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

An investigation into the curing of urushi and tung oil films by thermoanalytical and mass spectrometric techniques

Urushi is the oldest and most precious lacquer used since antiquity in East Asia. For artistic purposes, in order to obtain suitable rheological properties, the lacquer is usually mixed with a vegetable oil. In this work we investigated the curing process of urushi/tung oil mixtures in order to highlight the chemical interactions at the molecular level between the two materials. A multi-analytical approach was adopted, based on thermogravimetry (TG), differential scanning calorimetry (DSC), gas chromatography-mass spectrometry (GC-MS), evolved gas analysis mass spectrometry (EGA-MS), analytical pyrolysis coupled with gas chromatography and mass spectrometry (Py-GC-MS) and high performance liquid chromatography-mass spectrometry (HPLC-MS). Fresh and aged mixtures were analysed and the results were compared with those obtained from the analysis of the individual materials. The data highlighted that different polymerization and oxidation mechanisms take place in oil/urushi mixtures compared to the pure materials. Py-GC-MS and GC-MS showed that the profile of aliphatic mono- and di-carboxylic acids was drastically different for the aged film of pure tung oil compared to the mixtures. The ratio between the relative content of azelaic and palmitic acids was much lower in the mixtures than in the pure oil, highlighting a lower level of oxidation. On the other hand, the relative content of short chain carboxylic acids, which are produced by pyrolysis of the cross-linked oil network, increased as the concentration of urushi in the mixtures increased, thus indicating an increasing level of reticulation. HPLC-MS showed a relatively higher amount of triglycerides with hydroxylated fatty acids – the intermediate oxidation product of polyunsaturated fatty acids - in the mixtures with respect to pure tung oil

The role of organic and inorganic indoor pollutants in museum environments in the degradation of dammar varnish

This paper investigates the effects of inorganic (NO2 and O3) and volatile organic acid (acetic acid) pollutants on the degradation of dammar varnish in museum environments. Model paint varnish samples based on dammar resin were investigated by Gas Chromatography Mass Spectrometry (GC-MS), Dynamic Mechanical Analysis (DMA) and Atomic Force Microscopy (AFM). Dammar is a natural triterpenoid resin, commonly used as a paint varnish. Samples were subjected to accelerated ageing by different levels of pollutants (NO2 and O3 and acetic acid) over a range of relative humidities (RH) and then analysed. The results revealed that as the dose of the pollutant was increased, so did the degree of oxidation and cross-linking of the resin. Most interestingly, it was shown for the first time that exposure to acetic acid vapour resulted in the production of an oxidised and cross-linked resin, comparable to the resin obtained under exposure to NO2 and O3. These conclusions were supported by the analyses of model varnishes exposed for about two years in selected museum environments, where the levels of pollutants had been previously measured. Exposures were performed both within and outside the selected microclimate frames for paintings. Results showed that varnishes placed within the microclimate frames were not always better preserved than those exposed outside the frames. For some sites, the results highlighted the protective effects of the frames from outdoor generated pollutants, such as NO2 and O3. For other sites, the results showed that the microclimate frames acted as traps for the volatile organic acids emitted by the wooden components of the mc-frames, which damaged the varnish

Scientific investigation into the water sensitivity of twentieth century oil paints

In order to develop a better understanding at the molecular level of water sensitivity in twentieth century oil paintings, water sensitive Winsor & Newton oil paint swatches and twentieth century oil paintings were characterised using gas chromatography–mass spectrometry (GC–MS) and direct injection electrospray ionisation mass spectrometry (ESI-MS), and the data were analysed using principal component analysis. Liquid chromatography coupled with tandem mass spectrometry based on quadrupole and time of flight mass detectors and electrospray interface (HPLC-ESI-Q-ToF) was also used to obtain a better insight into the molecular composition of a selection of samples. The study highlights a strong relationship between the molecular composition of the binding medium and the type of pigment present in the paint, which relates to water sensitivity. Consistently non-water sensitive lead white, titanium white, and zinc white paints [all containing zinc oxide] contained a relatively low proportion of extractable diacids, and a relatively high proportion of extractable short chain monoacids. These paints also contained a relatively low level of unsaturated and hydroxylated glycerides. Water sensitive iron oxide and ultramarine paints are associated with both a relatively high degree of oxidation and a high proportion of extractable diacids, as well as a relatively high content of unsaturated and hydroxylated glycerides. Water sensitive cadmium red, yellow and orange paints were generally not highly oxidised, but they also contained a relatively high content of unsaturated and hydroxylated glycerides. It is hypothesised that water sensitivity relates to a low degree of saponification and crosslinking and possibly, on the relative content of dicarboxylic acids

Interactions between inorganic pigments and rabbit skin glue in reference paint reconstructions

The thermal degradation of rabbit skin glue, a collagen-based proteinaceous material used as a paint binder in paintings, was investigated in this paper. Paint reconstructions of the glue on its own or mixed with azurite (Cu3(CO3)2(OH)2), calcium carbonate (CaCO3), hematite (Fe2O3nH2O) and red lead (Pb3O4) were analysed using a thermoanalytical approach. This method enabled us to investigate the interactions between the glue and pigments before and after artificial indoor light ageing. The study was carried out using differential scanning calorimetry, thermogravimetry and thermogravimetry/FTIR analysis already successfully employed to characterize the paint binders. The results highlighted that all the inorganic pigments interact with rabbit skin glue, thus decreasing the thermal stability of the binder. Light ageing further decreased the thermal stability of pigmented paint replicas, suggesting a moderate increase in the rate of the degradation

Fourier transform infrared spectroscopic study of rabbit glue/inorganic pigments mixtures in fresh and aged reference paint reconstructions

We studied the interactions of rabbit glue, a collagen-based proteinaceous binder, with azurite (Cu3(CO3)2(OH)2), calcium carbonate (CaCO3), hematite (Fe2O3·nH2O), red lead (Pb3O4) and cinnabar (HgS) by Fourier transform infrared spectroscopy (FT-IR). The research was carried out on a set of paint reconstructions, which were analysed before and after artificial light ageing. A deconvolution of the amide I FT-IR absorption peak was performed with a written-in-house LabVIEW program to study the secondary structure of the glue.The changes in the glue conformation highlighted that all the inorganic pigments interact with the proteinaceous binder. The conformational changes were correlated with a loss of stability of the collagen structure, especially after ageing, likely due to the interlayer coordination of metals salts and oxide with protein functional groups. These results were correlated with the lower thermal stability of the glue/pigment mixtures with respect to the pure glue, evidenced by thermogravimetry (TG) and differential scanning calorimetry (DSC) analyses performed in a previous step of this work

An insight into Gandharan Art: Materials and Techniques of Polychrome Decoration

Gandharan art developed in the Himalayan area in the early centuries CE. It has been investigated mostly from an iconographic point of view, missing, until very recently, a systematic technical investigation of materials and techniques. Recently our team began performing chemical analyses of the traces of the polychromy originally covering statues, reliefs and architectural decorations, to discover the ancient painting techniques and artistic technologies. This paper presents the results of the analytical investigation (optical microscopy, Raman spectroscopy and gas chromatography coupled with mass spectrometry) of pigments, ground layers and binders of a new group of samples taken from stucco architectural decorations (2nd–3rd/4th centuries CE). The samples were collected directly at an archaeological site in the Swat Valley, ensuring the exact knowledge of their stratigraphic provenance, as well as the absence of any restoration treatment applied prior sampling. The results are discussed in the wider context of Gandharan polychromy investigated so far by our team, as found in sculptures and architectural decorations preserved in museums (in Italy and France) and in archaeological excavations in Pakistan. The aim of this research is to shed light on the materials and techniques of this Buddhist ancient art from this region and on the influences exerted on it from Eastern and Western artistic traditions

Identification of proteinaceous binders in paintings: A targeted proteomic approach for cultural heritage

Abstract Identification of proteins in paintings and polychrome objects is a challenge, which requires the development of tailored analytical approaches. In the present study, a targeted proteomics approach was developed for discriminating among the three most common proteinaceous materials used as paint binders, i.e. milk, egg, and animal glue. In this study a specific database of peptides was created based on tandem MS analyses of tryptic digests of several paint samples collected from a variety of art objects of different ages and conservation conditions. Specific peptide markers of each protein were then selected and monitored by LC-MSMS in Multiple Reaction Monitoring (MRM) ion mode, together with their specific precursor ion-product ion transitions, as defined by their unique amino acid sequence. The developed method enabled a sensitive and reliable detection of the target peptides in a selection of case studies, leading to the unambiguous identification of the proteins used as paint binders. The method showed greatly increased sensitivity compared to currently available strategies

Analyses of the brown stain on the Parthenon Centaur head in Denmark

In 1688 two sculptural fragments, a head of bearded man and a head of an unbearded youth, arrived in Copenhagen, sent from Athens as a gift to King Christian 5. They were placed in the Royal Kunstkammer, their provenance given as the Temple of Artemis in Ephesos, one of the Seven Wonders of the World. Almost a hundred and fifty years later, in the early 1820’s they were noticed and studied by two scholars independently visiting the Kunstkammer. However, both concluded that the two heads belonged to one of the metopes decorating the south side of the Parthenon temple on the Acropolis in Athens, showing fighting between Greeks and the mythical Centaurs, part man and part horse. In the 1830’s another sculptural fragment, a horse’s hoof, obtained through the German archaeologist and state antiquary of Greece, Ludwig Ross, reached Copenhagen. It was forwarded by the Danish consul to Athens, C.T. Falbe, as a gift to King Christian 8. The inventory reads: ‘… was found on the Acropolis near the Parthenon temple and is supposed to belong to one the Centaurs on the metopes.’ The present paper focuses solely on the head of the Centaur. A brown stain was noticed on the Parthenon marbles as early as 1830 by the British Museum and has ever since eluded a deeper understanding of its genesis despite many investigations and attempts of analyses. A quite similar brown stain can be observed on the Centaur’s head in Copenhagen as well. The present study reports analyses by LA-ICP-MS, SEM–EDX, µXRD, GC–MS, and LC–MS-MS, as well as optical microscopy of five small samples sequestered in 1999 from the Centaur head curated by the National Museum of Denmark. Our analyses show that the brown stain consists of two consecutively added surficial layers of the calcium oxalate minerals whewellite and weddellite. Despite a thorough search using proteomics, we have found no viable organic precursor material for the oxalates. Our results do not solve the mystery of the formation of the brown stain, but they do further qualify the structure and characterization of the brown stain

Proteomic Characterization of Collagen-Based Animal Glues for Restoration

Animal glues are widely used in restoration as adhesives, binders, and consolidants for organic and inorganic materials. Their variable performances are intrinsically linked to the adhesive properties of collagen, which determine the chemical, physical, and mechanical properties of the glue. We have molecularly characterized the protein components of a range of homemade and commercial glues using mass spectrometry techniques. A shotgun proteomic analysis provided animal origin, even when blended, and allowed us to distinguish between hide and bone glue on the basis of the presence of collagen type III, which is abundant in connective skin/leather tissues and poorly synthetized in bones. Furthermore, chemical modifications, a consequence of the preparation protocols from the original animal tissue, were thoroughly evaluated. Deamidation, methionine oxidation, and backbone cleavage have been analyzed as major collagen modifications, demonstrating their variability among different glues and showing that, on average, bone glues are less deamidated than hide glues, but more fragmented, and mixed-collagen glues are overall less deamidated than pure glues. We believe that these data may be of general analytical interest in the characterization of collagen-based materials and may help restorers in the selection of the most appropriate materials to be used in conservation treatments