Characterization of calcium sulfate grounds and fillings of applied tin-relief brocades by Raman spectroscopy, Fourier transform infrared spectroscopy, and scanning electron microscopy

摘要

Applied tin-relief brocade (commonly called applied brocade) refers to a decorative painting technique using tin leaf applied over a supporting relief mass (filling) which is glued to the artwork to simulate gold and silver textile brocades. This originated in Germany ca 1415-1430 and spread across Europe from the mid-15th century to the mid-16th century. This study focuses on six early 16th century altarpieces in the Basque country in the present province of Guipúzcoa, Spain. Cross sections of the ground and applied brocade were initially examined with optical microscopy and staining tests for proteins and lipids to assess the layering structure and materials present. Further examination with Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy with energy dispersive X-ray spectroscopy identified the inorganic and organic components of the various layers. Raman spectroscopic mapping was used to image the location of phases in selected cross sections. Five altarpieces from Spain had calcium sulfate grounds, whereas one thought to come from Flanders had a calcium carbonate ground. Raman and FTIR spectra showed that the thick, coarse lower ground layer (yeso grueso) is anhydrous calcium sulfate (anhydrite) whereas the fine, thin upper ground layer (yeso fino) is calcium sulfate dihydrate (gypsum). The filling masses consisted of different mixtures of inorganic (chiefly gypsum or anhydrite but occasionally with other pigments or additives) and organic (protein and/or oil or beeswax) materials. Comparison of the documented historical techniques with the materials found provides insight into local variations of the technique.