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CERAZUL: ASSESSMENT AND DEVELOPMENT OF MATERIALS AN D TECHNIQUES FOR THE CONSERVATION OF HISTORIC AZULEJO S

Sílvia R. M. Pereira, Laboratório Nacional de Engenharia Civil, DM-NPC, spereira@lnec.pt

Lurdes Esteves, Museu Nacional do Azulejo, lurdestevesbrito@gmail.com Marta T. Mendes, Laboratório Nacional de Engenharia Civil, Laboratório Hércules -

Universidade de Évora, martamagnini@gmail.com Jessica Musacchi, University of Bolonha, jessica.musacchi@gmail.it

Delgado Rodrigues, Laboratório Nacional de Engenharia Civil, DM-NPC, delgado@lnec.pt João M. Mimoso, Laboratório Nacional de Engenharia Civil, DM-NPC, jmimoso@lnec.pt

ABSTRACT

CerAzul aims to study, evaluate and optimize the treatments used in the conservation and restoration of architectural tiles (azulejos). The interventions that will be considered in more detail are the consolidation of the ceramic matrix, the re-adhesion of the glaze and biscuit fragments, and the volumetric reintegration of azulejo lacunae. The scientific state of the art on the application of these treatments to architectural ceramics will be discussed. The most common materials and techniques used nowadays in these interventions will be depicted together with some considerations about their use. The strategy to be followed intends to evaluate the efficacy, compatibility and durability of the treatments with the azulejo substrate in order to obtain systematic information that will allow professionals in the field to choose the best solution for their interventions. A set of existing and promising materials for the conservation of tiles is planned to be tested. Effectiveness and compatibility tests will be performed on treated azulejo samples and these ones will be subject to accelerated ageing in a climatic chamber where their degradation will be monitored. New formulations, alterations to existing ones, or new materials will be developed in order to tentatively obtain treatments with improved properties. KEYWORDS: CerAzul, conservation, azulejo, durability, compatibility.

1. INTRODUCTION

Azulejos are a major Portuguese contribution to the cultural heritage of the World. However, a sizeable part of this rich heritage is lost every year through neglect and continuous decay. The degraded panels can be subjected to various restoration measures: cleaning; desalination; removal from the support; biscuit consolidation; tile fragment adhesion; glaze to ceramic adhesion; re-attachment of the tiles to the support; tile and panel lacunae filling; and chromatic reintegration. The main concern nowadays is on how to select compatible and durable products and methods to use for the conservation and restoration of architectural glazed ceramics – mainly in the consolidation, adhesion and lacunae filling interventions. This is the subject we propose to address within the FCT funded CerAzul project (PTDC/CTM-CER/119085/2010), so that the azulejo heritage may be passed on to future generations, as much as possible as it was received from our forefathers.

As mentioned before, three main types of restoration interventions will be considered: tile consolidation, re-adhesion of glaze layers and fragments; and lacunae repair. The first intervention type requires a consolidating material, the second an adhesive, and the third a filler, or filler plus a coating. Treatment materials are available on the market but limited information is available to direct the choice of the conservator-restorer about their effectiveness, compatibility and long-term

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performance (commonly called “durability”). This lack of knowledge is particularly felt for glazed ceramics in architectural usage which are subjected to very harsh environmental conditions.

This Project will strive to provide information on the long-term performance of the commercial treatments available for the restoration of historic azulejos and develop specifically improved products and methods for azulejo treatment. The preservation of our azulejo heritage for future generations is a responsibility which we hope to help fulfil with CerAzul and call upon all the professionals interested in the field to co-operate towards results that all of us will share.

3. RESEARCH ON CONSERVATION TREATMENTS OF HISTORIC AZULEJOS In order to define proper restoration and conservation treatments for azulejos it is important to

know the basic constituent materials of the silica-lead based glaze and the biscuit of azulejos. Regarding this characterization the following works should be highlighted; Antunes [1], Coroado Gomes [2] and Pereira et al [3], where analytical results on the physical chemical and mineralogical characterization of Portuguese azulejos are reported. The studies on the degradation of the glaze layer and its connection with intrinsic defects should also be highlighted [4,5,6,7]; and finally the work by Pérez-Rodriguez et al. [8] in the characterization of the ceramic sculptures from Seville cathedral to establish their decay mechanisms shall also be mentioned.

While scarce studies exist on the characterization and degradation of architectural glazed ceramics such as azulejos, even more limited technical information exists on the study of their conservation and restoration methods [9].The existing studies of the conservation and restoration treatments of ceramic substrates are mainly directed to non-architectural objects usually from a museum setting [10,11,12,13]. In an architectural setting, the restoration treatments need to be able to endure long exposures to harsh environmental conditions, requiring knowledge about the degradation mechanisms of the treated azulejos and their ageing behaviour that does not exist nowadays.

Some initial research has been however performed on the consolidation of Portuguese azulejo ceramic base [1,14]. These works have tackled some aspects of the effectiveness and compatibility of the commonly used Paraloid B72® acrylic resin. The work of Vaz et al [14] also studied the application of two alkoxysilanes as consolidants of azulejos. However the compatibility and ageing behavior issues of the interventions in azulejos have never been to our knowledge assessed. A project entitled "Desenvolvimento de Novos Produtos e Tecnologias para Restauro e Conservação de Azulejos Antigos" (PRAXIS/2/2.1/HIS/13/94) has run from the year 1995 to 1998. From this project some publications have been issued [e.g. 15 , 16 , 17 , 18 ] mainly relating to intervention case studies, desalination, and tiles characterization, but the main developments obtained in the project regarding the study of treatments used on the conservation and restoration of tiles are difficult to find. Other publications describing the methodologies used on the conservation and restoration interventions of architectural tiles and some considerations about the methodology used have also been published [19,20,21,22,23,24,25,26]. However, to our knowledge there is not a consistent study developed on the efficacy, compatibility and durability of treatments used on the conservation and restoration of tiles considered in an architectural setting.

Several other studies have been made on the individual materials or in other substrates such as stone. These can be helpful when considering their application into azulejos. For instance Paraloid B72®, a methacrylate organic polymer is the most widely used organic material in the conservation and restoration of tiles, being applied as a consolidant, adhesive and filler (as the binding agent together with an bulk inorganic material). However, some studies concerning its ageing behavior on stone treatments indicated its susceptibility to UV radiation [27] and sensitivity to biocontamination-biodegradation. Several other polyacrylates used in conservation have also been tested allowing the selection of the best performing formulations with regards to their photo-oxidative and/or thermal

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stability [28,29,30]. However, none of these studies considered the application of these treatments to glazed ceramics. The stability of five different types of commonly used polymers after exposure to thermal and relative humidity variations, UV radiation and SO2- was researched with spectroscopy by López- Ballaster et al. [31]. This study indicated that polymeric alterations were detected on the polymers structure such as chain scissioning, cross-linkage changes and oxidation of the main chains or side groups. Down et al [32] have done an extensive research on poly(vinyl acetate) and acrylic adhesives used in conservation and restoration allowing the distinction of several commercial products in regards to their ageing behavior. Extensive studies with epoxy polymers have been performed by Tennent et al. [33,34,35], who have elaborated on their thermal and photo degradation mechanisms. The existing information on the isolated polymers ageing behavior is important to understand the degradation mechanisms but the interaction of the azulejos treated with these polymers is essential.

4. MATERIALS USED FOR CONSOLIDATION, ADHESION AND L ACUNAE FILLING

TREATMENTS OF AZULEJOS Several types of materials have been, or can be, used on consolidation and/or adherence

treatments of ceramic materials. These can be divided in inorganic products, organic polymeric materials, and mixes of organic-inorganic materials [11,36]. Besides some interventions that have been published with technical details [19-26], the information obtained about the materials and methods used was based on consultation with professionals from the field and existing reports of previous interventions. 4.1. Consolidation of ceramic biscuit

Consolidation of ceramic biscuit is not the most common restoration method applied on tiles but it is indispensible when there is a disaggregated biscuit ceramic matrix which in many cases is caused by the presence and action of water and/or soluble salts. Consolidation is generally applied specifically on the fragile areas of the tile ceramic support or, less commonly, to the whole tile when the ceramic matrix is in a highly disintegrated state. For circumscribed adhesion, the consolidants are applied by brushing, while for integral and more in-depth consolidation immersion is usually used. The higher efficiency of in-depth impregnation using the immersion method when applying a Paraloid B72 solution in relation to brushing has been however questioned by Prudêncio et al [37].

The most common material applied for consolidation used nowadays is undoubtedly the acrylic resin Paraloid B72® on a low dissolution (5-25% on a solvent such as acetone), however epoxidic resins such as HXTAL NYL-1®, and acid silicate esters (such as TEOS – tetraorthosilicate) are also sometimes used.

Figure 1. Consolidation of ceramic biscuit by brushing (a) and immersion (b).

4.2. Re-adhesion of glaze and ceramic fragments

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The tile fragmentation is usually the result of mechanical stresses applied to the tiles. This could be inflicted by movements and load in the wall support, incision through the surface or even the result of differential water or thermal expansibilities and salts crystallization. For the glaze detachment from the biscuit several causes could be contributing such as hydrolysis reactions between the materials in the glaze-ceramic interface and the different hydric and thermal expansibilities of the ceramic matrix and the glaze that can cause shear stress in that interface. The adhesion treatment is usually made by applying organic adhesives. An exception is made when the tile fragments are re-applied onto the support with a mortar. This type of intervention is however not going to be considered since the re-aplication of tile fragments does not try to maintain the full integrity of the tile, and the tile fragments are handled as new units (Figure 2a). The material most commonly used nowadays for the re-adhesion of fragments and glaze is the acrylic resin Paraloid B72®. The adhesion of fragments is usually made with a solution of Paraloid B72® of 40-50% in acetone or other solvents, while the adhesion of glaze to biscuit is made with a lower concentration around 10-20%. Other acrylic resins are also sometimes used such as Primal AC33® and Paraloid B67®, which possess a higher TG (50ºC) than Paraloid B72® (40ºC), and therefore it is considered as more appropriate for use in the Mediterranean climates. In some cases the use of epoxy resins is also reported (e. g. Devcom 5min®, HXTAL NYL-1®), together with the use of cellulose nitrate resins such as UHU Hart® (usually in interior architectural settings since they are hygroscopic and sensible to UV radiation yellowing with time [11]. The adhesives are usually applied with a brush or directly from the adhesive tube in loose fragments and via injection to reach fractures such as lifting glaze (Figure 3).

Figure 2. Fragmented tiles. a) Re-application of the fragment on to the wall, b) Fragmented tile,

c) tile after re-adhesion treatment.

Figure 3. Azulejo fragments (a) and glaze to biscuit (b) re-adhesion.

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4.3. Lacunae fillings Tile lacunae are formed by the loss of glaze, with or without loss of ceramic material and on

their treatment a diverse range of solutions is used. These solutions comprise the use of organic or inorganic binders together with an aggregate. The inorganic binders most commonly used are aerial lime and hydraulic lime. Gypsum is also sometimes applied but its use is not recommended due to its behavior in the presence of humidity and due to soluble salts (its use is only acceptable in interior settings without humidity). As regards the organic materials several resins are usually applied such as acrylic resins (e.g. Paraloid B72®, Primal AC33®), vinylic resins (e.g. Mowlith®, Vinnapas®), Poliesters (AIROCOLL®) and epoxy resins (e.g. Icosit k101®, Milliput®). Due to low its low viscosity some binders are mixed together with an aggregate to form a paste. The pastes are usually applied with a spatula/scalpel and then smoothed with fine abrasives such as abrasive papers or cloths. The aggregates used can be of a wide variety of materials such as fine sand, powder silica, glass microspheres, chamotte, stone powder, etc.

Figure 3. Lacunae filling.

5. PROPOSED METHODOLOGY TO STUDY THE TREATMENT SOL UTIONS Within this project a selection of commonly used or promising materials will be made and subsequently studied in detail for the consolidation, adhesion and volumetric reintegration treatments. These treatments will be tested in terms of their effectiveness for azulejo interventions. The best treatments will be assessed as to their compatibility based on the PRODOMEA indicators methodology [38]. The most efficient and compatible treatments will then be the subject to accelerated ageing in a climatic chamber and their degradation mechanisms monitored. After ageing, their effectiveness and compatibility indicators will be reassessed. If any of the products previously studied is considered promising but requiring some adjustment to be applicable to the historic tiles, its properties will be optimized as appropriate. Eventually new materials will also be developed and assessed in relation to their efficacy, compatibility and durability for interventions in azulejos. 6. NEW MATERIALS AND METHODS Some authors have tried to study and optimize the polymerization processes to attain better properties of the consolidation materials for general use in conservation practice. For instance, Salazar et al.[ 39] have studied the influence of the solvent type and addition of colloidal silica particles on the polymerization of tetraorthosilicate (TEOS), and concluded that the solvent type influences the polymer degree of condensation, while the presence of silica particles influences its porosity . Zhai et al [40] have studied the effect of nanoparticles addition on the adhesion of an epoxy resin having concluded that adhesion strength has largely increased. And Mosquera et al [41] have studied the

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positive effect of surfactants addition to decrease the cracking behavior and hydrophobicity of stone consolidant silica materials. Depending on the results obtained from the study of existing and promising new treatment materials, it is our intention to optimize product formulations towards improved properties in order to better adjust them for azulejos restoration. Promising consolidants, adhesives and/or coatings materials will be therefore chosen for further in-depth investigation. New formulations, or alterations to existing ones, will be developed in order to improve some of their properties (e.g. viscosity, cracking susceptibility, penetration capacity, adhesion) and to optimize products formulations to better adjust them to azulejos conservation and restoration. Promising new materials will also be considered such as sol-gel methods and geopolymers. This will hopefully result in tailored treatments for glazed ceramics. 7. CONCLUSIONS

CerAzul aims to cover a gap of technical information existing in this field and direct the choice of the professional conservator and restorers towards the best treatments for azulejo interventions. This Project will therefore try to provide information on the long-term performance of commercial treatments available for the restoration of historic azulejos and develop improved products and methods specifically for azulejo treatment. The preservation of our azulejo heritage for future generations is a responsibility which we hope to help fulfil with CerAzul and call upon all the professionals interested to collaborate with the project.

8. ACKNOWLEDGEMENTS: The authors credit FCT for financing the CerAzul project (PTDC/CTM-CER/119085/2010). Marta T. Mendes also credit FCT for her PhD grant. Jessica Mussachi credits the Fondazione Flaminia Ravenna for her Masters grant. This research is also integrated into LNEC Planned Research Program within the Project 0205/11/17684 (Conservation of historic azulejos).

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