importance of establishing a connection between degradation phenomena affecting

wall paintings and their causes is expressed in the literature (Calicchia and Cannelli 2005, 116; Mora

1974, 11; Mora, Mora and Philippot 1999, 173). Consequently, conservation treatments implemented

in-situ without subsequent environmental control and amelioration of causes are deemed insufficient

to achieve long-term effects (Brajer and Larsen 2008, 219). Whilst this concept appears to be

powerfully articulated in conservation literature, descriptions of causes of structural delamination are

scarcely found with only a few articles and books generally describing this issue (Calicchia and

Cannelli 2005; Mora 1974; Torraca 2009; Torraca, 2005). It is remarkable to observe that several

areas of research appeared to be centred around the different identification methods and remedial

treatments of structural delamination, rather than around their causes (Biçer-Şimşir and Rainer 2011;

Hinsch et al. 2009; Dandolo and Jepsen 2016; Kordatos et al. 2013).

Therefore, this chapter aimed at elaborating the general statements stipulated in the literature to

provide a more meaningful and complete understanding of the correlation between intrinsic, extrinsic

and environmental factors and the development of structural detachments. Additional fields of

research should involve laboratory testing to corroborate the chemical dissolution of calcite and

subsequent formation of delamination, induced by acidic conditions and the migration and re-

crystallisation of salts.

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4. METHODS OF IDENTIFICATION OF STRUCTURAL DETACHMENTS 4000

Assessing the preservation state of wall paintings is one the most challenging and important technical

problems faced by conservators (Tornari et al. 2013, 3). Proficient approaches not only require the

identification of damages but should also provide reliable results about their progress. Only through

the understanding of materials characteristics, the technology of execution of the murals and by the

accurate recording of their damages, rate of degradation and associated causes, an effective treatment

plan can be established (Pereira et al. 2011, 2145). Therefore, achieving a precise knowledge of the

overall subsurface condition of wall paintings is necessary for conservation purposes. The following

chapter provides a description of the current methods of identification of structural detachments,

outlining the associated advantages and disadvantages. It is crucial to note that any investigative

method should be ‘non-destructive’, i.e. it must not cause any irreversible alteration to the artefact.

Lastly, further studies should focus on widening the scope of results obtainable by including

qualitative information about delamination and understanding of the possible correlation between

structural and surface damages affecting the wall painting.

4.1. VISUAL APPROACH AND PERCUSSION METHOD

The initial examination of the structure of a wall painting is often executed by means of visual

observations, followed by the involvement of the so called percussion method (Fig.4.1). According to

Mora (Mora and Philippot 1999, 23), the initial inspection of the adherence between the pictorial layer

and the intonachino is easily achievable with a raking light examination; this way the overall surface

topography of the mural can be observed, and deformed raised areas should be considered indicative

of the presence of detachments. The adhesion and cohesion between mortar layers and between these

and the building support can be verified with the abovementioned percussion method. This, otherwise

known as the ‘tap and listen’ or acoustic approach, represents the most common technique involved

in conservation practice (Hinsch et al. 2009, 96). Generally, it implies the gentle tapping of the painted

surface with a nail, a finger, knuckle or a suitable light weight instrument (Hinsch et al. 2009, 96; Mora,

Mora and Philippot 1999, 23).5 However, different ways of executing this approach are described in

the literature, some mentioning the traversing of the surface with a pencil or a stick rather than its

tapping. The acoustic response generated needs to be interpreted: usually a hollow sound is attributed

5 A demonstration of how to carry out a percussion test for the identification of structural detachments is shown

by Mora in a video describing the conservation of the Tomb of Queen Nefertari. See ‘Conservation process of the

wall paintings in the Tomb of Queen Nefertari’, minute 16:00: https://www.nefertaritomb.com/process

https://www.nefertaritomb.com/process

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to the presence of detachments, however, differences in intensity can be specific of a large (low-

pitched) or relatively small delamination (high-pitched).

Whilst this method appears to be an extremely common approach for the identification of

detachments in wall paintings, only a few publications describe it in a meaningful way (Hinsch et al.

2009, 96; Guelker, Hinsch, and Joost 2001, 188; Fricke-Begemann, Gu, and Joost 2000, 538; Mora and

Philippot 1968, 170), with many others simply mentioning it (Lasyk et al. 2012, 3; Tornari et al. 2013,

4; Collini 2011, 868; Mancinelli 1991, 57; Adams 2006, 35) and potentially, believing to refer to a

standardized method. Eventually, the quality of the results and interpretations directly corresponds

to the experience of the conservator. Hinsch provides a detailed description of the different features

observed during a test performed on a wall painting (Hinsch et al. 2009, 96). These should be regarded

as an indicative suggestion of the variety of sounds originated by this type of test. Additionally, its

suitability relies on the condition of the pictorial layer, since mechanical pressure is inevitably exerted,

involving some degree of unintentional damage.

4.1.1. DISCUSSION

To this day, the use of the percussion method, despite being referred to as cumbersome, empirical and

subjective, appears to be widespread during diagnostical investigations in conservation projects

(Tornari et al. 2013, 4). The lasting presence of this method might be associated with the will to retain

traditional approaches and also, due to its ease of execution. Furthermore, it represents a customized

way of approaching this type of damage: the presence of structural detachments is almost always

associated with an intrinsic and unquestionable need for their treatment, putting in second place the

need for a correct diagnosis and quantification of these. Whilst results obtained by this technique do

not inform about the rate of deterioration or the extension of the detachments, its use, combined with

thorough observations of the surface, can provide a holistic picture of the general condition of the

mural in question (see Table 4.1) (Hinsch et al. 2009, 94).

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Table 4.1. Advantage and disadvantages of percussion method

Advantages Disadvantages

Cost – executing the technique is free Cost – reaching the pictorial surface might

involve costly scaffolding

Ease of execution Need for expert interpretation to somewhat

good but still subjective results

Results provide an overall indication of the

state of preservation of the painting

Mechanical pressure exerted on the surface

can lead to damages

No information about the rate of

deterioration or dimensions of the

detachments

Poor repeatability and time consuming

Fig.4.1: Drawing illustrating the execution of the percussion method. Photo

credit: Mora and Philippot, 'The Conservation of Wall Paintings' in The

Conservation of Cultural Property, with Special Reference to Tropical

Conditions, 171.

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4.2. INFRARED THERMOGRAPHY (IRT)

The presence of subsurface damages can be identified and mapped through the use of thermal

imaging; the temperature variation of structural materials is monitored allowing the assessment of

damage distribution (Kordatos et al. 2013, 1261). The technique is characterised as a non-destructive,

non-contact method and can be achieved by two different approaches: active and passive

thermography.

4.2.1. PASSIVE THERMOGRAPHY

The temperature distribution within structural materials in wall paintings or, more generally, in

historical buildings, can be monitored by means of passive thermography (Hinsch et al. 2009, 97). A

camera capable of detecting thermal radiations is used to generate visual images of the temperature

distribution within an object. Thermal imaging is considered a useful tool for the evaluation of

buildings performance, where heat input, heat loss, solar gain and air infiltration can be monitored.

However, its use for the identification of structural detachments is somewhat difficult to obtain. Due

to the low conductivity of building materials and lime-based mortars, a lack of consistent temperature

variations, within a homogeneous temperature environment, between detached and integral layers is

observed (Drácky and Lesák 2006, 594).

4.2.2. ACTIVE THERMOGRAPHY

More accurate results can be acquired with the use of active thermography (Mercuri et al. 2015, 1189).

This technique is based on the excitation of the surface of the wall painting with infrared radiation

emitted by a light source. Four subcategories are described: pulsed phased, transient, vibro-

thermography and lock-in thermography. Generally, active thermography delivers structural

information of an object by documenting the emitted infrared reflection which is detected by an IR

camera; this provides two-dimensional mapping of the reflected radiation (thermograms), showing

the temperature distribution within the mural. Material discontinuities, like cracks, voids and

detachments, can either decrease or accelerate the propagation of the thermal waves generating

visual contrasts in thermograms (Ibarra-Castanedo et al. 2009, 2).

Differences between subcategories involve variations in the methods of surface irradiation and the

elaboration of results. For instance, during the ‘Lock-in Thermography’ (LT) the surface of the mural

is periodically subjected to the thermal excitation of a halogen lamp, which produces a signal at a fixed

frequency, i.e. a sinusoidal wave (Ibarra-Castanedo et al. 2009, 10). The IR camera monitors the

emitted signal and the hardware, which is ‘locked’ on the known excitation frequency, elaborates it by

measuring the phase variation which is then transformed into a visual representation. Differently, in

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‘Pulsed Phased Thermography’ (PPL), structural damages are identified by the monitoring of the

thermal response of the object, once submitted to a heat pulse of a high power source, such as Xenon

flash. Therefore, both the response to increase and decrease in heat can be monitored, allowing the

visual representation of damages according to the principle that these can conduct thermal waves

differently. Similarly, ‘Transient’ or ‘Gradual Thermography’ involves analogous elaboration of results

as PPL by using similar light sources as LT for a longer duration of time (Kordatos et al. 2013, 1262).

Therefore, the wall painting is heated through a halogen lamp until a predetermined increase in

temperature is reached (between 3C to 10 C) and its subsequence decrease is recorded. This

subcategory is regarded as the cheapest and easiest to operate among the other active thermography

techniques. Finally, the ‘Vibro-thermography’, a technique also known as ultrasound thermography,

exploits the use of sonic or ultrasonic waves to detect internal defects (Ibarra-Castanedo et al. 2009,

13). This mechanical signal will produce a complex combination of absorption, scattering and

dispersion of the waves once in contact with material inhomogeneities, and their main consequent

reaction will be the manifestation of heat. According to Kordatos et al. (2013, 1264), most effective

results can be obtained by the combined used of different subcategories (see 4.3).

4.2.3. DISCUSSION

The applicability of active thermography for the identification of structural detachments can be a

rather complex procedure, however, excellent results have been obtained in conservation practice

when different subcategories are integrated (Kordatos et al. 2013, 1264) or coupled with other non-

destructive techniques, such as DSPI (Sfarra et al. 2014, 3480) or ultrasonic (Amanatiadis et al. 2019,

472). The main drawbacks entail: high costs associated with the equipment, the extensive amount of

time required for both test executions and interpretations of data; the need for experienced

professionals to elaborate results, which can be severely disturbed by the moisture content within

building materials (Drácky and Lesák. 2006, 596); ununiform heating of the structural materials;

reflectivity of the surface (due to a highly glossy surface) and surface topography (highlighted

undulations or surface damages) (Ibarra-Castanedo et al. 2009, 29). Finally, active thermography is

primarily involved to investigate the structural properties of materials to a depths up to 10 cm

(Dandolo and Jepsen 2016, 199). Table 4.2 offers a summary of associated advantages and

disadvantage.

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Table 4.2. Advantage and disadvantages of active thermography

4.3. ACOUSTIC METHODS

Non-destructive sonic and ultrasonic techniques have been implemented for the past thirty years to

monitor the state of conservation of building materials (McCann and Forde 2001, 73). Both techniques

exploit the transmission and interpretation of elastic waves at sonic and ultrasonic frequencies, to

document the mechanical behaviour and the structure of materials. Unlike electromagnetic waves

(see 4.4), mechanical elastic waves do not propagate in a vacuum and travel faster in solids and liquid

than through the air; for this reason, acoustic methods require a coupling medium (such as a water-

based gel or a sheet of aluminium) between the transducer (piezoelectric sensor) and the surface of

the artefact (Ibarra-Castanedo et al. 2009, 12). Therefore, these techniques, despite being non-

destructive, require the direct contact with the surface of the artefact.


Results give valuable information about the

building performance Costly and time consuming technique

Non-contact technique, no risks of damages

caused by pressure

Raw data are difficult to interpret and

elaborate

Best results are obtained by integration

with other subcategories/techniques

Results might be disturbed by moisture

content of materials, irregular heating of the

surface, high surface gloss and irregularities

of its topography

Allows the inspection of large surface areas Need for expert or professional figure to

interpret data

Quantitative results Risks associated with potential overheating

of the surface

Technique allows reproducible results

(objectiveness)

High depth of measurements of up to 10

cm depths

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Typical frequencies for sonic techniques are between 20 Hz and 20kHz whilst the range for ultrasonic

waves is between 20kHz and 1MHz (Ibarra-Castanedo et al. 2009, 12). Higher frequencies have

shorter wavelengths, which lead to greater attenuation from the material analysed.6 Therefore, high

frequencies can detect small defects and generate high resolution images, but will only provide

topographical information of shallow depths (Stockwell Berriman 2004, 32). Oppositely, low

frequencies, with long wavelengths, will result in less attenuation, further penetration but a reduced

sharpness of defects. For this reason, sound techniques became less desirable and relatively low

ultrasonic frequencies, above 20 kHz and up to 200 kHz, prevailed in practical applications.

Variations in ultrasonic techniques can be classified according to the different arrangements of

sensors or based on the different modes of propagation of the ultrasonic waves (Karaiskos et al. 2015,

4). The different positions of the piezoelectric transducer and receiver are influenced by the

accessibility of the object tested and can be: (I) direct, where these are on opposite faces of the object

(Fig 4.3); (II) semi-direct, where the emitter and receiver are located at a given angle (Fig.4.4); (III)

indirect, where these are on the same surface (Fig.4.5)(Menéndez 2016, 187). Based on the way

particles oscillate, ultrasonic waves can propagate through a material in three modes: (I) longitudinal

waves (or compressional), which vibrate in parallel direction to that of the wave; (II) shear waves (or

transverse), which oscillate perpendicularly to the direction of wave propagation; (III) surface waves

(or Rayleigh), which propagate along the surface vibrating in an elliptical motion (Karaiskos et al.

2015, 4). The ultrasonic signal analysis can occur in three methods: (I) ultrasonic pulse velocity; (II)

ultrasonic pulse echo; (III) ultrasonic tomography.

6 This factor can be heavily enhanced by the presence of heterogeneous materials like lime-base mortars.

Fig.4.3, 4.4, 4.5: Representation of the different arrangements of piezoelectric transducers and receivers in a wall

painting: direct, semi-direct and indirect. Restrict accessibility might influence the choice of one arrangement over

another. Photo credit: Valentina Gatto

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4.3.1. ULTRASONIC PULSE VELOCITY

This method, one of the most common non-destructive technique in civil engineering used for the

detection of material defects, exploits longitudinal waves and consists in the monitoring of the travel

time of such waves between the source point and the receiver (these can be positioned according to

Fig.4.3,4.4 and 4.5) (Menéndez 2016, 187). By measuring different points in the material, the wave

velocity can be determined and compared: material inhomogeneities, such as cracks, small cavities

and detachments, are characterised by low ultrasonic velocity, oppositely to their surrounding

integral areas (Fais et al. 2017, 72). Acoustic velocity is considered one of the easiest and most reliable

techniques for in situ characterisation of building materials.

4.3.2. ULTRASONIC PULSE ECHO

The identification of structural flaws in building materials can be achieved by the use of ultrasonic

pulse-echo. This method might be preferred due to the restricted accessibility of the wall painting, as

it consists of a single piezoelectric element that acts both as a transducer and a receiver (Karaiskos et

al. 2015, 7). Ultrasonic stress waves emitted are reflected and backscattered once in contact with

material defects, which can be detected and documented. The efficacy of this form of testing is

somewhat reduced, due to the presence heterogeneous materials in wall paintings with high

attenuation characteristics, which will result in the need for low frequency signals (around 20 kHz)

(McCann and Forde 2001, 76). The main drawback of this technique is the formation of scattering, due

to the material heterogeneity, which can lead to confusing results.

4.3.3. ULTRASONIC TOMOGRAPHY

This type of application, despite being widely used for the examination of metals and mainly involving

high frequencies (around 1MHz), have been successfully adopted for the analysis of building materials

using low frequency transducers (20kHz to 70kHz) (Menningen et al. 2018; Maev, Green, and Siddiolo

2006, 200; Menéndez 2016, 190). Ultrasonic tomography, like X-ray tomography, can provide images

of any desired layer through a solid object, by performing a large number of measurements of different

wave paths covering the volume in need of investigation (Menéndez 2016, 189). The technique

generates either 2D or 3D images portraying damaged regions like detachments, by documenting

areas of lower ultrasonic velocity. Due to the need for multiple measurements, this technique is

regarded as expensive and time consuming.

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4.3.4. DISCUSSION

Acoustic techniques are commonly used to document the elastic characteristics and dynamic

properties of building materials; the interpretation of data is rather complex with several factors

considered highly disadvantageous for the successful propagation of sonic and ultrasonic waves

(McCann and Forde 2001, 73). The presence of moisture, the material heterogeneity, the density,

porosity and anisotropy of composite materials can generate backscattering of the waves that can lead

to confusing results (Fais et al. 2017, 72). Moreover, one of the main drawbacks of this type of

techniques is their need to maintain contact with the surface of the mural involving a coupling

medium, usually a water-based gel. Therefore, the state of preservation of the pictorial layer of the

mural and its topography have a great influence on the suitability of acoustic techniques: flaking paint

can be further damaged by the mechanical pressure applied by the sensor, whilst the presence of

undulations can hinder the correct contact of the transducer with the surface, leading to gaps in the

measurements.

Images of high resolution are the main advantage of involving acoustic techniques for the

identification of structural delamination, despite an approximate range of 2 to 10 cm depth of

measurements can be achieved (Dandolo and Jepsen 2016, 199). Some hindrances, like the high signal

attenuation spawned by material heterogeneity, can be overcome by the use of shear waves, instead

of longitudinal: these can produce images with higher resolution since they have lower propagation

velocity than longitudinal waves and, thus, smaller wavelengths that can generate less attenuation

from heterogeneous materials like lime-based mortars. Furthermore, the development of non-contact

ultrasound techniques is still a prominent area of research and resulted in the advancement of

techniques such as pulse laser generators and electromagnetic techniques (see 4.4). Table 4.3

illustrates a summary of the main advantages and disadvantages.

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Table 4.3. Advantage and disadvantages of acoustic techniques

4.4. ELECTROMAGNETIC IMAGING TECHNIQUES

Several non-destructive techniques exploit the properties of electromagnetic waves to document both

the surface and structure of building materials. Oppositely to acoustic waves, electromagnetic waves,

which are the result of interactions between an electric and a magnetic field, can propagate in a

vacuum and, hence, do not require a medium to travel (Ibarra-Castanedo et al. 2009, 12). This

component is considered a significant advantage especially in wall painting conservation practice,

where in situ logistics and restricted accessibility might prevent a close contact with the surface of the

artefact. Similar to acoustic methods, electromagnetic imaging techniques consist in the propagation

of electromagnetic waves within a material, and the subsequent monitoring of the reflected signals

received. Two electromagnetic imaging techniques, working at different frequencies, are involved for

the monitoring and mapping of the structural integrity of building materials: Ground penetrating

radar (GPR) and TheraHertz imaging (THz).


High resolution images of structural

defects and inhomogeneities

Cost –reaching the pictorial surface might

involve costly scaffolding

Semi-quantitative or quantitative results

Contact technique – the transducer needs to

be in contact with the surface of the painting

together with a coupling medium to

propagate both sonic and ultrasonic waves


with other techniques

Surface irregularities can hinder the correct

contact between the transducer and the

surface, generating gaps in the

measurements

Allows the inspection of large surface areas

Flaky and vulnerable pictorial layers are

prone to damages induced by the pressure

applied by the sensor


(objectiveness)

Results might be hindered by the presence

of moisture, the material heterogeneity, the

density, porosity and anisotropy of

composite materials

Signal attenuation – the use of shear waves

could overcome this

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4.4.1. GROUND PENETRATING RADAR

GPR is a special radar (RAdion Detection And Ranging), which exploits the propagation of microwaves

(signals with a frequency between 300MHz and 300GHz) to investigate composite building materials

reaching a depth of a few centimetres up to some meters (according to the wavelength

involved)(Catapano et al. 2020, 677). GPR is regarded as a suitable technology to characterise the

inner structure of wall paintings and identify anomalies such as cracks, voids and iron elements. Two

antenna pairs are used to send microwave pulses within the material investigated and record the

returning echoes. As the two antennas move over the surface of the artefact, a radargram is generated.

This is a bi-dimensional graph depicting the positions of the antenna as x-coordinate and the two-

ways travel times as y-coordinate (these indicate the time employed by the signal to travel along the

sensor, go through the material, be reflected and eventually be registered by the receiver)(Nuzzo et

al. 2008, 3). Eventually, 2D and 3D images can be developed.

4.4.2. THERAHERTZ IMAGING

THz techniques have been employed since 2010 to document subsurface features of wall paintings

(Dandolo and Jepsen 2016, 199). The device exploits radiations belonging to the spectral range from

about 1011 to 1013 Hz to possibly characterise the structural delamination of wall paintings, proved

that these are occurring within 1cm from the surface, i.e., the maximum depth of measurement.

Despite this limitation and the strong influence of the thickness of the overall layered structure of the

wall painting investigated, it is essential to remind that such technique allows the characterisation of

internal features and defects with a sub-millimetric spatial resolution (Catapano et al. 2020, 675).

The signal analysis is based on the same mechanism as GPR: the propagation of terahertz waves within

the material through a transducer and the consequent interpretation based on the reflected signal.

Both 2D and 3D representations of results can be obtained, with accurate characterisation of inner

features, position and thickness of inner layers (Catapano et al. 2020, 676).

4.4.3. DISCUSSION

Electromagnetic imaging techniques are based on the propagation of waves of different frequencies

(300MHz to 300 GHz for GPR and around 1 THz for THz Imaging). These technique are affected by the

same main drawbacks associated with the acoustic methods, due to the similar mechanisms of

obtaining results through the exploitation of waves. In depth penetration up to a few metres can be

achieved with GPR, whilst high resolution images of subsurface defects can be obtained with THz

Imaging; both techniques can generate 2D and 3D representations of data.

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Specifically, the successful performance of GPR is influenced by the electromagnetic features of the

material tested (Catapano et al. 2020, 677). The propagation of the microwave signals depend on the

dielectric permittivity and electrical conductivity of the material: the first, , describe the capability of

materials to polarise themselves under the influence of an electromagnetic field, whilst the second, ,

quantifies the material capability to allow the flow of an electric current. Since these parameters

depend on the atomic, molecular and granular behaviours of composite materials, it is understood

how the presence of heterogeneous materials like lime-based mortar can hinder the flow of

microwave signals.

Furthermore, backscattering and signal attenuation are limitations usually associated with both

techniques. Generally, the interpretation of radargrams is a complex task, which heavily depends on

the experience of the user.

Table 4.4. Advantage and disadvantages of electromagnetic imaging techniques


High resolution images of structural

defects (THz Imaging) Low depth of measurements – THz Imaging

High depth of measurements – GPR Less resolution and sharpness of images

obtained – GPR


with other techniques


of crystallised salts, the material

heterogeneity, the density, porosity and

anisotropy of composite materials

Allows the inspection of large surface areas Signal attenuation and backscattering


caused by pressure

The thickness of the structure can hinder

the efficacy of the technique – THz Imaging

Need for expert or professional figure to

interpret data

Moderately high costs – higher than acoustic

techniques

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4.5. DIGITAL SPECKLE PATTERN INTERFEROMETRY (DSPI)

The use of Digital Speckle Pattern Interferometry (also knowns and Electronic Speckle Pattern

Interferometry, Laser Speckle Interferometry or TV Holography) is well documented in the literature

as a non-destructive technique for monitoring deformations and delamination in historic buildings

and monuments (Paoletti and Spagnolo 1993; Boone et al. 1997; Boaglio et al. 2012; Lasyk et al. 2012;

Gülker et al. 1996; Guelker, Hinsch, and Joost 2001; Fricke-Begemann, Gu, and Joost 2000; Hinsch et

al. 2009; Graves et al. 2017). In particular, the size of material discontinuities can be accurately

identified before the occurrence of any macroscopic damage (Zanetta and Facchini 1997, 222).

This technique, which is used for the first time by Casazza (and Giovannoni 1991, 15) to monitor loose

structural areas in the wall paintings in the Brancacci Chapel in Florence, is composed of four basic

parts: a laser light source (usually a continuous wave Nd:YAG laser in the green spectral range or a

HeNe laser, both with a variety of wavelengths)7, a video camera, a hardware (to process the resulting

images), and a loud speaker to introduce mechanical energy within the structure of the wall painting

and induce the oscillation of the detachments. The laser beam is passed through a microscope

objective, before being divided by a beam splitter. One beam irradiates the artefact, whilst the second

is used as a refence one: the first beam, once in contact with the rough surface of the wall painting, is

reflected in all sorts of directions and its superimposition with the reference beam creates the speckle

pattern image (Hinsch et al. 2009, 98; Guelker, Hinsch, and Joost 2001, 188). The latter is created by

an evaluation of phase change between the reflected beam and the reference one, once the surface of

the wall painting is deformed by the acoustic excitation of the speaker (usually in the frequency range

between 100 and 1000 Hz). Therefore, the phase change of the optical waves is recorded by the

camera and final images are processed by the hardware.8

4.5.1. DISCUSSION

DSPI has been used in the field of conservation to map structural detachments in a variety of artworks

(Boone et al. 1997). The high sensitivity of this technique allows the registration of vibration patterns

generated by extremely low sources of sound like voices and the air movement. Generally, results can

be altered by unfavourable ambient conditions such as: mechanical instability of the floor, adjacent

traffic, external acoustic and optical interferences (i.e. echoing of external noises and natural light

7 Generally the longest the wavelength, the lowest the frequency and the lowest the energy irradiated on the

surface of the wall painting.

8 The Getty Conservation Institute posted a short video explaining the basic mechanism of DSPI. This briefly

describes its uses, limitations and further improvements. See ‘Laser Speckle Interferometry for Conservation

Science’, https://www.youtube.com/watch?v=aDzMB27XVxQ.

https://www.youtube.com/watch?v=aDzMB27XVxQ

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irradiating the mural from a window)(Hinsch et al. 2009, 103). Furthermore, data processing and

interpretation of results require specialised professionals or trained conservators.

The use of DSPI allows to accurately identify structural detachments in wall paintings without

requiring contact and, hence, expensive scaffoldings or risks associated with the pressure exerted on

the pictorial layer are avoided. However, as pointed out by Hinsch et al. (2009, 103), the scope of the

results obtained is limited; delamination is precisely mapped, although no categorisation among

different types and degree of delamination is possible. This is considered a potential area of research

in the diagnosis of structural detachments affecting wall paintings (see 4.6). Finally, the technique

provides quantitative results and their interpretation together with previous results from percussion

tests and the evaluation of other damages observed and monitored, were crucial to determine

whether delamination was in progress in the wall paintings in St. John’s convent at Müstair,

Switzerland (Guelker, Hinsch, and Joost 2001; Hinsch et al. 2009).

Table 4.4. Advantage and disadvantages of DSPI

4.6. CONCLUSION

The use of non-destructive techniques to monitor and identify structural delamination in wall

paintings is documented in the literature, with many examples showing the advantages usually

obtained when these are combined (Menéndez 2016, 198; Amanatiadis et al. 2019, 472-481; Sfarra et

al. 2014, 3480; Casazza and Giovannoni 1991, 15). Generally, the disadvantages associated with one



(objectiveness)


of unfavourable ambient conditions:

mechanical instability of the floor, adjacent

traffic, external acoustic and optical

interferences

Quantitative results No categorisation among different types and

degree of delamination is possible

Allows the inspection of large surface areas Need for expert or professional figure to

interpret data


caused by pressure

Moderately high costs – these are likely to

be reduced should the technique become

portable and more commonly adopted

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technique can be overcome by the advantages of another. For example, both passive and active

infrared thermography were used to identify possible regions of material inhomogeneity in two

frescoes in Santa Maria della Croce di Roio in L’Aquila (Italy) (Sfarra et al. 2014). Successively, these

were documented and categorised as delamination with the use of DSPI. Moreover, the use of a

technique that provides an holistic overview of the state of preservation of both the wall painting and

the adjacent architectural elements is described as favourable. GPR and Infrared Thermography can

both give valuable information about the internal features of structural elements and the building

performance, thus indicating possible weak areas and suggesting potential causes of delamination.9

The identification of structural delamination in wall paintings is a complex and difficult procedure,

which only reflect the intricate nature of this type of damage. Whilst the non-destructive techniques

described in this chapter offer several methods of achieving a more holistic knowledge of the

structural preservation of the wall painting considered, their involvement during conservation

practice is still quite scarce and most results are based on the traditional percussion method (Hinsch

et al. 2009, 96). Furthermore, results obtained with any of the techniques available are limited: no

qualitative information are provided and no categorisation among different types and degree of

delamination is offered (Hinsch et al. 2009, 103). Finally, understanding the correlation between

structural detachments and the presence of cracks or surface irregularities is a dynamic that needs to

be further researched and studied: hitherto, this potential connection is expressed by Tornari (et al.

2013, 2-3) and Graves (et al. 2017, 219-222).

It can be concluded that obtaining an accurate identification of structural detachments is one of the

most difficult parts of the diagnostic investigation of damages affecting wall paintings. Finally, Table

4.5 is included as a final part of this chapter, to provide a general comparison of the results obtainable

with the described techniques.

9 For instance, cold regions on a thermogram can indicate areas of the building that underwent permanent

deformation and might facilitate the access of external water.

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Table 4.5. Comparison of results, advantages and disadvantages of current techniques

Technique Main Advantages Main Disadvantages Results Costs

Visual approach and percussion method

Ease of execution, results

provide an overall indication

of the state of preservation

of the wall painting

Subjective results, mechanical

pressure exerted on the surface,

poor repeatability

Graphic representation

based on the subjective

interpretation of results

Moderate/low – executing the

technique is free, the need of

being able to reach the surface

might involve costly scaffolding

Infrared thermography (IRT)

Valuable information about

the building performance,

non-contact technique, high

depth of measurements (up

to 10 cm depths)

Results might be disturbed by

moisture content of materials,

ununiform heating of the surface,

high surface gloss and irregularities,

need for experts to interpret data

Thermograms –

illustration of the

temperature distribution

Moderately High – Active IRT

Low – Passive IRT

Acoustic methods

High resolution images of

structural defects and

inhomogeneities

Contact technique, results might be

hindered by moisture, the material

heterogeneity, the density, porosity

and anisotropy of materials, surface

irregularities and flaky paint

2D or 3D visual

representation

Moderately High

Electromagnetic imaging techniques

High resolution (THz), high

depth of measurements

(GPR), non-contact

Low depth of measurements (THz),

results might be hindered by

crystallised salts, the material

heterogeneity, the density, porosity

and anisotropy of materials

2D or 3D visual

representation High

Digital speckle pattern interferometry (DSPI)

Non-contact, portable and

easy to operate, quantitative

results easily interpreted

Results might be hindered by

unfavourable ambient conditions:

mechanical instability of the floor,

adjacent traffic, external acoustic and

optical interferences

2D visual

representation/video Moderately High

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5. DECISION-MAKING: DETERMINING TREATMENT NEEDS

Despite the current unsatisfactory state of knowledge of both causes of delamination and methods for

their identification, conservation treatments have been executed in the past mainly by injecting

different types of grouts (Biçer-Şimşir et al. 2009). However, since any conservation treatment

implemented in-situ for the preservation of wall paintings, if carried out without any climate control,

monitoring and mitigation of causes, is considered insufficient to obtain long-term effects (Brajer and

Larsen 2008, 219), questions regarding the successful development of a conservation strategy to treat

this type of damage are raised.

Discussing general conservation approaches of wall paintings can be complex, since both passive and

active treatments can only be formulated according to specific needs and characteristics of the wall

painting examined. This subject is out of the scope of this thesis although it is briefly discussed in 5.1.

This chapter argumentation is based on the following notions: (I) the introduction of current

conservation methodology of structural delamination; (II) the need for developing a decision-making

model to establish specific treatment needs for structural detachments including risks and

limitations of present methods.

5.1. CONSERVATION METHODOLOGY OF STRUCTURAL DELAMINATION

Wall paintings are integral elements of building structures and form an interface between the fabric

of the building and its environment (Mora, Mora and Philippot 1999, 3). Correspondingly, the stability

of both buildings and their environmental conditions are directly correlated to the preservation state

of the wall painting. Therefore, any attempt at treating wall paintings as isolated items, separate from

their building structure and their environment is doomed to failure (Kaszewski 2018).

Traditionally, conservation methodologies and treatments of wall paintings were mainly executed to

fulfil aesthetical requirements and were often based on assumed needs and preconceived

assumptions about condition (Kaszewski 2018; Wong and Rickerby 2013). According to Cather (2003,

412) both causes and risks associated with major damages frequently found in wall paintings have

not been sufficiently studied nor understood, and thus, not all previous treatments might have been

necessary, with some causing further long-term damages. The literature describes a general shift in

both approaches and treatments from remedial interventions to preventive and passive ones (Cather

2003, 412; Tringham et al 2013, 86). These are defined as interventions implemented to address

causes of decay (preventive) and their activation mechanisms (passive). However, it must be

highlighted that these principles should be based on the assumption that conservators can intervene

only to ameliorate ongoing detrimental change and not stop it altogether (Cather 2003, 413).

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Denoting this general change in methodology is essential when considering how structural

delamination in wall paintings was diagnosed and treated. Whilst this tendency led to a general

reduction of remedial interventions regarded as potentially invasive (such as overpainting or

extensive retouching), the investigation of structural delamination involved improvements brought

by technological advances (see Chapter 4) whilst their treatment underwent changes mainly related

to the composition of the grouts injected. According to Arnold’s hierarchy of conservation approaches,

remedial treatment, like injection grouting, are only effective at strengthening the object (Cather 2003,

413). Therefore, in the following paragraph the practice of executing such treatment is brought into

question: it felt necessary to outline a method to determine whether remedial treatment is indeed

required and eventually understand whether its effectiveness can be enhanced by passive and

preventive measures or even exceeded by these.

5.2. THE ESTABLISHMENT OF NEED FOR STRUCTURAL TREATMENT

The aim of this section is to elaborate a method of determining the treatment needs for wall paintings

affected by structural delamination, taking in consideration the limitations outlined in Chapters 3 and

4.10 Since each wall painting is considered a unique case study, with specific needs and challenges, this

argumentation is developed as a broad point of reflection. The need for structural treatments should

be formulated by the interpretation of results obtained through a diagnostic investigation comprised

of four procedures (Cather 2003, 416; Tringham et al. 2013, 86; Wong et al. 2013, 215):

I. Conservation history of the wall painting: to document previous conservation treatments,

gather archival photos to evaluate the progress of decay of the wall painting and identify

previous adverse remedial treatments. A research on the composition of these materials and

their potential influence on the development of delamination should outlined.

II. Identification of original and previous conservation materials: to obtain information

about binders and aggregates used, to identify previous conservation materials, allow their

distinction from original ones, assess their stability and potential compatibility with current

conservation materials. Ideas on the overall materials stability and potential intrinsic and

extrinsic causes of damage and delamination should be formulated.

III. Condition monitoring: ideally carried out over an extended period of time (at least one year)

to allow documentation and mapping of changes in relation to seasonal cycles, by obtaining

10 i.e. difficulties in obtaining reliable results regarding the identification of detached areas and the challenge of

determining the causes or activation mechanisms that generated the structural detachments.

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environmental measurements of RH and T (ideally both external and internal), and

understand the buffering function of the building. An overall picture of the relation between

building and wall painting should be framed together with an hypothesis on the potential

causes of structural delamination.

IV. Assessment of rate of change: this should provide information on the evolution of

delamination over time. One or a combination of technical analysis (as described in Chapter

4) should be implemented at regular intervals over an extended period of time (at least one

year) to understand if delamination is in progress, quantify its severity and frequency and

reinforce or weaken the previously formulated hypothesis on the potential causes of damage.

Due to the complexity of the system analysed (formed by the building, the wall painting and both

external and internal environments) this initial diagnostic approach should be flexible and allow the

re-assessment and re-interpretation of data accumulated (Cather 2003, 416). Executing diagnostic

research in situ is far more easily advocated than practised, due to limitations of site-specific logistics

and potential complications in implementing technical analysis. Generally, diagnostic investigation

can be extremely challenging and the possibility of retaining certain questions unanswered is likely

(Wong et al. 2013, 215). Consequently, the need for elaborating a decision-making model that can

incorporate case-specific characteristics is expressed. The model should not provide a single and

definitive answer regarding the severity of structural delamination, but it should be designed in such

a way that all questions can be independently answered and that only by the ultimate interpretation

of results, the user can allocate a ‘type of delamination’11 and its consequent need for treatment

(Bolhuis et al. 2017, 1-7). To further outline the necessity of establishing a decision-making model to

assess the need for remedial treatment of delamination in wall paintings, a brief evaluation of the risks

associated with the current treatment (i.e. injection grouting) is provided in the next paragraph.

5.2.1. INJECTION GROUTING: RISKS AND LIMITATIONS

Currently, structural treatments of delamination in wall paintings involve the injection of grouts, a

bulked fluid material with adhesive properties, through pre-drilled holes specifically executed to

allow the access of either a syringe or a small tube. Despite being an irreversible treatment, grouting

is considered an important method for the stabilisation of architectural surfaces in situ (Biçer-Şimşir

et al. 2009, 3). The latter started to be implemented as an alternative to detaching wall paintings and

mosaics approximately in the late 1970s, with the earliest published research dating back to the

11 This should not be confused with the different types of delamination described in 3.1. The name ‘type’ here is

taken from the terminology ‘type of risks’ expressed by Waller in his article about risk assessments, and it is used

to describe the extent of damage (i.e. structural delamination) based on its severity and frequency (see 5.3).

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1980’s. Because of the invasive and irreversible nature of injection grouting, much of its development

focused on two main parameters: the compatibility with original materials and the re-treatability.

Therefore, a variety of materials combination, usually involving a binder, a filler, a suspension medium

and sometimes some additives (such as plasticisers, fluidisers and retarders) together with a variety

of commercially available products have been used to reinstate the adhesive and cohesive properties

of mortars and renderings (Pasian et al. 2018, 1).

Generally, conservators tend to generate grouts with similar material composition of the mortar

involved in the structure of the wall painting treated, with the aim of replicating its working properties

and performance characteristics. However, these types of grout would not produce properties

desirable for injection and would possess stronger adhesive properties then the original mortar, since

this would be in a weak and deteriorated state (Biçer-Şimşir et al. 2009, 4). As a result, additives were

involved with the aim of increasing flow and modify the curing properties of grouts. Currently, only

one analytical study regarding their working properties and performance characteristics exist (Biçer-

Şimşir and Rainer 2011).

A few general risks, both intrinsic and extrinsic, can be described. According to Rickerby et al. (2004,

481), the successful injection of grouts mainly depends on the experience and judgment of the

conservator. Therefore, the level of inexperience of the practising conservator can be a factor of risk,

especially when considering that the treatment involves the drilling of injection holes. Other adverse

effects can be caused by the specific material composition of the grout injected (Biçer-Şimşir et al.

2009, 4). In particular, the need for creating a fluid material required the addition of both fluidisers

and diluents, like water. The latter can potentially generate the harmful re-dissolution and re-

crystallisation of salts (see 3.3.1 and 3.3.3) . Finally, as aforementioned, grouts can exhibit excessive

adhesive and mechanical strengths, due to their composition. This can generate mechanical damages

with consequent permanent deformations, due the to the different reaction of grouts and original

mortar to thermal, external or internal stresses (Torraca 2009, 69,86; Torraca 2005, 119). Ironically,

it can be concluded that injection grouting can hypothetically result in additional damages to the

structure of wall paintings.

5.3. DEVELOPMENT OF DECISION-MAKING MODEL

The identification and treatment of structural delamination is a substantially complex task to execute.

A decision-making model should be formulated as an aiding tool to help conservators to address this

phenomenon from two perspectives: (I) diagnostic investigation; (II) comparison of remedial, passive

and preventive interventions. The first part of the decision-making model should be framed to

accurately document and interpret data acquired during a diagnostic investigation, assess the severity

of damage and a correspondent treatment needs and urgency. Ideally, the model should be flexible

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and allow a ‘custom-made’ design so that it can be shaped around case specific characteristics. Finally,

to help conservator in assessing the need for treatment of delamination, a table, such as the one

illustrated by Waller (1994, 12), depicting different grades of frequency and severity of damage is

included in the model (see Table 5.1). This way, the user would be able to allocate a ‘type of

delamination’ and therefore judge the treatment need for the wall painting studied. The second part

of the decision-making model should be formulated to allow the comparison of different existing

remedial, passive and preventive interventions. This should include an evaluation of grouts

composition, their affinity with original materials and application as indicated by Turk et al. (2019).

The latter is out of the scope of this thesis, however its elaboration and implementation during

conservation practice are considered advantageous and represent an area of study in need of further

research.

Examples of decision-making models (The Foundation for the Conservation of Contemporary Art

(SBMK) 1999; Bolhuis et al. 2017, 1-7), guidelines for the conservation of wall paintings (ICOMOS

Principles for the Preservation of Wall Paintings 2003; Historic England 2002) and other publications

(Wong et al. 2013, 15; 2012, 1) have been consulted to allow the initial argumentation of the first part

of the decision-making model. As suggested by Bolhuis et al (2017, 2), the decision-making model

should be designed as a series of open questions that need to be answered and weighed so that the

user can both interpret the results obtained and also decide if certain aspects are more or less

important to the specific case-study analysed. The questions composing the model should be built

around the four categories outlined in 5.2: (I) Conservation history of the wall painting; (II)

Identification of original materials; (III) Condition monitoring; (IV) Assessment of the rate change.

Other parameters, such as, budget and significance, would be included in the second part of the

decision-making model.

Lastly, the structure of the model should allow the user to make his/her own assessments based on

the interpretation of results and to allocate a ‘type of delamination’ as shown in Table 5.1. The

flexibility of the questionnaire would allow the creation of a table with specific characteristics of

severity and frequency, that are within the scope of the wall painting studied and that reflect the

results obtained. This is due to the ascertained complexity of the phenomenon of structural

delamination and the need for elaborating results and interpretations that are individual to the case

study. As a result, the conservator would be able to add detailed observations peculiar to the type of

delamination identified or to damages triggered by this, especially in relation to the presence of

surface cracks and irregularities.

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Table 5.1: Example of table involved to define treatment needs for structural delamination

Finally, the conservator should be able to allocate one of the four treatment needs described in the

Guidelines for Assessment and Recommendations of Wall Paintings in Danish Churches of the National

Museum of Denmark (n.d.). It must be reminded that the ultimate choice on the type of treatment to

implement should be executed by completing the second part of the decision-making model.

I. Acutely in need for treatment: indicating an enhanced plaster instability and urgency of

treatment together with the need for ameliorating causes and activation mechanisms of

delamination (passive/preventive measures) (as in the case of the Cave 85 in the Mogao

Grottoes, Rickerby et al 2004, 480-486).

II. In need for treatment: referring to an unstable condition of the structure of the wall

paintings where a combination of passive and preventive interventions need to be

implemented with the aim of reaching a status as described in sections III and IV.

III. Possibly in need for treatment: where delamination is only moderately present and any

intervention should aim at stabilising the system by addressing causes and activation

mechanisms (passive/preventive measures).

IV. Satisfactory condition: describing a situation where delamination might be identified as

stable, thus, indicating the need for maintaining a regular inspection and monitoring strategy.

Severity

Fre

qu

ency

Structural

Delamination

1: Ongoing

progress: unstable

2: No visible signs of

ongoing progress: unstable

3: Inactive:

stable

1: Widespread

2: Major

3: Moderate

4: Limited

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5.4. PROTOTYPE OF THE DECISION-MAKING MODEL

An initial prototype of the decision-making model described in the previous section is here presented.

This should only be regarded as a first layout of the model, which would require consequent editing

and re-formatting, both of its content and shape. Ideally, the model would consists of a total of four

sheets, each indicating an area of diagnostic investigation, as described in section 5.2. Moreover, the

model would include a manual for use and a glossary (see Appendix VIII) .

The four tables comprising this prototype are reported in this section (Fig. 5.1, 5.2, 5.3, 5.4) and a list

of all the questions composing the model is reported in Appendix VII. Whilst all the information

gathered from the decision-making model are considered essential for the purpose of establishing the

need for treatment of delamination, several questions are highlighted throughout the model. These

are deemed particularly meaningful to achieve the aforementioned scope of the model and ultimately

appoint one of the four treatment needs described in 5.3. Further research should focus on

establishing the suitability of the questions included and the overall effectiveness of the model in

conservation practice.

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Fig.5.1: Table 1 of the prototype of decision-making model. Photo credit: Valentina Gatto

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6. MARIA CHURCH AND THE WALL PAINTINGS: A REVISION

The case study of the wall paintings in Maria Church in Nisse exhaustively represents the limitations

encountered when approaching the diagnosis of structural detachments. The identification of this

type of structural damage is often approached in conservation practice with the percussion method,

a cumbersome technique, which often generates incongruent results (Guelker, Hinsch, and Joost 2001,

184). Furthermore, conservation treatments, often based on preconceived assumptions of the state of

preservation of the murals, mainly involve the injection of different type of grouts. As described in

Chapter 5, the long-term effectiveness of such remedial treatments are in need of further research and

evaluation.

The aim of the final chapter of this thesis is to accomplish an assessment of the information gathered

from the previous conservation reports of the wall paintings in Maria Church, to elaborate the

succeeding concepts: (I) potential causes of structural delamination; (II) suitable identification

methods for the identification of delamination. Lastly, this case study offers the opportunity to

implement and test, for the first time, the potential effectiveness of the prototype of the decision-

making model outlined in section 5.4.

6.1. POTENTIAL CAUSES OF STRCUTURAL DELAMINATION

As described in Chapter 3, the causes usually associated with the development of structural

delamination are complex and their occurrence is often simultaneous and interdependent. Generally,

it can be argued that the current condition of the wall paintings in Maria Church have been severely

affected by both environmental conditions and previous adverse conservation treatments.

The main environmental factors considered potentially responsible for the formation of detachments

are the previous leaking of water from the roof and fluctuations of relative humidity (Crijns,

Morelissen, and Duivenvoorden 2020). Moisture in building is considered the main cause of

degradation of wall paintings and a fundamental trigger of secondary reactions responsible for the

disintegration of their structure (Mora, Mora and Philippot 1999, 174). Therefore, the role of moisture

is essential for the phenomenon of migration and re-crystallisation of salts and for the consequent

chemical degradation and dissolution of lime (see 3.3 and Appendix VI). Moreover, the formation of

structural detachments in the wall paintings in Maria Church can be correlated to the presence of bats.

Bats droppings and urine can have deleterious effects on porous materials like lime, marble or stone

(Hales 2014, 98). Droppings were documents on the upper area of the wall paintings on the triumphal

arch (Jorien Duivenvoorden, email to author, March 19, 2020) and on the door below the wall painting

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of St. Christopher (Fig. 6.1, 6.2, 6.3). The main influence of bats dropping and urine on the formation

of delamination are attributed to the subsequent deposition of salts and dissolution of lime.

Additionally, the location of the wall painting of St. Christopher is considered favourable for the

development of structural delamination. The mural is positioned on the north side of the church and

is integrated within a wall that is directly exposed to the outer environment. The formation of

condensation, caused by the positioning of the wall painting on the coldest area of the church and the

direct exposure of the building support to atmospheric agents, might contribute to the development

of delamination.

Previous conservation treatments executed in Maria Church are argued to have strongly influenced

the development of delamination in the structure of the wall paintings. The cleaning of their pictorial

surface with acetic acid (80% in aqueous solution) is believed to have generated structural damages.

Acetic acid can dissolve calcium carbonate to form a salt, calcium acetate (Fig. 6.4),thus contributing

to the formation of structural delamination.

2CH3COOH + CaCO3

acetic acid + calcium carbonate

(CH3COOH)Ca + CO2 + H2O calcium acetate + carbon dioxide + water

Fig.6.4: Reaction of acetic acid and calcium carbonate. The latter is dissolved to form a salt, calcium acetate

Fig.6.1, 6.2, 6.3: Door below the wall painting depicting St. Christopher showing signs of bats droppings and

possible degradation of the polychrome surface due to urine and high relative humidity.

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Furthermore, the injection of a two-component synthetic adhesive to re-establish the adhesive and

cohesive properties of the structure of the wall paintings, the filling of lacunae with a hardened

gypsum-based material and the final application of a PVA resin to consolidate the pictorial surface

could have strongly contributed to the formation of delamination. Whilst all these materials possess

different permeability properties, only the synthetic grout and PVA resin become hydrophobic, once

cured. As a result these materials can generate the deflection or interruption of moisture passage with

the subsequent change of the evaporation zone leading to localised internal stresses that can cause

the mechanical failure of the surrounding original material.

The accurate identification of the causes of structural delamination in the wall paintings in Maria

Church would require a through in-situ assessment and, potentially, the interpretation of samples to

obtain more precise information regarding the layer archaeology of the murals and the hydraulic and

mechanical properties of its constituent materials. Therefore, the argumentation of this paragraph

represents an initial assumption of the potential causes of delamination.

6.2. SUITABLE METHODS FOR THE IDENTIFICATION OF STRUCTURAL DELAMINATION

The wall paintings in Maria Church significantly exemplify the need for implementing, during

conservation practice, a more scientific and objective method to document structural detachments.

The involvement of different executions of the percussion method to document delaminated regions

in the wall paintings on the triumphal arch are described by Crevecoeur (2017) and by Crijns,

Morelissen, and Duivenvoorden (2020) respectively (see 2.4). The results obtained are strikingly

conflicting, with one implementation indicating the presence of 96 locations of structurally detached

areas (Crevecoeur, 2017) and another one detecting only 44 (Crijns, Morelissen, and Duivenvoorden

2020). The scarce repeatability of this method and the pronounced influence of the experience of the

conservator involved during the investigation are clearly outlined in the remarkable difference of

these results.

The layout of the historical building, the surface topography and the pictorial preservation state of the

wall paintings considered can have a profound influence on the choice of the identification technique.

The wall paintings in Maria Church are differently integrated within the building and, thus, different

approaches might be in need. The wall painting of St. Christopher appears more easily reachable than

the murals on the triumphal arch. As a result, techniques that require the direct contact with the

surface of the mural, such as the ultrasonic method, could be considered for this mural (see Chapter

4). The wall paintings on the triumphal arch are more difficult to reach, hence, the implementation of

a technique that can be used remotely, such as DSPI, GPR and THz Imaging, would be ideal. As

aforementioned, the state of preservation of the pictorial surface and its topography are crucial factors

to consider, when selecting a suitable technique. Whilst the pictorial surface of all murals is considered

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in a good state of conservation, hence, possibly indicating the suitability of a technique that requires

the close contact with their surface, their topography appears quite irregular and undulated (see

section 2.4 and Fig.2.11). This aspect might hinder the adequate contact between the sensor and the

surface, thus interfering with the successful attainment of results. Moreover, the use of an ultrasonic

technique for the identification of structural detachments in the wall paintings on the triumphal arch

would require the construction of either a scaffolding or a movable platform. The potential influence

of the vibrations generated by a movable platform could produce an increase of the deterioration rate

of the wall paintings and worsen its structural instability (see 3.4.2).

Consequently, during a preliminary inspection of the church, conducted with Merel Schrojenstein

Lantman12 and Roger Groves13, the most suitable technique to identify structurally delaminated areas

in the wall painting in Maria Church was deemed to be DSPI. However, this method presents some

disadvantages that require some adjustments. The distance between the floor and the wall paintings

on the triumphal arch is too great and the equipment needs to be placed closer by using a ladder or a

zip-up scaffold. Furthermore, the wall painting of St. Christopher is located on the north wall, in front

of a glass window. The solar light might interfere with the laser beam of the technique, ultimately

compromising the results. For this reason, testing of the wall painting of St. Christopher, might have

to be performed either after sunset or with the use of a shielding material to be placed over the

windows. Due to its positioning on the north area of the church and the direct exposure of its building

structure to the outer environment, structural delamination in the wall painting of St. Christopher is

hypothesised to be more enhanced than in the wall paintings on the triumphal arch.

Lastly, the lower area of the wall painting of St. Christopher can be easily reached and inspected with

a ladder; this will allow the additional assessment of the suitability of the ultrasonic method, to verify

whether the transducer can be appropriately placed in contact with the surface. Specifically, the

method of phased array ultrasonic testing will be involved in this phase. This portable equipment

consists of many small ultrasonic transducers that can be activated independently, thus allowing the

progressive steering of the beam and the formation of images showing material inhomogeneities

(Roger Groves, personal communication, 26 March, 2020) (see 4.3). The main advantages of this

method are the following: the absence of any gel or liquid between the transducer and the surface (as

this is already present within the transducer) and the possibility of controlling the direction of

ultrasonic beam electronically (and not manually), thus conveying results in a faster manner.

12 Lecturer of the Conservation of Historic Interior specialisation at the University of Amsterdam and freelance

accredited conservator.

13 Associate Professor of the Aerospace Engineering Department at TU Delft.

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6.3. STRUCTURAL DELAMINATION: HOW TO DETERMINE TREATMENT NEEDS?

The treatment needs for a wall painting affected by structural delamination should be elaborated by

the interpretation of results obtained through a diagnostic investigation based on the four procedures

described in 5.2. and 5.4 (see Fig. 5.1- 5.5). The RCE started a diagnostic investigation to determine

the general treatments needs for the wall paintings in Maria Church in 2019. Only the initial phase of

this investigation was executed and, as a result, an argumentation to whether the wall paintings are

in need for structural treatments is currently unattainable. However, this case-study offers the

opportunity to test the implementation of the prototype of the model for the first the time and,

potentially, assess its effectiveness. Furthermore the initial steps of the decision-making model can be

completed, thus allowing the elaboration of hypothesis regarding causes of delamination (see 6.1) and

the identification of a suitable method for their identification (see 6.2).

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7. CONCLUSION AND FURTHER RESEARCH

The main purpose of this master’s thesis is to obtain an insightful understanding of the process of

formation of structural detachments in wall paintings. This study is considered a literature review of

potential causes of structural detachments and methods of their identification. Additionally, it is

regarded as a starting point for the development of a decision-making model to establish specific

treatment needs for structural detachments.

Detachments are physical forms of damage induced by either external or internal mechanical stresses

resulting in the progressive loss of adhesive and cohesive properties of heterogenous layers in the

structure of wall paintings (Wang 2011, 121; Fricke-Begemann, Gu, and Joost 2000, 538). Causes of

delamination are complex and their occurrence is generally concurrent and interdependent (Mora

1974, 11). As a result, their precise identification can be a rather intricate task. A correlation of

intrinsic, extrinsic and environmental factors can cause physical and chemical degradation processes

that can generate structural delamination (D’Ossat 1982, 8-24). Future research should focus on

corroborating the influence of acidic conditions and the migration and re-crystallisation of salts on

the chemical dissolution of calcite. Lastly, an elevated presence of moisture in building materials is

regarded as a fundamental activation mechanism of causes of delamination: its monitoring and

control should always be prioritised.

In conservation practice, the identification of structural detachments in wall paintings is

predominately obtained by documenting the acoustic response generated by the tapping of the

pictorial surface and its subsequent interpretation (Hinsch et al. 2009, 96; Guelker, Hinsch, and Joost

2001, 188). This method, known as the percussion approach, can generate confusing results due to its

poor repeatability and subjectivity; the case study of the wall paintings in Maria Church exhaustively

illustrates the limitations of this methods. Suitable and effective techniques to identify structural

damages involve the implementation of: IRT (Infrared Thermography), ultrasonic techniques, GPR

(Ground penetrating radar), THz Imaging (Terahertz Imaging) and DSPI (Digital Speckle Pattern

Interferometry). All the above-mentioned techniques, excluding ultrasonic ones, are non-contact and

offer a variety of resolution of images (the sharpest being THz Imaging) and depth of measurements

(the highest being GPR). Signal attenuation and backscattering are considered the most significant

disadvantages and are caused by the heterogeneity of structural materials of wall paintings.

Furthermore, results obtained with any of these techniques are limited: no qualitative information are

provided and no categorisation among different types and degree of delamination is offered (Hinsch

et al. 2009, 103). Finally, understanding the correlation between structural detachments and the

presence of cracks or surface irregularities is a dynamic that needs to be further researched (Tornari

et al. 2013, 2-3; Graves et al. 2017, 219-222).

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The case study of the wall paintings in Maria Church is considered to extensively illustrate the present

problematics encountered in conservation practice. Current conservation treatments mainly involve

the injection of different types of grouts to re-establish the structural cohesiveness of wall paintings

affected by delamination (Biçer-Şimşir et al. 2009). However, the implementation of this irreversible

procedure is rarely justified and long-term effects of the adhesive mixtures injected need to be

evaluated; it is hypothesised that some of these materials could have adversely affected the structural

stability of murals. Therefore, interests for the development of a decision-making model to establish

the specific needs for treatment of structural delamination is deduced: a prototype of decision-making

model is outlined in this thesis and the effectiveness of both contents and layout needs to be assessed.

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8. REFERENCE LIST

8.1. BOOKS, CHAPTERS OF EDITED BOOKS, JOURNAL ARTICLES, CONFERENCE PAPERS, AND

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Rickerby, Stephen, Lisa Shekede, Fan Zaixuan, Tang Wei, Qiao H ai andYang Jinjian. 2004.

"Implementation of Grouting and Salts-Reduction Treatments of Cave 85 Wall Paintings." In

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Conference on the Conservation of Grotto Sites, Mogao Grottoes, Dunhuang, People’s Republic of

China, June 28–July 3, 2004, edited by Neville Agnew, 480–487. Los Angeles: The Getty


Rickerby, Stephen, Lisa Shekede, Fan Zaixuan, Tang Wei, Qiao Hai, Yang Jinjian and Francesca Piqué.

2004. "Development and Testing of the Grouting and Soluble-Slats Reduction Treatments of

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Second International Conference on the Conservation of Grotto Sites, Mogao Grottoes,

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1256-1267.

Sfarra, S., C. Ibarra-Castanedo, D. Ambrosini, D. Paoletti, A. Bendada, and X. Maldague. 2014. "Non-

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Science and Engineering 39 (5): 3461–3480.

Stambolov, T., and J.R.J. van Asperen de Boer. 1976. The Deterioration and Conservation of Porous

Building Materials in Monuments: A Review of the Literature. Rome: ICCROM, International

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Stockwell Berriman, James Robert. 2004. "The Application of Air-Coupled Ultrasonic Systems and

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Suneson, Carola. 2001. "Hydraulic Lime Based Injectable Mortars." In Conservation of Mural Paintings,

edited by Ulf Lindborg, 39–44. Stockholm: National Heritage Board.

The Foundation for the Conservation of Contemporary Art (SBMK). 1999. The Decision-Making Model

for the Conservation and Restoration of Modern and Contemporary Art.

Tornari, V., E. Bernikola, E. Tsiranidou, K. Hatzigiannakis, M. Andrianakis, V. Detalle, and J.L. Bodnar.

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Wall Paintings." International Journal of Heritage in the Digital Era 2 (1): 1–22.

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edition. Rome: ICCROM, International Centre for the Study of the Preservation and the

Restoration of Cultural Property.

Torraca, Giorgio. 2009. Lectures on Materials Science for Architectural Conservation. Los Angeles: The

Getty Conservation Institute.

Tringham, Sibylla, Charlotte Martine de Fonjaudran, Sharon Cather, and Cynthia de Giorgio. 2013.

"Passive and remedial approaches to salt damage and biodeterioration of wall paintings and

monuments in the Crypt of the Grand Masters, St. John's Co-Cathedral, Valletta." In

Wandmalereien in Krypten, Grotten, Katakomben: Zur Konservierung Gefasster Oberflächen in

Umweltgeschädigten Räumen ; Symposium, Germain National Committee of ICOMOS,

Quedlinburg, Germany, 3-6 November, 2011, edited by Thomas Danzl, Matthias Exner, Ralph-

Günther Patocka, Elisabeth Rüber-Schütte, Uwe Steinecke, and John Ziesemer, 85–96.

Quedlinburg: ICOMOS.

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"Decision Support Criteria and the Development of a Decision Support Tool for the Selection

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Waller, Robert. 1994. "Conservation Risk Assessment: A Strategy for Managing Resources for

Preventive Conservation." Studies in Conservation 39 (sup2): 12–16.

Wang, Wanfu. 2011. "Detection of Delamination in Wall Paintings by Ground Penetrating Radar." In

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Correia, Pamela Jerome, Marcial Bondet, Maddalena Achenza, 1–14. Lisbon: Argumentum.

8.2. WEBSITE CONTENTS AND UNPUBLISHED DOCUMENTS

BibleGateway. N.d. “King James Version (KJV).”Accessed August 20, 2020.

https://www.biblegateway.com/passage/?search=1%20Kings%2011%3A36&version=KJV

Crevecoeur, Robert. 2017. “ Schilderingen in de Mariakerk te Nisse: Nader Bekeken.

Crijns Bernice, Rutger Morelissen, and Jorien Duivenvoorden. 2020. “Mariakerk Nisse DRAFT Report.”

Dupa-grund: Fissativo consolidante a solvente per esterni. N.d. “Prodotti.” Accessed August 20, 2020.

https://www.caparol.it/prodotti/preparazioni-risanamenti-e-rasature-armate/fondi-a-

solvente-trasparenti/dupa-grund

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Information.” http://www.proyectopatrimonio.info/wp-content/uploads/2018/11/Wall-

painting-conservation-documents.pdf

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Preservation and Conservation/Restoration of Wall Paintings.”

https://www.icomos.org/en/what-we-do/focus/179-articles-en-

francais/ressources/charters-and-standards/166-icomosprinciples-for-the-preservation-

and-conservationrestoration-of-wall-paintings

Kaszewski, Lucy. 2018. “Wall Paintings and Environment.”

https://www.buildingconservation.com/articles/wallpainting/wallpainting.html

Mariakerk Nisse: Protestant Congregation in Nisse. 2020. “Church Building, History.”

https://www.mariakerk-nisse.nl/de-kerk/historie/

Mariakerk Nisse: Protestant Congregation in Nisse. 2020. “Church Building, Exterior.”

https://www.mariakerk-nisse.nl/de-kerk/exterieur/

https://www.biblegateway.com/passage/?search=1%20Kings%2011%3A36&version=KJV

https://www.caparol.it/prodotti/preparazioni-risanamenti-e-rasature-armate/fondi-a-solvente-trasparenti/dupa-grund

https://www.caparol.it/prodotti/preparazioni-risanamenti-e-rasature-armate/fondi-a-solvente-trasparenti/dupa-grund

http://www.proyectopatrimonio.info/wp-content/uploads/2018/11/Wall-painting-conservation-documents.pdf

http://www.proyectopatrimonio.info/wp-content/uploads/2018/11/Wall-painting-conservation-documents.pdf

https://www.icomos.org/en/what-we-do/focus/179-articles-en-francais/ressources/charters-and-standards/166-icomosprinciples-for-the-preservation-and-conservationrestoration-of-wall-paintings



https://www.buildingconservation.com/articles/wallpainting/wallpainting.html

https://www.mariakerk-nisse.nl/de-kerk/historie/

https://www.mariakerk-nisse.nl/de-kerk/exterieur/

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Mariakerk Nisse: Protestant Congregation in Nisse. 2020. “Church Building, Interior.”

https://www.mariakerk-nisse.nl/de-kerk/interieur/

National Museum of Denmark. N.d. “Wall Paintings in Danish Churches: Assessment and

Recommendation.” Accessed August 20, 2020. https://en.natmus.dk/sales-and-

services/museum-services/wall-paintings-in-danish-churches/assessments-and-

recommendation/

Nefertari Tomb. N.d. “Conservation Process of the Wall Paintings.”


Rijksdienst voor het Cultureel Erfgoed (RCE), National Monuments Register. 2020. “Monument

number: 10008, Dorpsplein 49, 4443 AG in Nisse.”

https://monumentenregister.cultureelerfgoed.nl/monumenten/10008

TIS Climate/Humidity Table. N.d. Accessed August 16, 2020.

https://www.tis-gdv.de/tis_e/misc/klima-htm/

Wong, Lori, and Stephen Rickerby. 2013. “Conservation and Management of the Tomb of

Tutankhamen: Conservation Planning and Implementation.”

https://www.getty.edu/conservation/our_projects/field_projects/tut/planning.html

YouTube, Getty Conservation Institute. 2013. “Laser Speckle Interferometry for Conservation

Science.” https://www.youtube.com/watch?v=aDzMB27XVxQ

https://www.mariakerk-nisse.nl/de-kerk/interieur/

https://en.natmus.dk/sales-and-services/museum-services/wall-paintings-in-danish-churches/assessments-and-recommendation/




https://monumentenregister.cultureelerfgoed.nl/monumenten/10008

https://www.tis-gdv.de/tis_e/misc/klima-htm/

https://www.getty.edu/conservation/our_projects/field_projects/tut/planning.html

https://www.youtube.com/watch?v=aDzMB27XVxQ

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SUMMARY

Structural detachments in wall paintings: Causes, methods of identification and decision-

making – Case study of Maria Church, Nisse.

Valentina Gatto, University of Amsterdam, August 2020.

Detachments are physical decaying processes that gradually cause the loss of adhesion and cohesion

of heterogeneous layers in the structure of wall paintings. Consequently, the partial separation

between pictorial layers, plaster layers and their support is a phenomenon frequently affecting the

structure of murals. Common conservation approaches are based on the identification of areas of

delamination by means of subjective and empirical methods, such as the percussion approach. The

latter is based on the gentle tapping of the surface of the wall painting and the manual documentation

of the acoustic response generated. Successively, delaminated regions of the structure of murals are

consolidated by injecting a variety of adhesive mixtures. This procedure is frequently described in the

literature as an obvious treatment and very little justification and evaluation of the long-term effects

of such adhesives are provided.

Therefore, three main areas of research and related aim were determined: (I) a description of

potential causes associated with structural detachments is found in Chapter 3; (II) an evaluation of

traditional and modern methods of identification with an overview of associated advantages and

disadvantages is formulated in Chapter 4; (III) the need for developing a decision-making tool to

establish treatment needs for structural detachments is expressed in Chapter 5. Additionally, the case

study of the wall paintings in Maria Church exemplifies the need for investigating the aforementioned

areas of research; Chapter 6 elaborates the probable causes, methods of identification of structural

detachments and the possibility of implementing a prototype of decision-making model to determine

their treatments needs.

Generally, causes associated with structural delamination are induced by either internal or external

mechanical stresses. Their main causes are: (I) intrinsic causes related to inherent fault of materials;

(II) environmental factors, primarily associated with the presence of moisture and fluctuations of

relative humidity; (III) extrinsic causes related to human factors. Suitable and effective methods to

characterise delamination usually involve the implementation (often combined) of: IRT (Infrared

Thermography), ultrasonic techniques, GPR (Ground penetrating radar), THz Imaging (Terahertz

Imaging) and DSPI (Digital Speckle Pattern Interferometry). Lastly, the formulation of a prototype of

decision-making model to determine the need for treatment of delamination is outlined: its

effectiveness needs to be evaluated.

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APPENDIX I: ICONOGRAPHY OF THE WALL PAINTINGS IN MARIA CHURCH

The detailed study and interpretation of the saints and symbols depicted on the wall paintings in Maria

Church is out the scope of this thesis. However, it was deemed necessary to include an introduction of

the main themes represented to provide further information regarding the historical and artistic

context of this fascinating church.

I.I. THE WALL PAINTINGS ON THE TRIUMPHAL ARCH

The wall paintings on the triumphal arch represent three scenes depicting the life of the Virgin Mary,

to whom the church is dedicated.

The Coronation of Mary is a popular religious theme expressed in Christian art since the twelfth

century (González 2013, 4). In Maria Church, this subject is represented on the centre of the triumphal

arch (Fig.2.6, 2.7); here two figures are portrayed standing on a black and white chequered tile floor,

the only pictorial component preserved able to convey spatial suggestion and perspective. The two

figures are respectively: Mary, depicted on the left side wearing a red robe, a pink undergarment and

a crown, and God the Father represented with an adorned yellow brocade dress and two crowns, one

on his head and Mary’s crown, held by his hand. Jesus Christ was assumingly part of this composition

and was originally painted on the lap of his mother, Mary. Furthermore, two other elements are

depicted: a peculiar harp is discernible on the left of Mary whereas the remnants of a red stick with a

yellow flag and a red pennant can be seen on the right side of God. The composition is framed within

two pink pillars on either sides and a pink and yellow stripe with stylised plant motives on the top.

The lower right side of the triumphal arch exhibits another recurrent artistic Christian subject; the

announcement by the Archangel Gabriel to the Virgin Mary of the conception and further incarnation

of Jesus (Fig.2.8). In the church, this theme is portrayed by Mary and the Archangel Gabriel painted

next to each other in a seemingly simple room. The latter is exemplified by the presence of a small

stained glass window on the left. A few letters can be distinguished around the shoulder of the

Archangel Gabriel; a,i, and either a T or an F. Spatial suggestion is furthermore enhanced by the

presence of a red and white chequered floor.

The last theme represented within the arch is located on the lower left side. Here the Tree of Jesse, the

schematic representation of the ancestors of Jesus, can be observed (Fig.2.9). A total of eight figures

are depicted surrounded by flowers, leaves and branches, therefore forming a tree-like composition,

showing some of Mary’s descendants. The first figure visible at the bottom of the tree is Jesse of

Bethlem, now hardly recognisable, followed by his son, King David, who is portrayed as a bearded

youth wearing a crown, a red cloak and holding a harp. His name is written on a nameplate to the right

of his head and it is currently still visible. The composition terminates with the representation of Mary,

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the last descendant of David. Mary is represented wearing a red dress, holding Jesus Christ, within a

pointed oval shape, made of branches and leaves. Special pictorial elements of this wall paintings are

the burning candles, painted next to several figures of the tree; their symbolic meaning is potentially

related to a biblical passage (1 Kings,11:36, from the King James version) which states as follow: “and

unto his son will I give one tribe, that David my servant may have a light always before me in

Jerusalem, the city which I have chosen me to put my name there (BibleGateway n.d.).”

I.II. THE WALL PAINTING ON THE NORTH WALL A wall painting depicting Saint Christopher, patron saint of travellers and pilgrims, can be seen on the

left side of the main entrance (Fig. 2.10). The location of this is tightly connected to the significance of

this saint; worshippers could appreciate viewing a representation of the martyr responsible for the

protection of pilgrims the moment they entered the church. In Maria Church, the saint is depicted

standing nearby a river. On the background, two rows of palm trees and a building with a round dome

are visible. A hermit is painted exiting the this building, wrapped in a yellow cloak, admiring the

gesture of the martyr whilst he crosses the river carrying the infant Jesus Christ on his right shoulder,

now almost entirely disappeared. Saint Christopher is portrayed wearing a red dress, a turban

wrapped around his curly hair and a golden-yellow beard. The martyr is holding a tree trunk with

both hands, while his head is leaned over his shoulder and slightly turned to the right, to look at the

infant Jesus Christ.

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APPENDIX II: LOCAL NEWSPAPER ‘PROVINCIALE ZEEUWSE COURANT’

Fig. I.I: Local newspaper ‘Provinciale Zeeuwse Courant’, issued on December 9th, 1986.

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Fig. I.II: Local newspaper ‘Provinciale Zeeuwse Courant’, issued on November 4th, 1986.

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APPENDIX III: HISTORY AND RESTORATION HISTORY OF MARIA CHURCH

The following information were gathered during the RCE investigation in 2019, and are the outcome

of an archival research. They are here included and summarised in Table II, to provide a further

description of the history and previous restorations of Maria Church and its wall paintings.

Table II. History and Restoration History of Maria Church

Year Event Source

2nd half of the 15th

century

Potential date of execution of the wall

paintings commissioned by the Lord of

Borssele

De Ruiter 1987: 26

2nd half of the 16th

century

Potential date of lime-washing of the

wall paintings because of the

Reformation (first minister appointed in

Nisse on December 1, 1578)

De Ruiter 1987: 41

Before 1920

Water infiltration on the north wall,

behind and on the roof of the wall

painting of St. Christopher;

Further water leakage above wall

paintings on triumphal arch, loss of

paint, cracks

Por 1920: 1

Post 1984: 2

Por 1920: 2

1920 Paintings uncovered by restorer Jacob

Por Por 1920: 1

1920

Restoration of paintings by Jacob Por

Work (inter alia):

St. Christopher:

• Removal of whitewash that covered

the painting

• Restoration of northern wall from

outside

• Fillings on yellow band at the bottom

• Entire painting "treated and

preserved"

• Fillings, retouches

Triumphal arch:

• Renovation of column

• Wall paintings "treated and preserved

as a fragment"

Por 1920: 1-3

Post 1984: 2

Before 1984 Metal hanging system for curtains

installed for triumphal arch paintings

RCE photos 252,044, 252,045

RCE Image library

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Between 1984 and

1986 Removal of hanging system

RCE photo 296,905, 262,955

RCE Image bank

1984-86

Restoration of paintings by John Post:

St. Christopher (based on a

restoration plan):

• Injection glue ("Korrekor van Tollens")

• Clean with 95% alcohol prior to

injection

• Fixing original paint and retouches

(with “Dupa-Grund from Caparol”)

• Removal of covering lime and stucco

• Retouches (with “ETA paint from

Talens” or Keimverf)

• Replace of previous cement

restorations

• Cleaning (80% acetic acid in distilled

water)

Both paintings:

• 6 month restoration project

• "Hundreds of thousands" of holes

drilled for injection

• Use of electric drill and injection

needle with 2-component adhesive

Post 1984

Provinciale Zeeuwse Courant

December 9, 1986

Provinciale Zeeuwse Courant

November 4, 1986

2009

"However, the condition of these

paintings [on the north face and

triumphal arch] leaves something to be

desired" W.H.P. Scholten

Mail exchange W.H.P. Scholten

(Stichting Cultureel Erfgoed

Zeeland) and M. Polman

(Specialist color and paintings,

RCE)

2010 Restoration exterior and tower

Mail exchange Renate Pekaar

(Regional Consultant

Architecture, RCE) and M.

Polman

(Specialist color and paintings,

RCE)

2011 Restoration roof

Church website,

https://www.mariakerk-

nisse.nl/de-kerk/historie/

2012-2013 Restoration interior choir church and

ceiling

Church website,

https://www.mariakerk-

nisse.nl/de-kerk/historie/

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Before 2017

Placement of Plexiglas separation

between the choir church and the nave

(probably during 2012-2013

restoration);

Instability of the plaster layers were

observed and reported

Crèvecoeur 2017: 2

2017

Condition assessment and injection

grout tests.

Observations regarding condition:

Triumphal arch:

• Many detached areas

• "Quite thick and glossy fixative layer

[...] that never seemed interrupted"

• No injection holes were observed

• Small cracks “with fixative coverage”

• Large, open, older crack starting to the

right of the middle at the vault

• Bat debris in the middle above the arch

• "Very large tratteggio retouchings that

over the entire painting"

St. Christoffel:

• Full of injection holes

• Severely retouched with tratteggio

technique

• Deposition of dirt at the top

• Presence of detachments, but less than

triumphal arch painting

Injection grout tests (i.s.m.

restoration painter of the Fa. Franse,

Kwadendamme):

• Bottom right triumphal arch with 40

cm diameter

• Drilled 2 mm holes at a distance of 10-

15 cm

• Injection 1: 1 ethanol: water mixture

• Injection of thickened Plextol B500

with 5% toluene

Cleaning tests and sampling fixer

layer:

• Tests with ethanol and acetone with

cotton swabs

Crèvecoeur 2017: 2-5


Crèvecoeur 2017: 8


Crèvecoeur 2017: 4

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• Sample of fixative layer to analyse for

composition that may show whether

there are long-term concerns, for

example with regard to yellowing or

becoming insoluble

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APPENDIX IV: SEM-EDX OF SAMPLES NO 1,2 AND 3

The following samples were taken by Jorien Duivenvoorden and analysed by means of SEM-EDX by Luc Megens in the RCE laboratories in the Ateliergebouw,

Amsterdam. Pictures of the SEM-EDX analysis and their interpretations are hereby included to provide a better understand of the material composition and

layer archaeology of the wall paintings in Maria Church, Nisse.

Nummer Overzicht Beschrijving locatie Monster 1

Pleisterschol rank rechts van Annunciatie Naamgeving

2019-205-1

Datum monstername 22 november 2019

Locatie monstername Mariakerk Nisse

Datum analyse 12 december 2019

Microscopie los monster Voorkant Achterkant Observaties

• 2-3 dunne pleisterlagen • Rode en donkere inclusies • Witte, glimmende inclusies

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Microscopie dwarsdoorsnede 2019-205-1

50x VIS 500x UV Observaties

• Twee lagen • Blauwe deeltjes (30

m), glasachtig Smalt? • Rode en gelige deeltjes

Okers? • Biologische structuur

(sterke luminiscentie onder UV) (70 m) met zeshoekige cellen en oranje materiaal erin Hout, plant?

• Doorzichtig deeltje (met felrode luminiscentie onder UV) (5 m) Meerkrap? Organische kleurstof?

Elektronenmicroscopie 65x 330x Observaties

• Twee lagen zichtbaar • Oranje deeltje (a) hoge

dichtheid • Celstructuur (b) lage

dichtheid

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SEM-EDX metingen 2019-205-1

Uitwerking metingen Interpretatie Uitwerking metingen Interpretatie

Area 1: Ca, Si, (Mg) Point 2: Ca, Fe, (Na, Si, S, Cl) Area 3: Ca, Si, (Na, Al, S, Cl) Area 4: Ca, Si, (Mg, S) Point 5: Si, (Ca, Na) Point 6: Si, Ca, (Mg, Fe, Na, Al, K) Point 7: Si, Fe, Al, K, Ca, (Mg, Na) Point 8: Ca, Si, (Mg, K) Point 9: Ca, Si

• IJzer in rode/gelige plekken • Sporen van Mg, Cl, Na, S

Zouten • Calcium overal aanwezig • Silicium overal in kleine

mate aanwezig • Punt 5 is waarschijnlijk een

kwartsdeeltje

Area 1: Ca, (Mg, Al, Si, S, Cl) Area 2: C, Ca, (Si, S, K) Point 3: Al, (S, Ca) Point 4: Ca, (Si, S)

• Aluminium in felrood luminscerend deeltje Organische kleurstof

• Zeshoekig cellulaire structuur is organisch

Conclusies monster 1 Twee dunne lagen kalkpleister die elementen van veelvoorkomende zouten, kwartsdeeltjes (zandkorrels) en waarschijnlijk een plantaardig vezel bevatten. Ook zijn er sporen van waarschijnlijk okerpigmenten in dit monster gevonden.

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Stopsel boven de annunciatie

Naamgeving 2019-205-4

Datum monstername

22 november 2019



Microscopie los monster Foto 1 Foto 2 Observaties

• Hard materiaal, monster was lastig te nemen

• Fijn, wit poeder • Donkere, glimmende

inclusies (100 m)

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Elektronenmicroscopie 2019-205-4 75x Observaties

• Ruw oppervlak omgeven door fijner oppervlak met een hogere dichtheid

SEM-EDX metingen Uitwerking metingen Interpretatie Conclusies monster 4

Area 1: Ca, S, (Si) Area 2: Ca, (Mg, Si, S, Cl, K) Area 3: Ca, (Na, Mg, Al, Si, S, K) Point 4: S, Ca, (Mg) Point 5: Si, Ca, (Al) Area 6: Ca, S, (Na, Mg, Al, Si)

• Calcium en zwavel zijn meest voorkomend

• Stopsels bestaat daarom waarschijnlijk uit gips (calcium sulfaat)

• Ook andere elementen van veelvoorkomende zouten zijn aanwezig

Stopsel is hoogstwaarschijnlijk gemaakt van gips. Dit komt overeen met het witte, gladde uiterlijk van de stopsels. Echter, gips is een zacht materiaal en het stopsel was erg hard. Het zou kunnen dat het stopsel is gemaakt van een mengsel van gips en een lijm. Een lijm is lastig te ontdekken met SEM-EDX. Ook zijn er elementen van veelvoorkomende zouten aanwezig in dit monster.

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Fixatief rechts van God de Vader

Naamgeving 2019-205-3

Datum monstername

22 november 2019



Microscopie los monster Foto 1 Foto 2 Observaties

• Glimmende, gladde laag met geel/oranje kleur op witte pleisterlaag

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Elektronenmicroscopie 2019-205-5

75x 150x Observaties

• Twee lagen • (a) fijn oppervlak met

kleine deeltjes erop • (b) ruwer oppervlak • Laag (a) lijkt onder laag

(b) te zitten

SEM-EDX metingen Uitwerking metingen Interpretatie Conclusies monster 5

Area 1: C, O, (Al, Si, S, Ca) Area 2: C, O, (F, Na, Mg, Al, Si, C, Cl, Ca) Area 3: Ca, (Na, Mg, Al, Si, S, Cl) Point 4: Ca, (F, Na, Mg, Al, Si, S, Cl, K) Point 5: -

• Materiaal (a) is een organisch materiaal

• Dat komt overeen met het vermoeden dat het een kunsthars is

• Materiaal (b) is kalkpleister

• Elementen van veelvoorkomende zouten zijn aanwezig op het fixatief en in de pleisterlaag

De SEM-EDX metingen bevestigen dat het fixatief een organisch materiaal is. Bovendien zijn er elementen van veelvoorkomende zouten op het flintertje fixatief aanwezig, wat duidt op een problematische combinatie van een afsluitende coating en een zoutprobleem.

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APPENDIX V: INTRODUCTION TO COMPOSITE MATERIALS IN WALL PAINTINGS

A brief description of the different materials historically used for the structural realisation of wall

paintings is included with the aim of achieving a better understanding of the formation process of

structural detachments.

Historically, the main constituent materials involved during the formation of structural foundations

of wall paintings were: clay, gypsum and lime (as binding agents) mixed with aggregates such as sand

of different granulometries, pozzolanic ash (or similar volcanic ashes), marble dust and different

vegetable or animal fibres. Generally, several applications of mixtures of the desirable binder and

aggregates, often starting with a low binder to aggregate ratio, were required to create a flat surface

suitable for the application of paint. Despite the enormous variety of techniques and material

combinations in use in different countries throughout history, the main artistic process diffused in the

West originated from the fresco technique (Mora, Mora and Philippot 1999, 116). The latter involved

the preparation of a structural support with different applications of lime, sand and marble dust at

varying ratios. Whilst this painting process reached its maximum expression in Greece, Italy and other

southern countries, the structural composition typical of frescoes can also be observed in wall

paintings executed with the secco painting technique.

Therefore, the preparation of a typical lime-based structural support for wall paintings consists in the

following three steps: (I) one layer of rough mortar called rinzaffo, composed of lime and sand of big

granulometry at a ratio of 3:1 applied directly over the wet bricks; (II) a denser layer called the

arriccio, where the sinopia, or preparatory drawing was usually executed, consisting of a mixture of

lime and sand of medium granulometry at a varying ratio of either 3:1 or 2:1; (III) finally the smooth

layer, the intonachino, often involving an even mixture of lime and very fine sand and/or marble dust

(Calicchia and Cannelli 2005, 116; Cennino Cennini 1940, 215). The lack of one or more of these

structural layers and variations in their thickness can be observed and can be correlated to the artist’s

background, purpose, and material availability, whilst composing the structure of the wall painting.

As elaborated in the introduction of this thesis, the main focus of this research is framed around the

understanding of the formation process of delamination in lime-based wall paintings. Hence, a

description of the cycle of formation of lime is considered essential to obtain an holistic

comprehension of potential damages affecting lime-based materials. Lime is a binding material

frequently found in architectural elements produced by the initial burning of limestone (Torraca

2009, 50). The manufacture of lime involves three steps: (I) calcination, or firing of the limestone,

where this is usually submitted to elevated heath (above 700 C) to allow the decomposition of

calcium carbonated and the formation of a paste-like material referred to as quicklime; (II) slaking,

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the process of submersion of quicklime paste in water, to create a smooth white lime-paste that mainly

contains calcium hydroxide, water and little calcium carbonate; (III) carbonation, the final process in

which the slaked lime or lime-paste is hardened by reacting with carbon dioxide present in the air and

is transformed back into calcium carbonate (Fig.V.I) (Torraca 2009, 51-53). The final step usually

involve a reduction in volume, which can be counteracted by the addition of the right proportional

amount of sand. Moreover, the presence of impurities in the limestone used in the firing process can

reduce the plasticity of the lime-paste formed during slaking and the ultimate failure of mortar

(Torraca 2009, 52).

Fig. V.I: The cycle of formation of lime involves three steps: firing, slaking and carbonation. Lime is formed by the firing

of limestone and is eventually transformed back into calcium carbonate by the final reaction of calcium hydroxide with

carbon dioxide in the air.

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APPENDIX VI: MOISTURE PASSAGE THROUGH POROUS BUILDING MATERIALS

Moisture in buildings is regarded as the main cause of degradation of wall paintings. The majority of

materials involved in the traditional construction of murals are porous and hydrophilic, hence their

direct susceptibility to moisture induced damage (Torraca 2009, 81). Generally, moisture is

considered a fundamental trigger of secondary reactions leading to the disintegration of both

structure and pictorial surface of wall paintings: its presence activates a sequence of physical and

chemical reactions which would simply not take place otherwise (Mora, Mora and Philippot 1999,

174). Therefore, since all chemical reactions damaging wall paintings need moisture, the identification

of its provenance and subsequent removal should be primary interventions.

The passage of moisture through building materials occurs due to their hydrophilic and polar natures.

The phenomenon of distribution of moisture through building materials is referred to as capillary

suction (Torraca 2005, 8). This force is indirectly correlated to the size of the pores in which moisture

is passing through: the smaller the diameter of the pore, the stronger the suction force. In this

circumstance, the average pore diameter is between 0.1 and 1 m and, as a result, the attraction of

water molecules to the walls of the pore is stronger than the attraction between water molecules.

Because of this, capillary rise in building materials with small pores can reach the height of many

metres (Torraca 2009, 82).

It is important to denote two fundamental concepts: (I) moisture is distributed within a porous

medium thus to reach the lowest energy state, which is achieved by reaching both an equilibrium of

the attraction forces between water molecules and building materials and a balance between the

amount of moisture uptake and subsequent evaporation; (II) the physical and chemical deterioration

of building materials induced by moisture is directly related to the pore size and pore distribution

within the material (Torraca 2009, 82). In conclusion, the most frequent way in which moisture is

distributed within building materials is the one that favours the equilibrium of attractions between

these elements. This is achieved in the following way: (I) only small pores (with diameter between 0.1

and 1 m) are completely filled with water and the nearby large pores (with diameter larger than 10

m) are dry; (II) small pores are completely filled, hence, water flows to the large pores where it is

only adhered to the surface and air fills most of the internal volume; (III) both small and large pores

are completely filled with water (Torraca 2005, 9- 11).

The movement of moisture within building materials can occur both in its liquid and gaseous phase

and is determined by several types of forces (Torraca 2005, 11). Table VI.I and VI.II summarise the

mechanisms responsible for the movement of moisture in both liquid and gaseous phase respectively,

as described by Torraca (2005, 11-15).

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Table VI.I Forces responsible for the movement of moisture in the liquid phase

Table VI.II Forces responsible for the movement of moisture in the gaseous phase

Type of force Description

Suction

Movement from a region in which water is occupying all small pores and only

adhering to the surface of the large ones to a dry (or drier) region. Air is

removed from the dry material so that water can occupy the vacant space.

Diffusion The natural movement of water from a region with higher water content to one

with lower water content.

Osmosis

This force occurs when salts are dissolved in water and are dissociated into ions.

These ions attract water molecules due to their electrical charge and

consequently, water moves from regions of low ionic concentration to regions of

high ionic concentration.

Heat The movement of water from warmer regions to colder ones.

Type of force Description

Condensation

If the surface temperature is below the dew point of the surrounding air,

water vapour in the air is released in form of water droplets on the surface

itself. The liquid water can then move within the porous system by one of the

mechanisms described (see Table VI.I).

Vapour diffusion

Water can move from regions with high vapour pressure (many water

molecules in gaseous form in the air) to regions with lower vapour pressure

(fewer water molecules in gaseous form in the air). This can generate the

evaporation of water from surfaces nearby air with high vapour pressure and

the condensation on surfaces nearby air with low vapour pressure.

Hygroscopicity

Also defined as ‘hygroscopic adsorption’, this describes the attraction

between hydrophilic materials and water molecules. This is strongly

influenced by the relative humidity: for instance, at RH below 100%, small

pores endure hygroscopic adsorption and can fill up with water.

Furthermore, soluble salts can adsorb water molecules from the atmosphere

at RH below 100%. For example, sodium chloride can adsorb water from the

air at RH above 75%.

Evaporation

This can occur on wet surfaces due to the presence of RH below 100% or

when the air is continuously moving, transporting water molecules away

once in gaseous phase.

Desorption

The release of water molecules due to low RH and air circulation. Desorption

always occurs at RH lower than the lowest RH at which adsorption is

possible. For example, sodium chloride can adsorb water at 75% RH but only

starts releasing it below 70% RH.

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The presence and passage of moisture through architectural elements can be classified according to

its origin: (I) rainwater affecting the outer building structure; (II) capillary rise due to rising damp

from the ground; (III) condensation on cold surfaces (below dew point temperature of the air); (IV)

diffusion of water induced by hygroscopic materials; (V) water vapours deriving from the ground

(Mora 1974, 38-43).

In conclusion, the identification of the source of moisture affecting building materials can be extremely

difficult and complex as this can be originating from different mechanisms occurring simultaneously.

Environmental monitoring is an essential component of the diagnostic investigation of historic

interiors and buildings and its main goal is the identification and documentation of sources of

moisture and their correlation to temperature parameters and outer environmental conditions.

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APPENDIX VII: QUESTIONS OF THE PROTOTYPE OF DECISION-MAKING MODEL

1. Conservation history

1.1. Documentation of previous conservation treatments:

1.1.1. Conservation phase executed in (year/s) by (name of conservator/s or company)

1.1.2. Report description of previous condition of the wall painting. Include archival photos and

original documents here.

1.1.3. Based on these initial observations and current condition of the wall painting how would you

describe its progress of decay?

1.1.4. Short summary of all investigation/research:

1.1.5. Short summary of all tests:

1.1.6. Shor summary of all treatments (remedial, passive, preventive):

1.1.7. Description of adverse remedial treatments:

- Short description of adverse treatments

- What material/s was/were used? For what purpose? Include composition of such

material/s, msds sheet/s and manufacturer information.

- What damage do you think this material caused?

- How can this material be connected with the formation of structural delamination? I.e.

epoxy resin as protective coating → deflection of moisture passage → salts → internal

stress → mechanical and chemical failure → structural detachment

2. Identification of original/ previous conservation materials

2.1. Summary of description of composite materials and technique as found from archival research:

2.1.1. Structural materials: binder and aggregates

2.1.2. Pictorial material: binder, pigments, varnish, glaze etc.

2.2. Investigation of original materials

2.2.1. Dino-lite/microscopic photos and observations.

2.2.2. Decision-making for sample taking:

- How many samples do you need?

- Sample location (coordinates)

- Location description

- Purpose/ remark

2.3. Investigation of previous conservation materials

2.3.1. Dino-lite/microscopic photos and observations.

2.3.2. Decision-making for sample taking:

- How many samples do you need?

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- Sample location (coordinates)

- Location description

- Purpose/ remark

2.4. Samples taken:

- Sample name/number

- Sample description before embedding

- Expected layer build-up and composition

- Embedding material

- Additional notes/remarks

2.4.1. Microscopic observations of sample

- Number of layers, thickness (m) and type (i.e. arriccio and intonaco)

- Description of each layer: particles and colour

- Visible light, raking light, UV pictures and observations

- Identification of materials: include analysis and images

- Notes/ remarks

2.5. Evaluation of re-treatability of previous conservation materials

2.5.1. Can previous adverse materials be removed? Can their removal be effectuated without risks

to the original materials?

2.5.2. What are the permeability/mechanical/chemical properties of adverse materials?

2.5.3. Based on these properties can you assess their compatibility with other conservation

materials?

3. Condition monitoring

A. Description of current condition

3.1. Based on which technique is delamination believed to affect the structure of this wall painting?

3.2. Where are delaminated areas believed to occur?

3.2.1. Is there an area/areas of the wall painting more heavily affected?

3.3. Where is the wall painting located within the church?

3.3.1. Is it integrated in a load-bearing wall?

3.3.2. Is the building support of the wall painting directly exposed to the outer environment?

3.4. How would you describe the overall state of preservation of the structure of the wall painting?

3.5. What damages are affecting the pictorial layer? Carry out a damage mapping at regular intervals.

Take regular pictures of damages on the same location of the painting. Include all files here.

- Cracks (describe size and location)

- Microcracks (describe size and location)

- Flaking paint (describe which area is more affected and which pigment/paint)

- Efflorescence/ Sub-efflorescence (if visible)

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- Paint loss (describe size, location and amount of material missing i.e. paint loss including

painted surface, intonaco and part of the underlying layer/ paint loss revealing part of the

building support)

- Surface irregularities (describe the irregularity and location)

3.6. How would you describe overall state of preservation of the pictorial layer of the wall painting?

B. Documentation of environmental monitoring

3.7. Document the following parameters for each month for at least a year

- Internal air temperature (T, C): average min, daily mean, average max

- External air temperature (T, C): average min, daily mean, average max

- Internal relative humidity (RH %)

- External relative (RH %)

3.7.1. In presence of heating system report any change of air temperature (T, C) induced

3.8. Document the surface temperature of the wall painting with passive thermography

3.9. Document the changes induced by the heating system/other to the surface temperature of the

wall painting with passive thermography

3.9.1. Do you think condensation is occurring on the surface?

3.10. Document the building performance with passive thermography. Include pictures here.

3.10.1. What are the weak areas surrounding the wall paintings?

3.10.2. Is water leaking or has leaked in the past from these weak spots?

3.11. What damages are affecting the architectural elements surrounding the wall painting?

- Cracks (describe size and location)

- Microcracks (describe size and location)

- Efflorescence/ Sub-efflorescence (if visible)

- Delamination

- Sulphation

3.12. Based on results obtained by sheet 1,2 and 3, can you hypothesise causes of delamination?

4. Assessment of rate of change

A. Interpretation of results obtained during condition monitoring

4.1. Compare damage mapping and pictures of damages and describe observation

- Are there more damages occurring?

- Which ones?

4.2. Compare images obtained with passive thermography. What differences do you see?

4.3. How would you describe the environmental conditions of the church/building?

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- Do you think RH and T are contributing factors to the decay process affecting the wall

painting? Specify which parameter should be addressed i.e. rising damp, condensation etc.

- Is RH and T affecting one particular damage?

B. Assessment of structural delamination

4.4. What is the most suitable technique?. Check the following parameters:

- Condition of the pictorial layer

- Surface topography

- Accessibility of the wall painting

- Budget

- In-situ logistics i.e. window in front of the mural, nearby source of vibrations, restricted

accessibility etc.

4.5. Once selected the most suitable technique/techniques this/these should be used at regular

intervals (at least twice in a year) to assess the progress of delamination. Document your

observations:

- Visually document delaminated areas and include photos here.

- Can delamination be quantified?

- Where in the layer archaeology is delamination occurring?

4.6. How would you describe the overall state of preservation of the structure of the wall painting

based on these results? Check difference with 1.4.

- Describe the severity by which the wall paintings is affected by delamination

- Describe the frequency by which the wall paintings is affected by delamination

4.7. How would you describe the progress of delamination?

- Create a customised table with severity and frequency (see Table 5.1).

4.8. Can you draw a parallel between delaminated areas and damages on the pictorial layer? Describe

this here, include pictures.

4.9. Based on the obtained results, can you confirm what stated regarding the causes of delamination

in 3.21?

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APPENDIX VIII: GLOSSARY

ABSOLUTE HUMIDITY (AH): Mass of water vapour divided by the mass of dry air in a certain volume

at a specific temperature. It is expressed as grams of water vapour/moisture per cubic metre of air

(g/m3). For instance: the maximum AH of warm air at 30 C is approximately 30 g/m3, whilst the

maximum AH of cold air at 0 C is approximately 5 g/m3. Warmer air holds more water, whereas cold

air hols less.

ACTIVATION MECHANISMS: This term is used to indicate all those mechanisms responsible to

initiate deterioration. The presence of condensation on the surface of a wall painting is an activation

mechanism of several forms of decay; therefore the causes of such decays are to be found in the causes

of the formation of condensation, most likely fluctuation of RH and the presence of microclimates.

ARRICCIO: Italian term, used to indicate the second fundamental structural layer of fresco and lime-

based wall paintings. The sinopia, or preparatory drawing is usually executed on this layer which is

formed of a mixture of lime and sand of medium granulometry at a varying ratio of either 3:1 or 2:1.

ATTENUATION: The reduction of the amplitude of a signal, electric current or other oscillation within

a material, in this case, within the structure of a wall painting. During the implementation of non-

destructive techniques to documents the presence of delamination, lime-based structural rendering

can cause signal attenuation due to their heterogeneity.

BAT DROPPINGS: Otherwise known as guano, it indicates the accumulated presence of excrement

of bats. It contains exceptionally high contents of nitrogen, phosphate and potassium.

BUILDING SUPPORT: The supporting material of a building (bricks, concrete, wood etc.) where the

structure of a wall painting has been applied.

CEMENT: A binder used for construction that hardens and adhered to other structural materials to

bind them together. Cements used in construction is composed of a mixture of inorganic materials

such as lime, calcium silicate and aluminates and can be non-hydraulic (do not require the presence

of water to harden) and hydraulic (in need of water to set).

CLAY: Fine-grain natural soil material that contains hydrous aluminium phyllosilicates that develop

plasticity when wet and become hard and brittle once dry. The typical pores arrangement of clay is

describe as platy and compressed; water molecules can be easily trapped between these plates of clay

mineral, hence explaining its plasticity.

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DECISION-MAKING MODEL: Series of questions, schemes and tables that need to be fill in to be able

to interpret complex results and information and possibly help the user to formulate a decision

regarding a specific topic.

DELIQUESCE: The process of change of phase of a water soluble solid to liquid by absorption of

moisture from the air. Soluble salts can often endure this mechanism in presence of high RH.

DEW POINT TEMPERATURE: Temperature the air needs to be cooled to achieve a RH of 100%. At

this point the air cannot hold more water vapour, therefore, if it was cooled even more (below the dew

point) water vapour would be released in the liquid form creating condensation or dew.

ELASTIC WAVE: A motion in a medium in which, when particles are displaced, a force proportional

to the displacements acts on these particles to restore them to their original position. For example: a

gas is an elastic medium and sound is transmitted through a gas as an elastic wave.

ELECTRIC FIELD: A region around a charged particles within which a force would be exerted on other

charged particles or objects.

ELECTROMAGNETIC WAVE: Waves created by the interaction of an electric field and a magnetic field.

Electromagnetic waves are composed of oscillating magnetic and electric fields perpendicular to each

other. They can travel in a vacuum and, therefore, do not need a medium to travel through (as opposed

to acoustic waves). As a result their involvement in non-destructive techniques to document

delamination in wall paintings is advantageous.

ENVIRONMENTAL FACTORS: Factors deriving from the environment that are deemed to cause decay

and damages to artefact, such as wall paintings. See 3.3.

EXTRINSIC FACTORS: Factors connected to both natural and human activities that are deemed to

cause decay and damages to artefact, such as wall paintings. These can include natural catastrophes

(earthquakes, tsunamis, storms etc.) and human activities that are considered adverse for the long-

term preservation of such artefacts (terrorism, vandalism, vibrations and adverse conservation

treatments). See 3.4.

FREQUENCY: In physics, it refers to the number of occurrences of a repeating event per unit of time.

It is measured in units of hertz (Hz) and 1 Hz equals to one occurrence of a repeating event per second.

This term is used in this thesis to described the different frequencies a wave can have.

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FROST DAMAGE/FREEZE THAW: The development of ice crystals within building materials

facilitated by low temperatures. This can cause severe increase of internal stresses with the

consequent mechanical failure of lime renderings and the ultimate formation of delamination.

FUNDAMENTAL CAUSES OF DAMAGE: Main triggering factors of damages occurring in wall

paintings. When describing structural detachments, the main fundamental causes are the following:

(I) intrinsic; (II) environmental; (III) extrinsic. See Chapter 3.

GYPSUM: Soft mineral implemented in the structural composition of architectural surfaces and wall

paintings. It is composed of calcium sulphate dihydrate, CaSO4 .2H2O.

HeNe LASER: Helium- neon laser, which most commonly operates at a wavelength of 632.8 nm, in the

red part of the visible spectrum.

HOLOGRAPHY: A method of creating a three-dimension image of an object by recording the pattern

of interference formed by a split-laser beam and then illuminating this pattern with either a laser or

ordinary light. It measures not only the intensity, but also the phase information of the light recorded.

It is the fundamental operating mechanism behind the non-destructive technique referred to as: DSPI,

ESPI or TV Holography. See 4.4.

HYDROPHILIC: A molecule or portion of this, that can interact with water or other polar substances.

They are typically charged-polarised and capable of forming hydrogen bonding.

HYDROPHOBIC: A molecule or portion of this, that repel water or other polar substances and can

interact with oil or other non-polar substances. They are typically non-polar.

INFRARED RADIATION: Electromagnetic radiation with wavelengths between 700 nm to 1 mn.

These extends from the red edge of the visible spectrum and are, therefore, invisible to the human eye.

INJECTION GROUT: Bulked fluid material with adhesive properties injected through pre-drilled holes

in a wall painting to treat structural delamination. A variety of material combination, usually involving

a binder, a filler, a suspension medium and sometimes some additives (such as plasticisers, fluidisers

and retarders) together with a variety of commercially available products have been used to treat this

form of damage.

INTERFEROMETRY: A family of techniques in which waves, usually electromagnetic waves, are

superimposed, causing the phenomenon of interference. This is used in DSPI, ESPI or TV Holography:

two laser beams are split into two with one being used a reference beam and the other being reflected

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by the wall painting. Once these are superimposed, their phase change is recorded and an interference

pattern is formed: this way the different optical path endured by the reference beam is documented

and is indicative of material inhomogeneities.

INTONACO/INTONACHINO: Italian term, used to indicate the last fundamental structural layer of

fresco and lime-based wall paintings. This layer often involve an even mixture of lime and very fine

sand and/or marble dust, to create a smooth surface optimal for the application of paint.

INTRINSIC/ INHERENT: Factors generating damages strictly connected to the origin and nature of

the artefact/ wall painting. In particular, faulty materials, defective techniques and the geological

composition of the ground of the building are elements considered to be directly responsible for the

formation of structural delamination. See 3.5.

LAYER ARCHAEOLOGY: The layered structure of a wall painting.

LIME: Term commonly used in the chemical language to indicate calcium oxide (CaO). However, it is

also used by architects and engineers to refer to a range of materials derived from calcium oxide that

can exhibit striking differences of mechanical and chemical properties. The correct use of the technical

names of lime materials is described below.

QUICKLIME: Calcium oxide, CaO.

LIME PUTTY: Calcium hydroxide + water, Ca(OH)2 + H2O

HYDRATED LIME: Calcium hydroxide, Ca(OH)2.

HYDRAULIC LIME: Calcium silicate and aluminates + calcium hydroxide and/or calcium

oxide, Ca(OH)2 + C2S + C3S + CH and others.

LOSS OF ADHESION: Failure of adhesion from a bonding surface or between two adhered layers.

Structural delamination is caused by failure of the adhesive properties between different structural

layers of the wall paintings or between the binder (lime) and its aggregates (sand, fibres etc.).

LOSS OF COHESION: Failure of adhesive properties within particles of the same substance. Structural

delamination is caused by failure of the adhesive properties of the binder itself.

MAGNETIC FIELD: A region of space near a magnet, electric current, or moving charged particle in

which a magnetic force acts on any other magnet, electric current, or moving charged particle.

Nd:YAG LASER: Neodymium-doped yttrium aluminium garnet laser, which most commonly operates

at a wavelength of 532 nm, in the green part of the visible spectrum.

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NON-DESTRUCTIVE TECHNIQUE: Wide groups of analysis and techniques used in science and

technology industry to evaluate the properties of a material without causing any irreversible

alteration.

PHASE DIFFERENCE: The difference in degrees (angle) or time of two sinusoidal waves with the same

frequency.

PIEZOELECTRIC TRANSDUCER/SENSOR: Device that generate or record electromagnetic and

ultrasound energy. These can be divided into three broad categories: transmitters, receivers and

transceivers. Transmitters convert electrical signals into ultrasonic or electromagnetic waves,

receivers convert ultrasonic or electromagnetic waves into electrical signal while transceivers can

execute both mechanism.

POROSITY: measure of the void spaces within the material. It is a fraction of the volume of the voids

over the total volume of the material and it is expressed as a percentage.

PVA RESIN: Polyvinyl acetate resin.

RELATIVE HUMIDITY: Ratio of the absolute humidity (AH) at a given time, compared to the

maximum amount of moisture that air can hold at a given temperature and pressure. It is expresses

as a percentage. For example: at 25 C and with an AH of 23 g/m3, the RH is 100%. This means that

the air is holding the maximum capacity of water vapour at that temperature. If the AH was 11.5 g/m3,

consequently the RH would have been 50 % (the air is holding half the amount of water vapour of its

maximum capacity at that temperature).

RINZAFFO: Italian term, used to indicate the first fundamental structural layer of fresco and lime-

based wall paintings. This layer of rough mortar is composed of lime and sand of big granulometry at

a ratio of 3:1 applied directly over the wet bricks/building support.

SECONDARY CAUSES: Decaying reactions spawned by the single or combined action of the main

causes. When describing structural detachments the secondary causes are the following: frost

damage, migration and re-crystallisation of soluble salts and fungi. See Chapter 3.

STRUCTURAL DETACHMENTS/DELAMINATION: Physical forms of damage induced by either

external or internal mechanical stresses. They are complex phenomena, considered among the most

frequent types of damage found in mural paintings. The most commonly observed types of

delamination can affect both structure and pictorial layer of a wall painting. Structural delamination,

detachment, defect or even disintegration are terminologies used to define damages involving: the

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loss of cohesion within the same structural layer, the loss of adhesion between two different layers

and the loss of adhesion between different applications of the same layer.

SPECIFIC SURFACE AREA: Property of solids defined as the ratio of the surface area of a material to

either its mass or apparent volume. Materials with rough surface texture have higher specific surface

area than materials with smooth surface topography.

SURFACE TOPOGRAPHY/TEXTURE/FINISH: The quality of a surface, in terms of its roughness,

microscopic pattern (lay) and macroscopic pattern (waviness).

WAVELENGTH: The distance between two successive crests of a wave, which may be electromagnetic,

ultrasonic or thermal. It is measured in units of lengths such as metres, centimetres, millimetres and

nanometres.

WELL-ROUNDED WELL-SORTED GRAINS: Pores with similar size and corners completely rounded.

Materials with these properties are generally porous, as opposed to poorly sorted sediments, which

have low porosity.

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