Characterisation of metal carboxylates by Raman and infrared spectroscopy in works of art

Abstract

This work introduces the complementary use of μ‐Raman and μ‐Fourier transform infrared (IR) spectroscopy for the detection of specific carbon chains and cations for the identification of metal carboxylates within oil paint microsamples. Metal carboxylates (metal soaps) form naturally when free fatty acids react with metal cations and may also be found as additives or degradation products. Twenty‐two metal carboxylates were synthesised, and their spectra assembled in a reference database. Metal salts of cations commonly present in oil paintings were used, including lead, zinc, calcium, cadmium, copper and manganese. The fatty acids selected were the saturated acids palmitic (C16 : 0) and stearic (C18 : 0) and the polyunsaturated oleic acid (C18 : 1). Azelaic acid (C9 diacid), a product resulting from autoxidation of polyunsaturated acids, was also included. Metal carboxylates were characterised by Raman and IR spectroscopy, and their structures were confirmed by X‐ray diffraction. Raman and IR spectroscopy proved to be complementary techniques for a full identification of the metal carboxylates in complex aged paint. Raman enables the differentiation of the carbon chain length in the C–C stretching region (1120–1040 cm−1), and IR distinguishes the metal cation in the COO− stretching absorption region (1650–1380 cm−1). Principal component analysis was applied to the spectra in order to facilitate a fast and accurate method to discriminate between the different metal carboxylates and to aide in their identification. Finally, spectra from case studies were successfully projected in the principal component analysis models built, enabling a higher confidence level for the identification of copper palmitate and copper azelate in two 19th‐century Portuguese oil paintings. Copyright © 2014 John Wiley & Sons, Ltd.

Disclosing Jackson Pollock’s palette in Alchemy (1947) by non-invasive spectroscopies

“…Variable amounts of metal oxalates are observed both on the greens and yellows as single band or doublet in the 1400-1200 cm −1 range. The white1 (ZnO/anatase), manganese violet and blue1 (copper phthalocyanine) paints are all characterized by a high content of zinc carboxylates (main bands at 1540 and 1400 cm −1 [32]) and small amount of zinc oxalates (signals at 1360 and 1316 cm −1 [33]) ascribable to saponification and oxidation processes involving zinc oxide and the oil-based medium (Fig. 8c).…”

Breaking down banners: analytical approaches to determining the materials of painted banners

“…The formation of the soaps is also confirmed by the reduced presence of the C=O ester and acid stretching at 1700-1750 cm −1 associated with the linseed oil components. Despite the presence of lead white detected on most of the samples from the banner the formation of lead soaps, on the samples analysed, was not detected as the doublets associated with lead soaps occur at lower frequencies 1540-1500 cm −1 [22] and these were not seen on the spectrum. However, this may be due to the use of macro FTIR and it may be that the use of micro FTIR may have found these soaps to be present.…”

Lead soaps formation and biodiversity in a XVIII Century wax seal coloured with minium

“…The position of the peaks, Fig. , at 1463, 1441, 1419, 1370, 1297, 1173, 1130, 1099, 1063, 923 and 891 cm −1 , are consistent with the formation of a metal soap mostly between lead and palmitic acid, that is, one of the major components of carnauba wax and are in very good agreement with the scientific literature (Robinet and Corbeil, ; Otero et al ., ; Sodo et al ., ).…”