Abstract
This paper describes a method for reliably identifying saccharide materials in paintings. Since the 3rd millennium B.C., polysaccharide materials such as plant gums, sugar, flour, and honey were used as binding media and sizing agents in paintings, illuminated manuscripts, and polychrome objects. Although it has been reported that plant gums have a stable composition, their identification in paint samples is often doubtful and rarely discussed. Our research was carried out independently at two different laboratories: the Getty Conservation Institute in Los Angeles, USA (GCI) and the Department of Chemistry and Industrial Chemistry of the University of Pisa, Italy (DCCI). It was shown in a previous stage of this research that the two methods give highly comparable data when analysing both reference paint samples and paint layers from art objects, thus the combined data was used to build a large database. In this study, the simultaneous presence of proteinaceous binders and pigments in fresh and artificially aged paint replicas was investigated, and it highlighted how these can affect the sugar profile of arabic, tragacanth, and fruit tree gums. The environmental contamination due to sugars from various plant tissues is also discussed. The results allowed the development of a new model for the reliable identification of saccharide binders in paintings based on the evaluation of markers that are stable to ageing and unaffected by pigments. This new model was applied to the sugar profiles obtained from the analysis of a large number of samples from murals, easel paintings, manuscripts, and polychrome objects from different geographical areas and dating from the 13th century BC to the 20th century AD, thus demonstrating its reliability.
Highlights
Saccharide materials are known to have been used extensively as binding media for paintings and adhesives for gilding, as testified by several medieval manuscripts from Western Europe and recipe books describing painting techniques and materials [1,2,3,4]
– fucose, in arabic and fruit tree gums, showed very low relative content, which was always below 1.0%; it was relatively abundant in all samples in tragacanth gum, subject to changes in the relative amounts depending on the pigment and ageing;
– mannose, in arabic and tragacanth gums, showed a very low relative content and was always below 1.0%; in fruit tree gum it was present above 1.0% in all samples, subject to changes in the relative amounts depending on the pigment and ageing, with the exception of all the samples containing malachite (0.0%);
Summary
Saccharide materials are known to have been used extensively as binding media for paintings and adhesives for gilding, as testified by several medieval manuscripts from Western Europe and recipe books describing painting techniques and materials [1,2,3,4] Despite their widespread and documented use, polysaccharide materials have been identified in very few works-of-art [5] such as mural paintings and polychromies in ancient Greece [6], Egypt [7,8,9], India [10], and Afghanistan [11]. Plant gums (like arabic, tragacanth, and fruit tree) are naturally occurring polysaccharide materials exuded by several species of plants or extracted from the endosperm of some seeds They are high molecular weight polymers consisting of aldopentoses, aldohexoses, and uronic acids joined together by glycoside bonds [15,16]. It is well known that amino acids and sugars may react leading to the formation of Maillard reaction products, some of which have been isolated in animal glue [27]
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