Late Gothic/early Renaissance gilding technology and the traditional poliment material “Armenian bole”: Truly red clay, or rather bauxite?

Abstract It is generally considered that the so-called “Armenian bole” used since the Antiquity as a medicament and later also as, e.g., a preparatory layer – poliment for water gilding in fine art, was the same material (both by source and composition). It is possible to assume that its alkalinity and strong absorption capacity corresponding most frequently to high content of smectites is beneficial for medicinal purposes, on the other hand, presence of smectites in poliments can cause mechanical instability of the gilding due to swelling effects under increased humidity. Further, pure smectites usually do not allow obtaining a compact layer. Other properties of the bole, as, e.g. elasticity, formability and intense red colour, were sought by painters and goldsmiths and therefore, the boles were more likely represented by mixtures of minerals. We have collected micro-samples of gildings on bole from significant panel paintings and polychrome statues belonging to the most prospective period of the 15th and the early 16th century, when the gilding on bole was occurring most frequently in workshops connected with the most attractive artistic centres of the Central Europe, as, e.g. Southern Germany or Danube region. Non-invasive and micro-analytical methods were applied, with particular attention given to the collection of laboratory micro-pXRD data. It was found out that at the turn of the 15th and the 16th century, clay-rich boles of various compositions started to be replaced by products of intense lateritic weathering with significant contents of free Al hydro-oxides (gibbsite, boehmite). It is the first discovery of natural Al-rich pigment (referring either to bauxite or Al-laterite) in historical paintings ever.