Interactions between inorganic pigments and proteinaceous binders in reference paint reconstructions

Abstract

The degradation of the proteinaceous binders, ovalbumin (OVA) and casein, and their interactions with azurite (Cu(3)(CO(3))(2)(OH)(2)), calcium carbonate (CaCO(3)), hematite (Fe(2)O(3)) and red lead (Pb(3)O(4)) pigments were studied. A multi-analytical approach based on Thermogravimetric Analysis (TG), Differential Scanning Calorimetry (DSC), Fourier Transform Infrared Spectroscopy (FTIR) and Size Exclusion Chromatography (SEC) was used. The research was carried out on a set of paint reconstructions, which were analysed before and after artificial light ageing. We highlighted that in most cases the inorganic pigments interact with both proteins by decreasing their thermal stability and their intermolecular β-sheet content, and that ageing induces aggregation. We hypothesized that pigments intercalate between protein molecules, producing a partial disruption to the protein-protein intermolecular interaction. In the case of casein, these phenomena continued during ageing. In fact, we observed a complete disappearance of intermolecular β-sheets and an increase in intramolecular β-sheets and random coil during ageing. This result is in agreement with the structural properties of casein, whose aggregation is known to be induced by hydrophobic interactions. On the other hand, in aged OVA paint replicas, we observed the formation of new intermolecular β-sheets and an increase in thermostability. In addition FTIR showed oxidation of the side chains of the aged OVA/hematite sample and aged casein pigment samples, and SEC highlighted hydrolysis phenomena in aged carbonate, azurite and red lead/OVA complexes and in aged casein/calcium carbonate and casein/azurite samples.

On the stability of mediaeval inorganic pigments: a literature review of the effect of climate, material selection, biological activity, analysis and conservation treatments

“…On the other hand, it is reported that medium-rich proteinaceous red lead paints are stable to light, the binder protecting the pigment [239]. Moreover, the pigmented layer has different properties than an unpigmented one [51]. It is soluble in diluted acids (nitric, chloridric, oxalic, acetic), with formation of the corresponding salts, and when heated it transforms to litharge [1,240].…”

“…In oil layers, Fe-ions promote photo-oxidative reactions, while Mn-ions act as a siccative [12]. In proteinaceous binder, interactions are reported between pigment and medium [51]. In rabbit skin glue, haematite shows a high degree of interaction with the binder, making the paint layer more stable than for other pigments [65].…”

“…Cucarboxylates are rarely formed, as leadwhite, often mixed with azurite, is more reactive; moreover, as azurite is preferred in a tempera medium, carboxylic acids are less present [56]. In proteinaceous binder, other interactions are reported [51]. Verdigris xCu(CH 3 COO 2 )·yCu(OH) 2 ·zH 2 O might be formed when high relative humidity interacts with an azurite containing paint layer, degrading the oil binder [64].…”

“…Calcium carbonate pigments are not suitable for use in oil medium, as they don't have hiding power [45], but they are used in mixtures with other white pigments to adjust the shade and hue, as well as in wall paintings [7]. In proteinaceous binder, interactions are reported between pigment and medium [51].…”

Development of analytical methods for the characterization of tempera paintings at micro- and nano-scale and their deterioration and biodeterioration processes.

“…Additionally, hydrolysis of triglycerides, which results in the release of long and short chain fatty acids and diacids as well as formation of cross-linked aggregates including cobalt-lecithin complexes has been reported (Mallégol et al, 2000). More recently, Duce et al have considered the ageing behavior of casein and ovoalbumin proteins in presence of cinnabar calcite, azurite, hematite and red lead (Duce et al, 2013) identifying alteration processes among which are induced aggregation of proteins by covalent cross-linking, oxidation of amino acids side chain and hydrolysis of the polypeptide chain in both pure and pigmented protein samples.…”