Revealing Degradation Patterns: Imaging Techniques for the Study of Metal Soap Formation on Painted Metal Objects

In attempting to document the degradation processes occurring on cultural heritage objects, imaging-based analytical techniques present many advantages, as they provide spatial and spectral information and allow the simultaneous investigation of the chemical and morphological characteristics of a sample. This study presents a protocol based on chemical imaging – Fourier transform infrared microspectroscopy (μ-FTIR) and hyperspectral imaging (HSI) – aimed at monitoring the formation of metal soaps on model metal coupons. Oil-painted metal supports are in fact not immune to degradation due to metal soaps formation, a phenomenon that affects all oil-painted surfaces from the initial curing of the paint film. Copper and zinc sheets were coated with cold-pressed linseed oil and artificially aged for one month in order to instigate the formation of metal soaps. Their reaction was then monitored by means of µ-FTIR. The chemical maps showed an increasing trend over time, elucidating some aspects and differences in the mechanism of formation of the organic salts for the two metal substrates. Additionally, the samples were analysed using two hyperspectral cameras, operating in the visible-near infrared and short-wave infrared spectral range. The appropriateness of the two cameras in the investigation of metal soaps, and the effect of the thickness of the coating on the data obtained, is discussed here.