On the way to tempera grassa: Unraveling the properties of emulsion-based paint binders

Abstract

Painting technique in Europe underwent a major change in the 15th century, when the use of egg yolk as a binder in tempera painting was gradually replaced by oil paint. This transitional period probably saw the occasional use of a mixed technique called tempera grassa, in which both types of binders, egg and oil, are mixed in the form of an emulsion. In order to better understand and document this historical painting practice, this article describes the physico-chemical aspects of emulsion-based binders, prepared with egg yolk and either raw linseed oil, or linseed oil partially saponified with lead oxide. Highly stable direct emulsions can be prepared with linseed oil (raw or lead-treated) and egg yolk. Remarkably, lead-treated oil also allows the formation of stable inverse emulsions. The rheological properties of direct emulsions are driven by the oil fraction: as this parameter increases from 0 to 70 wt% oil, the emulsion changes from a viscous liquid to a viscoelastic solid, which allows its flow to be easily tuned by the painter. The impact of egg yolk on the drying mechanism of the emulsion is elucidated thanks to the combined use of NMR relaxometry and FTIR spectroscopy.