Notes on Metal Soap Extenders in Modern Oil Paints: History, Use, Degradation, and Analysis

The long-term behavior of metal stearates used as additive in paints is not well known, and their detection is challenging. This paper presents some aspects of the production of technical metal stearates from stearin in twentieth-century oil paint and its use. Two fast direct analytical mass spectrometry (MS) techniques, evolved gas analysis MS and electrospray ionization MS, are introduced as useful tools for the detection of low (<2%) quantities based on the different palmitate/stearate (P/S) ratios of the stearates, oils, and other lipids in the paint. Early occurring fatty acid efflorescence on twentieth-century oil paints is caused primarily by metal stearates, as shown in one example from a modern painting by Mia Tarney. Metal stearates are not stable in the course of time; artificial aging of a set of paints shows the fast hydrolysis of aluminum stearates under elevated relative humidity.