Spectrofluorimetric study of the ageing of mixtions used in the gildings of mediaeval wall paintings

Abstract

In the Middle Ages, we could find gildings on mural paintings. Gold, silver or tin leaves were applied according to distemper or mixtion technique. For the first one, a binder as glue is necessary, and for the second, a lipidic binder is used to stick the metallic leaf. Studies of gildings materials characterization show that the mixtion technique, with a mordant, is the most common. Linseed oil seems to be the binder used. It is always mixed with a siccative agent as lead. Because of bad conditions of conservation, the gildings do not resist anymore, only remain traces of metal or the adhesive under-layer. Thanks to the binder fluorescence, we can nowadays detect ancient gildings. The purpose of this paper is to study the degradation of the linseed oil, generally mixed with lead white to give a mordant for the metallic leaf, by spectrofluorimetry. To understand in situ fluorescence, gildings recreations, linseed oil and lead white are aged in hydro-thermal and ultraviolet (UV) light (313 nm) climatic rooms and under UV irradiation. Irradiation wavelengths are chosen according to the maximum of absorption of linseed oil and the bibliography (296, 313 and 366 nm = mercury bands). In comparison with results (in situ UV lamp, spectrofluorimetry), excitation wavelength chosen is 366 nm. Irradiations at 366 nm of linseed oil and linseed oil mixed with lead white show the most degrading effect in the fluorescence to the big wavelength. Lead white plays an important siccative role; it increases the intensity fluorescence and accelerates the drying of linseed oil. This study also allows to show that 366 nm wavelength is good for the in situ observation.