Non-invasive X-ray investigations of medieval sculptures: New insights on “applied tin-relief brocade” technique

Abstract

Abstract Two polychrome sculptures from the late Middle Age were investigated by means of non-invasive techniques with the aim of characterizing sophisticated relief decoration named “applied brocade”. Here, we have used portable instrumentation solely in order to perform X-ray powder diffraction and fluorescence measurements at the surface of the sculptures. The results show a reliable localisation and identification of the different materials making up an applied brocade: a filler material, a tin-based foil and a gilding, even in the case of a largely overpainted sculpture. More particularly, a direct identification of the organic material used for the filler as natural beeswax is proposed. Differences between the two investigated artworks have also been identified: (i) in the structure of the calcium sulphate preparation layers (one or double layer of gesso) (ii) in the presence or absence of a priming paint layer under decors (iii) in the choice of the material used to make the gilding: pure gold or probable zwischgold. The characterization of the degradation of this type of decorations was also assessed, namely romarchite (SnO) and cassiterite (SnO2) were identified and their relative proportion estimated.