The Identification of Multiple Crystalline Zinc Soap Structures Using Infrared Spectroscopy

Abstract

The formation of crystalline zinc soaps (zinc salts of fatty acids) in oil paint layers is a common sign of paint degradation. In this study, we have used infrared spectroscopy to systematically identify differences in structure and composition of crystalline zinc soap phases, and report data analysis methods for structure attribution in challenging oil paint samples. Supported by reported crystal structures, it was possible to distinguish two distinct types of zinc soap geometry: a highly symmetrical packing for long-chain saturated soaps (type B) and an alternating packing for zinc soaps with short, unsaturated, or dicarboxylic chains (type A). These two types of packing can be identified by a single or split asymmetric COO stretch vibration band. With this new information, we studied the structure and composition of zinc soaps formed in a zinc white model paint and in a cross-section from the painting Equations in Space by Lawren Harris. Using non-negative matrix factorization, band integration and band position maps, it was possible to clearly identify zinc azelate in the model paint and map its spatial distribution. The same methods showed that the paint cross-section contained both types of zinc soap structure within the same paint layer, with the less symmetrical structure appearing only at the interface with the ground layer. The results give valuable information on the internal chemistry of oil paint layers, and the demonstrated methods can find widespread application for in-depth analysis of infrared microscopy data.

Fatty Acids and Their Metal Salts: A Review of Their Infrared Spectra in Light of Their Presence in Cultural Heritage

“…The infrared spectra of FAMS have been studied to some extent [ 78 , 87 , 133 , 138 , 147 ]. In support of the current discussion, selected sodium (representative of late-historical cleaning agents and modern additives in routinely used materials), calcium (typically expected in interaction products of FA with the calcareous environment) and zinc soaps (a usual case of metal soaps in oil paintings) were prepared in the laboratory (see Materials and Techniques).…”

“…The coordination geometry plays a significant role in the band maxima [ 147 , 148 , 149 ]. Moreover, the separation between the antisymmetric and symmetric band, often called the Δ (‘delta’) value, depends on the coordination symmetry; the lower the symmetry, the higher the Δ .…”

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

“…Lead soaps have been detected in lead-based layers of both the samples, and they are characterized by the strong absorption at 1515 cm −1 (ν as (COO − )) and a shoulder band at 1538 cm −1 [36,44,49,50]. Nevertheless, further detailed discrimination of the specific lead carboxylates (palmitate, stearate, azelate) from the ATR-FTIR spectra is not feasible, as (1) the ν as (COO − ) band is not as sharp as shown in literature [49,51], probably due to a not so well-defined coordination geometry of the metal soaps (poorly crystalline metal soaps or metal soaps not fully crystallized); (2) the other marker bands of lead soaps (such as the progression of bands associated with the wagging and twisting of hydrocarbon long chains in the range between 1350 and 1150 cm −1 [16]) are of low absorbance and/ or overlapped with the absorptions of other compounds; (3) possible co-presence of different lead soaps (starates, azelates, oleates); and (4) the possible formation of a lead ionomer both before or after the oligomerization of the drying oil, which partially contribute to the ν as (COO − ) mode detected in the ATR-FTIR spectra [1]. In fact, lead soaps are always detected in mixtures with at least two other substances, typically oil of the binding medium (1738 cm −1 , 2929 cm −1 , and 2855 cm −1 ) and the lead white pigment (1045 cm −1 ).…”

Technical examination of Liu Kang’s Paris and Shanghai painting supports (1929–1937)

“…However, while proteins were confirmed in one examined ground sample, their identification in the remaining two samples was inconclusive due to overlapping peaks of zinc soap at 1540 cm −1 (Fig. 11b) [26][27][28]. Nevertheless, it is difficult to determine if the examined type of grounds is semi-absorbent emulsion of animal glue and oil binders or absorbent ground bound with animal glue as the oil may have originated from the artist's paint.…”