Insight into the effects of moisture and layer build-up on the formation of lead soaps using micro-ATR-FTIR spectroscopic imaging of complex painted stratigraphies

Metal soaps are formed in paint layers thorough the reaction of metal ions of pigments and fatty acids of organic binders. In thisstudy, micro-ATR-FTIR spectroscopic imaging was used to analyse the formation of lead soaps in oil-based paint layers inrelation to their exposure to moisture sources. The investigations were carried out on authentic samples of complex stratigraphiesfrom cold painted terracotta statues (Sacred Mount, Varallo, UNESCO) and different IR-active lead white pigments, organicmaterials, and lead soaps were discriminated. The saponification of selected paint layers was correlated to the conservationhistory, the manufacturing technique, and the build-up of layers. The presence of hydrophilic layers within the stratigraphy andtheir role as a further water source are discussed. Furthermore, the modifications experienced by lead-based pigments from thecore of an intact grain of pigment towards the newly formed decay phases were investigated via a novel approach based on shiftof the peak for the corresponding spectral bands and their integrated absorbance in the ATR-FTIR spectra. Qualitative information on the spatial distribution from the chemical images was combined with quantitative information on the peak shift to evaluatethe different manufacture (lead carbonate, basic lead carbonate) or the extent of decay undergone by the lead-based pigments as afunction of their grain size, contiguous layers, and moisture source. Similar results, having a high impact on heritage science andanalytical chemistry, allow developing up-to-date conservation strategies by connecting an advanced knowledge of the materialsto the social and conservation history of artefacts.