Oxidation and Cross-Linking in the Curing of Air-Drying Artists’ Oil Paints

Abstract

In this study, the chemistry of air-drying artist’s oil paint curing and aging up to 24 months was studied. The objective is to improve our molecular understating of the processes that lead to the conversion of the fluid binder into a dry film and how this evolves with time, which is at the base of a better comprehension of degradation phenomena of oil paintings and relevant to the artists’ paint manufacturing industry. To this aim, a methodological approach based on thermogravimetric (TG) analysis, differential scanning calorimetry (DSC), gas chromatography–mass spectrometry (GC–MS), and analytical pyrolysis coupled with gas chromatography and mass spectrometry (Py–GC–MS) was implemented. Model paintings based on linseed oil and safflower oil (a drying and a semidrying oil, respectively) mixed with two historically relevant pigmentslead white (a through drier) and synthetic ultramarine blue (a pigment often encountered in degraded painting layers)were investigated. The oil curing under accelerated conditions (80 °C under air flow) was followed by isothermal TG analysis. The oxygen uptake profiles were fit by a semiempiric equation that allowed to study the kinetics of the oil oxidation and estimate oxidative degradation. The DSC signal due to hydroperoxide decomposition and radical recombination was used to monitor the radical activity over time and to evaluate the stability of peroxides formed in the paint layers. GC–MS was performed at 7 and 24 months of natural aging to investigate the noncovalently cross-linked fractions and Py–GC–MS to characterize the whole organic fraction of the model paintings, including the cross-linked network. We show that the oil–pigment combination may have a strong influence on the relative degree of oxidation of the films formed with respect to its degree of cross-linking, which may be correlated with the literature on the stability of painting layers. Undocumented pathways of oxidation are also highlighted.

Oil crystallization properties as an index for monitoring early stage curing of oil-based paints: DSC analysis on linseed oil systems

“…On the other hand, the slower kinetics observed in the case of UBLO (Fig. 3) is also in line with the literature that reports that the drying in the presence of ultramarine blue is slower than with lead white [1,19]. Ultramarine blue stabilizes the peroxides [19] slowing down the curing process and favouring alternative reaction pathways competing with the formation of a stable crosslinked network [19].…”

“…The monitoring of the crystallization properties of linseed oil was performed by running DSC cooling/heating cycles at different regular time intervals on samples which were left to age at indoor conditions. The results are here compared with those previously obtained by us on the same painting systems monitoring the DSC peroxides decomposition/recombination peak [19]. Through the investigation of the paint curing under limiting oxygen conditions, further details about the action of the pigments on linseed oil crystallization properties and hence on the whole curing process were evidenced.…”

“…Considering that lipolysis, autoxidation and photooxidation influence the DSC profiles [30], while the presence of pigments did not significantly affect most of the two melting profiles, we may argue that autoxidation chain reactions, that are the basis of the polymer network formation, have not occurred yet to a significant extent in the case of fresh paints. Indeed, considering the different chemistry of lead white and ultramarine blue on linseed oil curing process, we would expect differences on the kinetics of the formation of the polymer network [19] (see below for more details), and in turn differences in LWLO and UBLO overall curves. Instead, the curves shown in Fig.…”

“…an agent that catalyses the polymerization mechanism [31]. It decreases the stability of the peroxides [19] and induces a quick conversion of ROOH into RO• + •OH, which generally is the rate-limiting step in the propagation steps [39]. Consequently, a rapid and full consumption of the fatty acids' double bonds takes place, promoting a better and uniform drying throughout the entire film thickness as well as the formation of a well-established polymeric network [19,33].…”

“…Furthermore, the peak of peroxides decomposition/recombination is observable after a certain Silvia Pizzimenti and Francesca Saitta are contributed equally to the work. time (one or more days of drying) depending on the pigment included into the paint and thus on the mechanism involved in the formation of the polymer network [19].…”