Infrared chemical mapping of degradation products in cross-sections from paintings and painted objects

Abstract

Paintings and painted objects are quite susceptible to degradation, as paint layers are usually composed of complex mixtures of materials that can participate in chemical degradation processes. The identification of the constituent materials in paint (including binders, pigments, and fillers) and the degradation products within paint layers is of particular importance to ensuring the conservation of paintings, by providing important information both about their material history as well as their state of conservation. Metal fatty acid salts (metal soaps) are degradation products that can form in situ from interactions between inorganic pigments and free fatty acids in oil-based binding media, and can cause significant condition issues in paintings. Fourier transform infrared (FTIR) spectroscopy is one of the leading analytical techniques for the study of metal soaps. In this article, the materials analysis of several cross-sections from paintings and painted objects from works in Canadian collections is presented. Recent results on the use of external reflection FTIR (R-FTIR) spectroscopy to identify and map the distribution of paint components and metal soap degradation products is presented. In particular, zinc, lead, calcium, and copper fatty acid salts were all readily identified in paint cross-sections by R-FTIR spectroscopy, along with several pigments and the oil binding medium. The results shown here are among the first detailed examinations of these metal soaps in paint cross-sections using R-FTIR spectroscopy. The use of highly polished samples in which specular reflection is dominant allowed for spectral transformations to be applied to generate transmission/absorption-like spectra which facilitated identification of these species. The distribution of these species across the cross-sections was mapped by integrating characteristic absorption features in the R-FTIR spectra. Attenuated total internal reflection (ATR) FTIR spectroscopy was also performed on several samples, which provided additional compositional details at the interface of paint layers and degradation products.

The Identification of Multiple Crystalline Zinc Soap Structures Using Infrared Spectroscopy

“…The source of the zinc soaps responsible for the cracking and lifting are the lowest paint layers of the composition. 9,13…”

“…While the νa COO vibration shows a single band around 1538 cm −1 for zinc soaps of long-chain saturated fatty acids, the band has been described as a doublet for zinc soaps with unsaturated chains, 9,10 suggesting that there are at least two different coordination geometries possible. In oil paint samples, both the single and the double band types are regularly detected, 9,13,23 but a clear interpretation of these spectral differences in terms of crystal structure and fatty acid type has so far remained challenging.…”

Analytical study of Buddha sculptures in Jingyin temple of Taiyuan, China

“…According to the in-situ observation, at least two layers of paper were found at the three . Multi-layers paintings were also investigated in the paintings [17] and wooden statues [18]. The result not only helped us to deepen the knowledge of the historical and cultural value but also helped the restorers to determine whether the overlapping layers need to be removed.…”

Comprehensive research of the painted statues of Tutang Buddha and two attendants Buddha in Jingyin Temple, Taiyuan, China

“…According to the in-situ observation, at least two layers of paper were found at the three statues with three or four layers of paper in some parts. Meanwhile, the inscriptions proved the results that the statues were restored repeatedly in A.D.1541 (the reign of the Jiajing Emperor in the Ming dynasty) and A.D.1739 (the reign of the Qianlong Emperor in the Qing dynasty).Multi-layers paintings were also investigated in the paintings [17] and wooden statues [18] . The result not only helps us to deepen the knowledge of the historical and cultural value but also supports the restorers to determine whether the over layers need to be removed.…”