Atmospheric Environment 37 (2003) 1261–1269

Organic anions in damage layers on monuments and buildings

Cristina Sabbionia,*, Nadia Ghedinia,b, Alessandra Bonazzaa

a Institute ISAC�CNR Bologna, Via P. Gobetti 101, Bologna 40129, ItalybDepartment of Pharmaceutical Sciences, via Belmeloro 6, Bologna 40126, Italy

Received 20 July 2002; accepted 30 October 2002

Abstract

This article is focused on small (C1–C2) organic anions present in the damage layers on historic monuments and

buildings. Formate, acetate and oxalate are consistently found in black crusts, where atmospheric deposition

accumulates along with the products of the chemical transformation of stone and mortars. While sulphation processes

affecting building materials have been extensively studied, the importance of carbon compounds in black crusts is only

recently being realised. Recent data show carbon to be the second most important airborne element after sulphur in

damage layers on building exteriors. Total carbon is composed of carbonate-, organic-, and elemental carbon. The

organic fraction includes formate, acetate and oxalate; these are always detected in black crusts. Their origin, role and

measurement in the atmosphere and in the museum environment have been the subject of many studies, but little has

been reported concerning their presence in building exterior damage layers. This paper presents data on these anions in

damage layers on stones and mortars sampled on monuments and buildings at different urban, suburban and rural

European sites. Oxalate encountered in black crusts likely originates from the metabolism of micro-organisms and

protective treatments on surfaces. Primary and secondary atmospheric pollutants are likely the main sources of formate

and acetate anions.

r 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Damage; Black crusts; Formate; Acetate; Oxalate

1. Introduction

The effects of atmospheric pollution and acid deposi-

tion on monuments and historic buildings remain a topic

of considerable interest. Since the mid-19th century, the

degradation of construction materials has been acceler-

ated by air pollution. Atmospheric pollutants can be the

chief factor in building degradation, causing in some

cases a significant loss of cultural heritage (Nord and

Ericsson, 1993). The damage of calcareous materials in

urban areas is especially linked to atmospheric pollution

(Leysen et al., 1989).

The weathering of stone and mortar surfaces exposed

to atmospheric deposition leads to the formation of a

black crust, a characteristic degradation that signifies

both aesthetic and surface damage (Sabbioni, 1995).

Black crusts are the areas where atmospheric deposition

accumulates along with the products of the chemical

transformation of materials (Zappia et al., 1993). Their

exact composition varies from one sampling site to

another. Experimental data indicate that the local

atmospheric composition is the driving factor (Zappia

et al., 1998). The knowledge of damage layer composi-

tion is thus essential to the study of atmosphere–

materials interaction, to the identification of pollutant

sources, and to plan for the prevention, restoration and

conservation.

Many have studied the role of atmospheric sulphur

and nitrogen compounds in stone deterioration, but

few studies have examined the effects of carbon

compounds in promoting the growth of black crusts

and their potential catalytic action in gypsum formation

*Corresponding author. Tel.: +39-051-6399572; fax: +39-

051-6399649.

E-mail address: c.sabbioni@isac.cnr.it (C. Sabbioni).

1352-2310/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S1352-2310(02)01025-7

(Sabbioni, 1995; Benner et al., 1982). In order of

abundance, carbon is the second most important

anthropogenic element after sulphur contained in the

damage layers. Total carbon (TC) is composed of the

following fractions:

TC ¼ CCþNCC and NCC ¼ OCþ EC

where CC is carbonate carbon, basically due to the

underlying materials (stones and mortars), and NCC is

non-carbonate carbon originating from atmospheric

deposition. NCC is, in turn, composed of both natural

and anthropogenic organic carbon (OC), and elemental

carbon (EC), the latter mainly due to deposition of

combustion-derived aerosol.

Recent analyses on stones and mortars from Italian

and European historic monuments and buildings show

that formate, acetate, and oxalate are ubiquitous and

constitute a significant fraction of OC as defined above

(Sabbioni et al., 2000). The occurrence and origin of

these organic acids in the atmosphere and their

environmental effects at high concentrations have been

widely reported on. Formic and acetic acids are the most

abundant organic acids in the atmosphere (Talbot et al.,

1990; Grosjean, 1992; Facchini et al., 1992); the presence

of oxalic acid and oxalates has also been extensively

reported (Norton et al., 1983; Kawamura and Kaplan,

1987; Grosjean, 1989; De Santis and Allegrini, 1989;

Kawamura and Ikushima, 1993; Kerminen et al., 2000;

Rohrl and Lammel, 2001; Limbeck et al., 2001; Yao

et al., 2002; Boring et al., 2002). Acetic and formic acids

are known to corrode lead (Tennent and Cannon, 1993)

and bronze (Tennent and Baird, 1992), and cause

efflorescence on calcareous materials (Tennent and

Baird, 1985; FitzHugh and Gettens, 1971); these have

been studied in the museum environment (Ryhl-Svend-

sen and Glastrup, 2002). In contrast, reports of

occurrence of these anions in damage layers on buildings

are scant. More recently, the occurrence of calcium

oxalates on exposed urban and rural building surfaces

have been reported. The formation of a characteristic

yellow-ochre oxalate patina can sometimes act as a

protective layer. The most frequently found salts of

oxalic acid on outdoor monuments and masonry are the

monohydrate whewellite (CaC2O4 �H2O) and the dihy-

drate weddellite (CaC2O4 � 2H2O) (Matteini and Moles,

1986). A variety of natural and anthropogenic sources

has been proposed to explain the origin of these calcium

oxalates on stone and mortars (Realini and Toniolo,

1996). The presence of oxalic acid in black crusts and the

calcium oxalate patinas is likely correlated (Realini and

Toniolo, 1996). The data available to date do not allow

unambiguous assignment of the origin of these organic

anions in the damage layers; however, the following

sources have been suggested: (a) organic residues of

protective coatings used in the past (Rossi Manaresi,

1996), (b) deposition of primary and secondary atmo-

spheric pollutants (Saiz-Jimenez, 1993; Turpin and

Huntzicker, 1995; Zappia et al., 1998), and (c) biological

weathering (Sabbioni and Zappia, 1991; Saiz-Jimenez,

1995). Recently, performed field exposure tests support

the last hypothesis in that such anions were shown to

originate also from the metabolism of micro-organisms

like yeast (Mandrioli et al., 1999). Unlike sulphation

processes, no work has been performed on the interac-

tion between organic anions with materials except

for indoor environment studies (Ryhl-Svendsen and

Glastrup, 2002). To fill this gap, we present and

summarise here the results of our experimental work

on formate, acetate and oxalate measured in the damage

layers on stones and mortars from buildings of historic

and artistic interest located in different European urban

and rural areas.

2. Materials and methods

2.1. Sampling

Black crusts have been sampled on different building

materials: (a) low porous calcareous stone (mainly

composed of calcium carbonate); (b) lime mortar (i.e.

air-setting mortar prepared with lime and sand); and (c)

hydraulic mortar (i.e. mortar, with the property of

hardening under water, prepared with hydraulic com-

ponents such as cement, hydraulic lime, volcanic sand,

brick fragments), cement, hydraulic lime, pozzolanic and

cocciopesto mortars.

Black crusts on stone were collected from monuments

and buildings of historic or artistic interest in Italian

urban centres characterised by different local climate

and atmospheric composition. Samples of damaged

layers on mortars were collected from ancient and

modern buildings, at European urban, suburban and

rural sites.

For the purpose of analysing the carbon fractions, the

damage layers were sampled by scraping the surface

black crusts, as powder specimens, limiting as far as

possible the collection of the undamaged material. This

procedure is relatively easy in the case of low porosity

materials, where the interface between the damage layer

and the substrate is well-defined. However, in the case of

porous materials, particularly mortars, where the

damage process tends to penetrate into the structure

and the aggregate (sand) may be found embedded in the

damage layers, sampling just the damage layer is

difficult. For this reason, only samples containing more

than 40% of gypsum should be considered (Sabbioni

et al., 1997, 2001; Ghedini et al., 2000). In addition,

fragments containing both black crusts and undamaged

material were collected to permit the study of the

interface between the damage layer and the building

materials.

C. Sabbioni et al. / Atmospheric Environment 37 (2003) 1261–12691262

In particular, with reference to Table 1, the sampling

on stone was performed in five Italian towns as

follows:

* Rome: A large town that is highly polluted by

road traffic, but with low levels of emissions from

domestic heating and industries; samples were

collected from the (i) Coliseum, (ii) Tiburtina Gate,

and (iii) Maggiore Gate.* Pisa: A medium size historic city, B8 km from the

Tyrrhenian sea, with moderate pollution from

vehicular exhaust, but with an intermediate level of

industrialisation; samples were collected from the (i)

Leaning Tower and (ii) Baptistery.* Ravenna: Similar to Pisa in urban extension, micro-

climatic conditions, and distance from the sea

(Adriatic), but with a high level of pollution due

to petroleum refining in the surrounding area;

samples were collected from the (i) Rasponi Palace,

(ii) Adriana Gate, (iii) Modern Edifice, and (iv) St.

Vitale Church.* Venice: Famous historic tourist centre with local

pollution primarily from boat engine exhaust;

samples were collected from the Cavagnis Palace.* Syracuse: Chosen as representative of a less polluted

maritime site with negligible levels of emissions from

industrial and domestic combustion processes; sam-

ples were collected from the: (i) Column of Palace, (ii)

Dwelling House, and (iii) Rome Hotel.

Specimens of damaged mortars were collected in

different European sites (Tables 1 and 2) as follows:

* Bologna (I): A northern Italian city of medium

extension with moderate atmospheric pollution

from domestic heating and vehicular emissions;

samples were collected from the (i) Zamboni and

(ii) Mascarella Town Gates, both parts of the

Table 1

Mean concentration (mg/g) of formate, acetate and oxalate anions measured in damage layers on stone and mortar collected in some

European urban and suburban sites

Site Monument or Building Material CHO2� C2H3O2

� C2O42� Reference

Rome (I) Coliseum Travertino 225 335 2166 Ghedini et al. (2000)

Rome (I) Tiburtina Gate Travertino 321 2661 491 Ghedini et al. (2000)

Rome (I) Maggiore Gate Travertino 1107 149 2985 Ghedini et al. (2000)

Pisa (I) Leaning Tower St. Giuliano marble 194 495 446 Sabbioni et al. (2000)

Pisa (I) Baptistery Marble 59 136 742 Sabbioni et al. (2000)

Ravenna (I) Rasponi Palace Limestone 2299 2286 4153 Pantani (2000)

Ravenna (I) Adriana Gate Limestone 333 1555 956 Pantani (2000)

Ravenna (I) Modern Edifice Limestone 315 16476 1318 Pantani (2000)

Ravenna (I) Chapters St. Vitale Church Limestone 831 13718 15970 Gigli (2000)

Ravenna (I) Columns St. Vitale Church Limestone 771 13448 5164 Gigli (2000)

Venice (I) Cavagnis Palace Limestone 412 210 1593 Tavolin (1998)

Syracuse (I) Column of Palace . Calcarenite 897 612 512 Macchiarola et al. (1999)

Syracuse (I) Dwelling House Calcarenite 494 489 8737 Macchiarola et al. (1999)

Syracuse (I) Rome Hotel Calcarenite 804 524 453 Macchiarola et al. (1999)

Bologna (I) Zamboni Gate Lime mortar 275 1012 1401 Sabbioni et al. (1997)

Bologna (I) Mascarella Gate Lime mortar 694 1076 962 Sabbioni et al. (1997)

Venice (I) Arsenal Cocciopesto mortar 1070 7412 465 Piccolo (2000)

Venice (I) St. Nicol "o Church Lime mortar 344 218 406 Tavolin (1998)

Rome (I) Aureliane Walls Pozzolanic mortar 661 882 67 Sabbioni et al. (2001)

Rome (I) Anio Novus Aqueduct Pozzolanic mortar 808 1024 158 Sabbioni et al. (2001)

Rome (I) Aqua Marcia Aqueduct Pozzolanic mortar 702 783 109 Sabbioni et al. (2001)

Rome (I) Claudian Aqueduct Pozzolanic mortar 502 10224 199 Sabbioni et al. (2001)

Minturno (I) Minturnae Aqueduct Pozzolanic mortar 63 2254 62 Sabbioni et al. (2001)

Brussels (B) Cit"e Modern Cement mortar 1477 2343 172 Sabbioni et al. (2001)

Brussels (B) St. Augustin Church Hydr. lime mortar 481 1573 285 Sabbioni et al. (2001)

Brussels (B) Wielemans House Hydr. lime mortar 543 1959 370 Sabbioni et al. (2001)

Brussels (B) Wielemans Brewery Hydr. lime mortar 796 1424 293 Sabbioni et al. (2001)

Leuven (B) Van Balen Huis Cement mortar 1189 2600 270 Sabbioni et al. (2001)

Leuven (B) St. Michiels Church Lime mortar 561 660 168 Sabbioni et al. (2001)

Antwerp (B) St. Joris Church Cement mortar 1459 5282 525 Sabbioni et al. (2001)

Valladolid (SP) Senora del Pilar Church Cement mortar 1905 398 69 Sabbioni et al. (2001)

Madrid (SP) Modern Edifice Cement mortar 4349 953 359 Sabbioni et al. (2001)

C. Sabbioni et al. / Atmospheric Environment 37 (2003) 1261–1269 1263

14th-century medieval wall, now surrounded by

intense vehicular traffic emissions.* Venice (I): Samples were collected from the (i)

Arsenal, a 16th-century commercial and military

edifice and (ii) St. Nicol "o Church.* Rome (I): Samples were collected from the (i)

Aurelian Wall, brick wall fortification dating AD

271, located in an urban environment; (ii) Anio Novus

Aqueduct, Roman aqueduct from AD 34, in an urban

area; (iii) Aqua Marcia Aqueduct, Roman aqueduct

built in 144 BC, situated in a rural area, and (iv)

Claudian Aqueduct, a Roman aqueduct built between

38–52 BC, located in a suburban area of the city.* Minturno (I): Sampling was performed on the

Minturnae Aqueduct, a Roman aqueduct of the 1st

century BC, situated in an urban environment.* Brussels (B): Samples were collected from the (i) Cit!e

Modern, suburban housing complex (1922–1925), (ii)

St. Augustin Church, built between 1933–1935

situated in an urban location, (iii) Wielemans House,

a house in Brussels built in the early 20th century,

and (iv) Wielemans Brewery, an urban industrial

building of the 1930s.* Leuven (B): Samples were collected from the: (i) Van

Balen Huis, a late 19th-century dwelling house

situated in an urban location that is not heavily

polluted and (ii) St. Michiels Church, a typical

baroque church in the city centre, repaired and

restored during the 19th and 20th century.* Antwerp (B): Sampling was performed on the St.

Joris Church, a late-Gothic church with many

repaired parts, situated in an urban area in a wet,

shady environment.* Valladolid (SP): Sampling was performed on

the Senora del Pilar Church, a neo-Gothic church

(1905–1907) situated in an urban environment.* Madrid (SP): Sampling was performed on the Urban

Edifice, a modern dwelling house in the city centre

with significant pollution.* Caiazzo (I), Sampling was performed on the Caiatia

Water Reservoir, a Roman cistern built around 150

BC, located in a rural area.

* St. Salvatore Telesino (I): Sampling was performed

on the Telesiae City Wall, a section of the city wall

built in the 2nd century BC, situated in a rural

environment.* Tivoli (I): Samples were collected from the (i) Aqua

Marcia Aqueduct (144 BC) and (ii) Anio Novus

Aqueduct (38 BC), both situated in a rural area.* Sinuessa (I): Sampling was performed on the

Sinuessa Thermal Bath, a thermal bath built in the

3rd century BC, situated in a maritime environment.* Brussels (B): Sampling was performed on the Van

Steenbergen House, a 1930s house situated in a rural

area.

After sampling, all the specimens were dried and

stored at 201C under a nitrogen blanket until

analysis.

2.2. Analytical procedure

Formate, acetate and oxalate were measured by ion

chromatography (IC); the detailed procedure appears

elsewhere (Gobbi et al., 1995). Briefly, salts of low

solubility were dissolved by treating the sample with a

strong acid-type cation-exchange resin in H+-form.

Damage layers generally contain a carbonate and/or

sulphate matrix with variable quantities of other

components from atmospheric deposition. Specimens

containing poorly soluble salts, e.g., CaCO3 and

Ca(COO)2, or components imbedded in the crystalline

silica/silicate interstices, must be dissolved by the ion-

exchange resin treatment.

For the samples of black crust on stones collected in

Rome, Pisa, Ravenna, and Syracuse and for the damage

layers on lime mortars from the Zamboni and Mascar-

ella Gates in Bologna, the TC content and its NCC and

EC fraction were also measured through elemental

analysis by combustion, using a CHNSO Analyzer

(EA 1108 FISONS Instruments) and following a

methodology described by Ghedini et al. (2000). The

TC was obtained by combustion of the bulk samples,

while the non-carbonate and the EC were quantified

Table 2

Mean concentration (mg/g) of formate, acetate and oxalate anions measured in damage layers on mortar collected in some European

rural sites

Site Monument or Building Material CHO2� C2H3O2

� C2O42� Reference

Caiazzo (I) Caiatia Water Reservoir Pozzolanic mortar 252 14320 28 Sabbioni et al. (2001)

S. Salvatore Tel. (I) Telesiae City Walls Pozzolanic mortar 1549 648 24 Sabbioni et al. (2001)

Tivoli (I) Aqua Marcia Aqueduct Pozzolanic mortar 22 119 170 Sabbioni et al. (2001)

Tivoli (I) Anio Novus Aqueduct Pozzolanic mortar 414 428 253 Sabbioni et al. (2001)

Sinuessa (I) Sinuessa Thermal Baths Pozzolanic mortar 224 77 71 Sabbioni et al. (2001)

Brussels (B) V. Steenbergen House Cement mortar 174 626 101 Sabbioni et al. (2001)

C. Sabbioni et al. / Atmospheric Environment 37 (2003) 1261–12691264

after the elimination of CC and OC, respectively; the CC

and the OC were then calculated (Table 3).

In addition, all samples were microscopically exam-

ined in thin sections using transmitted light for miner-

alogy (Zeiss Pol III) and a stereomicroscope (Zeiss SV

8). The characteristic crystal growth patterns and their

inclusions were also observed by means of a scanning

electron microscope (SEM, Philips XL 20), linked with a

energy-dispersive elemental analyser (SEM-EDX).

3. Results and discussion

Optical microscopy of transversal thin sample sections

identified different crystalline habits, and showed a

black surface damage layer composed of gypsum

crystals and amorphous black and spherical particles

(Fig. 1a). The presence of carbonaceous particles was

evident in SEM observations; the reaction between

sulphur from atmospheric deposition and the building

material surface leads to the growth of a well-developed

crust of interlocking gypsum crystals that includes

spongy carbonaceous particles, as seen in Fig. 1b. The

spherical carbonaceous particles, rough and of irregular

porosity, appear entrapped in the gypsum crystals.

Elemental microanalysis indicates that these particles

are composed of a carbonaceous matrix, sulphur and

heavy metals, e.g., iron, nickel and vanadium.

Tables 1 and 2 report the mean concentrations,

obtained with the described procedure, of acetate,

formate and oxalate anions, for each monument and

historic building. They show the constant presence of

organic anions in damaged layers on stone and mortars

from outdoor buildings, located both in urban and rural

sites. In the Italian urban centres, black crust samples on

stone have concentrations in the order HCOO�

oCH3COO�oC2O

2�4 ; with the exception of the

Ravenna samples (Table 1) where CH3COO� is the

highest (mean 9.5mg/g). In particular, the samples from

Modern Edifice have the highest amount of CH3COO�

among all European samples analysed. In this town,

acetate is followed by oxalate (mean 5.5mg/g; the

maximum value was 16mg/g in the St. Vitale Church

samples), and formate (910 mg/g). Ravenna is a mediumsize historic city, B8 km from the Adriatic sea, with

moderate vehicular pollution and an intermediate level

of industrialisation in the vicinity. In this case, acetate

could originate from the surrounding petrochemical

industries; acetaldehyde is emitted by combustion

processes (Naldi, 2001) and atmospherically oxidised

to acetic acid (Khare et al., 1999). As reported in Table

1, the samples collected from mortars on buildings

located in urban and suburban sites in Italy and Belgium

show the highest mean CH3COO� concentrations (2.8

and 2.3mg/g, respectively) followed in order by HCOO�

(630 and 930 mg/g) and C2O2�4 (470 and 300mg/g).

Conversely, in the specimens from Spain (Valladolid and

Madrid), the order is HCOO� (mean 3.1mg/g)

>CH3COO� (mean 680 mg/g) and C2O4

2� (mean

210 mg/g). Madrid samples exhibited the highest

HCOO� concentration (4.3mg/g).

Table 2 reports the mean HCOO�, CH3COO� and

C2O2�4 concentrations measured in the black crusts on

hydraulic mortar in one Belgian and five Italian rural

sites. The concentrations of all three anions are generally

much lower than any of the samples from urban centres.

The sole exception is the acetate content of the samples

from the Caiatia Water Reservoir of Caiazzo (I), where

the mean acetate content of 14.3mg/g was the highest

found in damage layers on mortar. There are no

significant differences in the rural samples between the

two countries.

The TC ranges from a minimum of 2.01% in Bologna

to a maximum of 5.15% in Pisa. This variability is

primarily due to the high variability of CC deriving from

the substrate, whether stone or mortar. The NCC,

linked to anthropogenic sources, is less variable, from

1.04 in Syracuse to 2.04 in Rome. In all cases, OC is the

dominant fraction of NCC, representing 82.8% in

Rome, 50.3% in Pisa, 63.6% in Ravenna, 78.8% in

Syracuse and 59.6% in Bologna. Carbon from HCOO�,

CH3COO� and C2O

2�4 is part of the OC fraction, and

constitutes 5.9% of OC in Rome, 4.3% in Pisa, 47% in

Ravenna, 14% in Syracuse and 8.6% in the samples of

damage layers on mortars from Bologna, as shown in

Fig. 2. In all the sites studied, the contribution of these

Table 3

Mean concentrations (%) of total (TC), noncarbonate (NCC) and elemental (EC) carbon, measured by CHNSO analysis, on the black

crusts sampled in five Italian urban centres. Carbonate (CC) and organic (OC) carbon were calculated

Site Material TC CC NCC OC EC Reference

Rome Stone 4.35 2.31 2.04 1.69 0.35 Ghedini et al. (2000)

Pisa Stone 5.15 3.76 1.39 0.70 0.69 Sabbioni et al. (2000)

Ravenna Stone 2.52 0.68 1.84 1.17 0.67 Pantani (2000)

Syracuse Stone 5.04 4.00 1.04 0.82 0.22 Macchiarola et al. (1999)

Bologna Mortar 2.01 0.40 1.61 0.96 0.65 Sabbioni et al. (1997)

C. Sabbioni et al. / Atmospheric Environment 37 (2003) 1261–1269 1265

ions to OC (Fig. 3) is significant. In some cases, e.g., in

the Ravenna specimens, CH3COO� (3.9mg/g) and

C2O2�4 (1.5mg/g), are very high, respectively, represent-

ing 33% and 13% of the OC fraction.

4. Conclusions

Formate, acetate and oxalate are ubiquitous in

damage layers on building materials and are often

present in considerable quantities, regardless of the

prevailing local climate. While the exact amounts are

highly variable, generally higher concentrations are seen

on stones than on mortars. Chemical composition and/

or porosity are the primary variables in controlling the

interaction of building materials with the atmosphere.

Present results indicate that chemical composition are

more important. Calcium carbonate, much higher in

marble and limestone than in mortars, accumulate a

greater amount of the organic anions, presumably

because it is more susceptible to attack by the

corresponding acids than silica/silicate compounds.

Regarding the influence of the local air quality, all

three organic anions are more abundant in damage

layers from urban areas, compared to those from rural

sites. Formates are detected in the highest quantity in

the Belgian and Spanish samples on cement mortar,

whereas acetates show the highest mean values in Italian

specimens sampled in urban areas on stone (Ravenna),

in urban areas on cocciopesto and pozzolanic mortar

(Arsenal in Venice and Claudian Aqueduct in Rome), and

in rural areas on pozzolanic mortar (Caiatia Water

(a)

(b)

Fig. 1. (a) Transversal thin section (Nicols //) of a Ravenna stone sample with clearly defined surface black crust. (b) Scanning electron

micrograph of the surface of the damage layer, showing gypsum crystals with an embedded spongy carbonaceous particle.

C. Sabbioni et al. / Atmospheric Environment 37 (2003) 1261–12691266

Reservoir in Caiazzo). Oxalates are normally detected in

higher concentrations in black crusts on stone than in

those on mortar.

The significant presence of formate, acetate and

oxalate anions in black crusts on monuments and

historic buildings points to the need for more focused

and detailed studies on organic anions to identify their

specific origin and highlight their eventual role in

damage layer formation.

Acknowledgements

This work has been carried out within the EC Project

‘‘Carbon content and origin of damage layers in

European monuments–CARAMEL’’ (Ct. No. EVK4-

CT-2000-00029) funded by the 51 FP Research, Envir-

onment and Sustainable Development Programme–Key

Action ‘‘The City of Tomorrow and Cultural Heritage’’.

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