Did Van Eyck use white vitriol as paint drying agent? Zn and S micro-XANES of paint samples from the Portraits of Jan and Margaret van Eyck

Recently we have found Zn-containing paint in several Van Eyck paintings (15th c.) such as The Ghent Altarpiece, The Arnolfini Marriage and in Jan and Margaret Van Eyck’s portraits. Since Zn is not normally part of 15th c. artists’ pigments, its presence is highly unusual. We propose to examine by μ-XRF and μ-XANES at the Zn and S K edges, 5 original paint micro samples taken from paintings by Jan Van Eyck. Zn is present here in dark (brown-red) or red layers/areas that are either on the surface or buried below other paint; the latter can be visualized using macroscopic XRF. The Zn compound(s) are amorphous and not easily identified by μ-XRPD or conventional Raman microscopy.SEM and FTIR evidence suggest that the original Zn compound added to the paint may have been similar to K2Zn(SO4)2.6H2O or to goslarite (ZnSO4.7H2O), the source of white vitriol, a well-known oil-drying (i.e. secondary fatty acid cross-linking) agent in the 15th century.