Initial stages of metal soaps' formation in model paints: The role of humidity

The presence of metal soaps is well documented in paintings originating from the 15th century onward, and in numerous cases, the soaps'formation is attributed to environmental factors, like relative humidity, light exposure and/or high temperature. Understanding the formation mechanism, involving effects of these particular factors during artworks realization, is crucial for the search of suitable conservation and preservation strategies. In the present study, the role of humidity on the formation of metal carboxylates and other secondary products in mixtures of lead-based pigments (i.e., minium, lead-tin yellow and lead white) with linseed oil was described within the experimental drying of model paints. Samples were exposed to various relative humidity conditions and the processes were monitored by Fourier-transform infrared spectroscopy (FTIR) and powder X-ray diffraction (XRD) involving both the hourly measurements for the monitoring of immediate processes and long-term (up to two months) monitoring to evaluate secondary changes. The acquired data clearly demonstrated the influence that the different levels of humidity may have on the formation of lead carboxylates during the painting's drying. The selected methods were efficient in the reaction monitoring and provided new insights leading to a better understanding of factors affecting the metal soaps' formation in paints.