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

Abstract

Metal soaps are formed in paint layers thorough the reaction of metal ions of pigments and fatty acids of organic binders. In this study, micro-ATR-FTIR spectroscopic imaging was used to analyse the formation of lead soaps in oil-based paint layers in relation to their exposure to moisture sources. The investigations were carried out on authentic samples of complex stratigraphies from cold painted terracotta statues (Sacred Mount, Varallo, UNESCO) and different IR-active lead white pigments, organic materials, and lead soaps were discriminated. The saponification of selected paint layers was correlated to the conservation history, the manufacturing technique, and the build-up of layers. The presence of hydrophilic layers within the stratigraphy and their role as a further water source are discussed. Furthermore, the modifications experienced by lead-based pigments from the core of an intact grain of pigment towards the newly formed decay phases were investigated via a novel approach based on shift of 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 evaluate the different manufacture (lead carbonate, basic lead carbonate) or the extent of decay undergone by the lead-based pigments as a function of their grain size, contiguous layers, and moisture source. Similar results, having a high impact on heritage science and analytical chemistry, allow developing up-to-date conservation strategies by connecting an advanced knowledge of the materials to the social and conservation history of artefacts.

The stability of paintings and the molecular structure of the oil paint polymeric network

“…Artificial ageing under high relative humidity caused formation of amorphous metal soaps 18 , 19 —possibly, Mg salts—given concomitant changes to the hydromagnesite IR absorptions. Furthermore NWS presented a relatively higher content of unreacted free acids (HPLC).…”

“…As a result, the organic molecular composition of an oil paint layer evolves from polyunsaturated triglycerides to a significantly more complex system, whose composition evolves over years, even centuries, entailing the simultaneous presence of free fatty acids, free dicarboxylic acids, mono-, di- and triglycerides, and cross-linked fractions 2 , 3 , 5 , 7 , 10 , 11 , 13 – 17 . In addition, when metal soaps are formed, carboxyl moieties are bound to metal cations, in variable proportions depending on the age of the paint, the environmental conditions 18 , 19 , and the nature of the pigment 20 , leading to the formation of free metal soaps and ionomer-like networks 21 – 23 .…”

A Critical Review on the Analysis of Metal Soaps in Oil Paintings

“…[ 34 , 50 , 69 ] In addition, micro‐ATR‐FTIR imaging has been adopted in to detect and localise metal soaps, thanks to high chemical specificity and spatial resolution (down to ∼ 3–4 μm in the fingerprint region). [ 13 , 26 , 34 , 71 , 101 , 102 , 103 ] One of the first applications on lead soaps is by Spring and co‐workers at the National Gallery in London. [103] Thanks to the introduction of FTIR‐imaging, one of the main analytical limits (i. e. the bulk analysis of samples) was overcome since it can identify and localize phases in a region of interest in a non‐destructive way.…”

Complementary mapping techniques to characterize the wood finish of musical instruments

“…In addition, numerous spectra in the area of the sizing and the uppermost layers (D and E) bear the signals of the inorganic fraction (see Fig. SI5 see Online Resource): when the band at 1390 cm −1 , with a shoulder peak around 1360-1350 cm −1 is recognizable, it is ascribable to the lead white pigment [64,65], and when the stretching vibration of the carbonate group (ν(CO 3 −2 )) falling at 1420 cm −1 and the characteristic sharp peak at 870 cm −1 are present, they are related to the calcium carbonate (CaCO 3 ) [66]. The presence of overlapped peaks in the region between 1100-990 cm −1 related to the Si-O stretching vibrational mode also suggests the presence of silicates [62].…”