A versatile method to fingerprint and compare the oxidative behaviour of lipids beyond their oxidative stability

: In this work we propose the use of isothermal thermogravimetry to evaluate the oxidative stability of a lipid and to evaluate how the glyceride composition affects the entire oxidative process, to quantify the oxidation undertaken by the lipid, and numerically compare the oxidative behaviour of different lipids. The innovative aspect of the present method lies in the acquisition of a prolonged "oxygen uptake" curve (4000-10,000 min) of a lipid under oxygen and in the development of a semi-empirical fitting equation for the experimental data. This provides the induction period (oxidative stability), and allows to evaluate the rate of oxidation, the rate and the magnitude of oxidative degradation, the overall mass loss and the mass of oxygen taken by the lipid upon time. The proposed approach is used to characterize the oxidation of different edible oils with different degrees of unsaturation (linseed oil, sunflower oil, and olive oil) as well as chemically simpler compounds used in the literature to model the autoxidation of vegetable oils and lipids in general: triglycerides (glyceryl trilinolenate, glyceryl trilinoleate and glyceryl trioleate) and methyl esters (methyl linoleate and methyl linolenate). The approach proves very robust and very sensitive to changes in the sample composition

The chemistry of artists’ oil paints

Oil paintings from the 20th and 21st centuries present a range of conservation issues different from those of paintings realised in the previous centuries. These issues are believed to be correlated with the use of manufactured artists’ oil paints, which contained driers, fillers and stabilisers, modified oils, semi-drying oils and a range of pigments. The chemical nature of the changes occurring in the film former as a consequence of degradation is, in most cases, not fully understood at the molecular level. It has been hypothesised that some degradation phenomena taking place in oil paintings - especially those produced with modern manufactured artists’ oil paints - are related to the chemical nature of the oil cross-linked network, and in particular, to a low degree of polymerization and a high degree of oxidation. The polymeric as well as the highly complex multi-component nature of paint films do not allow a straightforward characterization of its molecular features. However, a useful approach to improve our understanding of the chemical structure of the paint film is the characterisation of the process leading to its formation: the curing. In this context, the principal aim of my Ph.D. project was to implement a methodological approach to monitor physico-chemical changes of oil paints upon curing and ageing. In particular, great effort was made to design a methodological approach able to characterise the effect of different oils, pigments and driers on the autoxidation kinetics, to monitor the evolution of oxidation products, to evaluate the magnitude of oxidative degradation and cross-linking phenomena, and to determine the structure of the cross-linked fractions

Unravelling the effect of carbon black in the autoxidation mechanism of polyunsaturated oils

Carbon black-based particles are widely employed as pigment and they are known to slow down the drying time of oil paints. In this work, the effect of amorphous carbon black on the mechanism and speed of autoxidation of a polyunsaturated oil was investigated. Moreover, the effect of the addition of aluminium stearate and zinc stearate was studied. These are two common additives used in the artists’ paint industry to facilitate pigment dispersion. The curing of the oil paints with and without the addition of the two stearates at 80 °C under airflow was followed by isothermal Thermogravimetry. The oxygen uptake profiles were fitted by a semi-empiric equation to comparatively study the kinetics of the oil oxidation and estimate oxidative degradation. Moreover, model paintings were left to cure at ambient conditions and Differential Scanning Calorimetry was then used to monitor their curing progress over time and to evaluate the stability of peroxides formed in the paint layers. Gas Chromatography–Mass Spectrometry was performed at 7 and 12 months of natural ageing of the model paintings, to investigate the non-covalently cross-linked fractions. Analytical Pyrolysis coupled with Gas Chromatography–Mass Spectrometry was used to characterise the whole organic fraction of the model paintings, including the cross-linked network. Amorphous carbon has an antioxidant effect and inhibits the radical chain propagation. The presence of aluminium and zinc stearates in the black paint affects the autoxidation process, by leading, in the first months, to a faster consumption of unsaturated moieties, and, accordingly, to accelerate and increase peroxides formation. After a few months though, the whole curing slows down, and active peroxides and radicals are still present even after 12 months

Disclosing the chemistry of oil curing by mass spectrometry using methyl linoleate as a model binder

The structure of the polymeric fraction in an oil painting is believed to be strongly connected to the stability of the paint layers over time, but its molecular the characterisation is extremely difficult given the complex composition of a vegetable oil-based polymer. In this study, we report the implementation of a methodological approach for the systematic mass spectrometric investigation of the molecular features of the products of oxidative degradation and cross-linking of oil paint layers upon curing. The approach is based on the use of methyl linoleate as a simplified model of an oil paint binder. Gas-chromatography coupled with mass spectrometry, solid phase microextraction gas chromatography-mass spectrometry, flow injection electrospray mass spectrometry and evolved gas analysis mass spectrometry, are used to analyse the evolution of compounds produced over seven months of natural ageing, from the volatile products to the macromolecular and cross-linked fractions. The aim is to improve our fundamental molecular understanding of the curing process of oil paints, and to investigate the balance between oxidative degradation and cross-linking when specific binder-pigment combinations are in place. Model paint layers were prepared using lead white and ultramarine blue as pigments. These two pigments are known to produce paint layers with different stability over time. The use of methyl linoleate as a model oil binder greatly simplifies the mass spectral features of the lipid paint fraction, enabling the detection of products of oxidation and cross-linking with a new high level of molecular detail. Data clearly show that that crucial differences between paints containing the two pigments establish with time, which are mostly related to the cross-linked fraction

DSC on ovalbumin-hematite “tempera” paints: the role of water and pigment on protein stability

The role of water and hematite (Fe2O3) on the stability of ovalbumin-based model paint layers was investigated by means of DSC and FTIR. The aim of this research is to improve our understanding of the stability of paint layers based on proteinaceous media, assessing the water content and the pigment presence effects. Pigments may play a fundamental role in determining the structure of proteins in paint layers, thus affecting the possible interactions among proteins and the external environment, including humidity. Previous studies revealed that hematite affects the secondary structure of OVA in paint layers, although no experimental evidence of hematite/ OVA covalent bonds have been reported in the literature. In this paper, we investigate the synergic effect of water and hematite on OVA structure and stability. DSC analyses coupled with FTIR measures on protein hydration revealed that below 30 % of humidity the amount of water strongly influences the protein structure and stability: the less the water content, the higher the protein stability. Furthermore, our results suggest that a water phase separation occurs in the presence of hematite for which, in water-limiting condition, the hematite’s hydration shell becomes almost negligible if compared to the bulk water available for the protein hydration because of the high protein-water affinity. Accordingly, the protein phase humidity is higher than the sample’s nominal value. Paints at the same overall humidity exhibit different protein hydration state following the pigment/binder ratio, and in turn different resistance to damages throughout ageing