Metal Soaps formationThe formation of metal soaps (metal carboxylates, metal stearates or metal palmitates) in both old and modern paintings is a topic that has received much attention in the field of conservation the past 15 to 20 years. Metal soaps are formed by the reaction of metals from pigments in the ground and paint layers with free fatty acids from the oil binding medium. The migration of metal soap components to the paint surface and subsequent remineralisation at the surface (with environmental conditions playing a serious role) can affect the stability of paint layers and result in disfiguring surface phenomena such as the formation of crusts, protrusions or aggregates or surface efflorescence. Metal soaps (often metal stearates) are intentionally added in small quantities to modern paints to act as driers, anti-settling, wetting, matting, or dispersive agents. Larger quantities are added to cheaper paints as extenders as they create a gel-like consistency reducing the proportion of pigment required in paint formulation. This can lead to issues such as increased drying time of paint layers, the formation and migration of fatty acid efflorescence and crumbling paint. The removal of metal soap crusts and efflorescence is starting to be researched and positive results have been reported using water-based gels containing chelating agents which sequester metal ions and break up the crust/efflorescence, and a controlled pH in the 5.5-8.5 range.
Wet (Non-drying) Paint