Non-invasive analysis of gilt leather gold varnish and protective layer by infrared reflection-absorption spectroscopy

Abstract

As part of a larger research aiming to develop analytical approaches to characterise gilt leather constitutive materials and identify material markers for their attribution, this paper focuses on the organic layers applied on the silver leaves: the gold varnish and the protective layer. Ancient gilt leather varnish recipes were first analysed and compared by ATR-FTIR and Py-GCMS to try to define markers allowing varnish discrimination. The research demonstrates that the two techniques cannot separate varnish recipes based on recipe-specific compounds present in minor concentration, but are sensitive to the variation in the oil-resin ratio. Therefore, quantification of the oil-resin proportion should be considered to help with the decor provenance attribution, as long as the gilt leather has not be lubricated. More importantly, the paper assessed the potential of infrared reflectance spectroscopy as a non-invasive analytical technique to characterise the organic layers applied on the silver leaves. Thanks to the metal leaf that acts as a mirror, infrared reflectance spectroscopy allows collection of high quality reflection-absorption spectra of the gold varnish and the protective layer in gilt leather. In the corpus of historical decors analysed, reflectance FTIR confirmed the presence of an oil-resin varnish in all gilt areas, and revealed, for the first time, the use of a protein compound as a protective layer in the silvered areas, as well as in some gilt areas. In the silvered area, different practices are evidenced, consequently a larger corpus of silvered gilt leather decor should be investigated to determine if these differences can be related to geographical practices.