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An approach to cleaning methods and materials for painted alabaster

The relief of Saint Jerome Penitent from the Cathedral of Salvador in Zaragoza (Spain)
Raquel Diogo, Elena Aguado-Guardiola, Fernando Costa and João Coroado

Abstracts

This research is aimed to obtain a deeper understanding of the chemical properties of painted alabaster sculptures as well as some of their ageing mechanisms in order to design cleaning protocols which take into account the particular changing nature of these dynamic and complex systems.Materials and some chemical properties (pH, conductivity and water solubility) of the case studyhave been characterised. Glass slides as well as mock-ups were elaborated and aged on alabaster, wood, and linen, with similar film forming materials as those that have been found in the case study.  

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The authors wish to thank Agar-Agar Productos de Conservación y Restauración, Christopher Marques, Ignacio Mustienes Sánchez, Ignácio Sebastián Ruíz, José Félix Méndez and Andrea Allué. Special acknowledge are to be made to all the researchers and professors that have shared with us their knowledge about both the aging/degradation of the film-forming materials involved in these complex and dynamic systems and the methods and materials for Cleaning Painted Surfaces.

Introduction

1This research is aimed to obtain a deeper understanding of some chemical properties (pH conductivity and water sensibility) of painted alabaster sculptures as well as some of their ageing mechanisms in order to design cleaning protocols which take into account the particular changing nature of these dynamic and complex systems.

2 A 16th century alabaster relief of Saint Jerome from the Cathedral of Salvador in Zaragoza, Spain, was taken as a case study and pH, conductivity and water solubility of their painted films has been measured and discussed.

Fig.1 Saint Jerome Penitent

Fig.1 Saint Jerome Penitent

16th century relief of Saint Jerome Penitent from the Cathedral of Salvador in Zaragoza (71,5 cm x 61,5 cm x 11 cm).

© photo Christopher Marques

The changing properties of painted alabaster during aging

3In addition, materials of the case study have been characterised with analytic technics such as light microscopy, Fourier transform infrared spectroscopy, x-ray diffraction, x-ray fluorescence and scanning electron microscopy. Glass slides as well as mock-ups  on alabaster, wood, and linen, were elaborated with some of the film forming materials found in the case study and aged.  The changing pH, solubility and conductivity of their surface, before and after ageing, have been studied.

4The behaviour of the polychromies on stone will depend on the specific painted materials used, the type of stone and the surrounding environmental conditions.  The accumulation of soils on the surface of the painted layers on alabaster could also modify the composition of the painted film.

5This combination of factors would dictate the chemical reactions that will eventually take place, with new chemical equilibriums being successively reached. A number of physical and chemical reactions can then occur in an unpredictable way. New and sometimes unexpected inorganic, organic and mixed products can be formed during such chemical-physical reactions.  The processes leading to the formation of these by-products on painted alabaster have not been fully understood yet but cause degradation over time and induce significant changes in properties such as pH, conductivity and solubility.

6During our research we’ve realize that painting materials don’t react over time with alabaster as they do with other organic or even inorganic supports. Exactly the same mixture of binder (oil or glue) and pigment appears to react differently whether the support is alabaster or a canvas with a ground of gypsum or chalk: the pH used to be similar to gypsum ones and higher than chalk ones but the conductivity of the films on alabaster used to be always too muchlower.

7Our results about pH, conductivity and solubility were obtained through standardized agarose gels being in contact with the surface for a fixed time range. They are not values ​​obtained by direct measurements and they are not absolute but relative values about pH and conductivity.

Fig. 2 Alabaster, oil and verdigris

Fig. 2 Alabaster, oil and verdigris

Aged Mock-ups oil and verdigris film on alabaster and on linen with chalk glue ground. 

© photos Christopher Marques

Surface cleaning as a function of surface properties

8The removal of soils and other substances deposited over time on painted surfaces is one of the treatments conservators usually must deal with when treating painted alabaster. Since these treatments involve direct contact with the films, they might have consequences that affect not only the material integrity of the sculpture but also the visual appearance of its surface. Therefore, prior their removal on painted surfaces, a good understanding of the forces of adhesion between the surface and the soils, the properties of the surface and the environmental conditions is necessary. As important as this prior knowledge about the surface is to control the properties, the form and the movement of our cleaning materials in order to create management strategies that minimize the risk for the surface.

Conclusion

9According to this, common surface cleaning protocols developed for painted surfaces on other supports might not suit well for the same kind of films on stone and especially alabaster, since the properties that the polychrome layers acquire over time are different according to the type of support, the painting materials and the environmental conditions.

10This is a preliminary step to understand and develop an approach to cleaning methods and materials for painted alabaster that respects and preserves the properties of such materials but, equally interesting, that guarantees the absence of toxicity for conservators.

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Attachment

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List of illustrations

Title Fig.1 Saint Jerome Penitent
Caption 16th century relief of Saint Jerome Penitent from the Cathedral of Salvador in Zaragoza (71,5 cm x 61,5 cm x 11 cm).
Credits © photo Christopher Marques
URL http://ceroart.revues.org/docannexe/image/4355/img-1.jpg
File image/jpeg, 8.7M
Title Fig. 2 Alabaster, oil and verdigris
Caption Aged Mock-ups oil and verdigris film on alabaster and on linen with chalk glue ground. 
Credits © photos Christopher Marques
URL http://ceroart.revues.org/docannexe/image/4355/img-2.jpg
File image/jpeg, 1.4M
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References

Electronic reference

Raquel Diogo, Elena Aguado-Guardiola, Fernando Costa and João Coroado, « An approach to cleaning methods and materials for painted alabaster », CeROArt [Online], HS | 2014, Online since 18 August 2014, connection on 20 October 2017. URL : http://ceroart.revues.org/4355

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About the authors

Raquel Diogo

Raquel Diogo is currently enrolled in the Master’s degree in Conservation and Restoration  with a profile specialization on integrated heritage at the Polytechnic Institute of Tomar (Portugal) and got the Bachelor’s degree in Conservation and Restoration in 2012 at the same school. Her Master’s thesis has been supported by Erasmus program  and included  in a research about the study of the materials of polychromed stone sculpture  developed at the Escuela Superior de Conservación y Restauración de Bienes Cullturales in Huesca (Spain). Author for correspondence: email:  raquelnogueiradiogo@gmail.com

Elena Aguado-Guardiola

Elena Aguado-Guardiola is an associate professor at the Escuela Superior de Conservación y Restauración de Bienes Culturales  in Huesca (Spain) and got the BA in Conservation and Restoration at the same school. She graduated in History of Art in 1992 from the University of Zaragoza (Spain) and has published different papers on French and Spanish burgundian stone-sculpture in 15th century.  She is currently enrolled in the PhD Program in Conservation and Restoration of Cultural Heritage at the Polytechnical University of Valencia (Spain) with a thesis dealing with the study of the materials of polychromed stone sculpture, their ageing and stabilization, with special emphasis in the study of the interactions between degradation materials and cleaning treatments.

Fernando Costa

Fernando Costa is currently an adjunct professor at the Instituto Politécnico in Tomar (Portugal) where he has taught since 1995, has a degree in Conservation and Restoration for the Escola Superior de Tecnologia in Tomar (Portugal) and a Master's Degree on Recovery of the Architectural and Landscape Heritage of the Universidade de Évora (Portugal) has participated on several projects and published a few articles in the areas of conservation and restoration of stone materials.

João Coroado

João Coroado has a degree in geology from the University of Coimbra (1990) and a Master and PhD in Geochemistry in Geosciences from the University of Aveiro (1994 and 2001 respectively). Is currently Coordinator Professor and Director of the School of Technology of Tomar, Polytechnic Institute of Tomar. Also collaborates in the Master Course in Applied Chemistry for Cultural Heritage, Faculty of Science at University of Lisbon. It is a permanent member of the Research Unit GeoBioTec Science and Technology Foundation and is the author of several publications related to the compositional characterization of ceramic materials and mortars.

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Copyright

Licence Creative Commons
CeROArt – Conservation, exposition, restauration d'objets d'arts est mis à disposition selon les termes de la licence Creative Commons Attribution - Pas d'Utilisation Commerciale - Pas de Modification 4.0 International.

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