Identification of lipidic binding media in plasterwork decorations from the Alhambra using GC-MS and chemometrics: Influence of pigments and aging

The potential influence of pigments and aging on the identification of lipidic binders in polychrome plasterwork was studied. Model plasterwork samples were prepared considering different binders applied alone and mixed with several pigments and subjected to different aging conditions. The relative content of methyl derivatives of the fatty acids (FAMEs) were determined by GC-MS after transesterification and extraction in benzene. From the preliminary exploration using principal component analysis (PCA) it was concluded that the differentiation between the binders could be possible regardless of the aging condition and the pigments present. Then, linear discriminant analysis (LDA) using a stepwise feature selection was tested, comparing its performance with the use of FAME ratios. The LDA method provided a more objective classification tool, showing a predictive ability of 100% for egg and linseed oil and 84% for poppy seed oil and the mixture egg + linseed oil. Finally, both approaches were used to identify the binders present in several historical samples from two chambers of the Alhambra complex (Granada, Spain).