Alteration of Asian lacquer: in-depth insight using a physico-chemical multiscale approach.

Oriental lacquer has been used in Asian countries for thousands of years as a durable and aesthetic coating material for its adhesive, consolidating, protective and decorative properties. Although these objects are made from an unusual material in Occident, Western museum collections host many lacquerwares. Curators, restorers and scientists are daily confronted with questions of their conservation and their alteration. The characterization of their conservation state is usually assessed through visual observations. However deterioration often starts at the microscopic level and cannot be detected by a simple visual inspection. Often, ageing and deterioration of artworks are connected to physical, mechanical and chemical transformations. Thus new insight into alteration of lacquer involves the monitoring of macro-, microscopic and molecular modifications, and this can be assessed from physico-chemical measurements. Non-invasive (microtopography and Scanning Electron Microscopy - SEM) and micro-invasive (infrared micro-spectroscopy using a synchrotron source - SR-µFTIR) investigations were performed to study the degradation processes of lacquers and evaluate their level of alteration. In particular, spectral decomposition and fitting procedure were performed in the 1820-1520 cm(-1) region to follow the shift of the C=O and C=C band positions during lacquer ageing. The present work proves the potential of this physico-chemical approach in conservation studies of lacquers and in the quantification of the state of alteration. It evidences chemical phenomena of alteration such as oxidation and decomposition of a lacquer polymeric network. It also demonstrates for the first time the degradation front of artificially aged lacquer and the chemical imaging of a more than 2000 years old archaeological lacquer by using SR-µFTIR.

Comparison of cryogenic and differential flow (forward and reverse fill/flush) modulators and applications to the analysis of heavy petroleum cuts by high-temperature comprehensive gas chromatography.

The development of new efficient conversion processes to transform heavy petroleum fractions into valuable products, such as diesel, requires improved chemical knowledge of the latter. High-temperature comprehensive gas chromatography (HT-GC × GC) has proven to be a powerful technique for characterizing such complex samples. This paper reports on an evaluation of the performances of four different differential flow modulators, including two original ones that have not been previously described in the literature, in terms of dispersion, peak intensity, peak capacity and overloading. These modulators, all of which are based on Agilent capillary flow technology (CFT), are forward fill/flush (FFF) differential flow modulators with an integrated collection channel or an adjustable channel (new) and reverse fill/flush (RFF) differential flow modulators with an integrated collection channel (new) or an adjustable channel. First, the optimization of the collection channel dimensions is described. Second, an RFF and an FFF differential flow modulator possessing the same collection channel were compared. The reverse differential flow modulation significantly reduced band broadening compared to forward differential flow modulation, and the peak intensity doubled for every modulated peak when an RFF differential flow modulator was used. Then, an RFF differential flow modulator and CO2 dual-jet modulator were compared. Whereas the percentages of separation space used were similar (61% with the HT-GC × GC method using a cryogenic modulator and 59% with the method using an RFF differential flow modulator), the peak capacities were at least three times more important with differential flow modulation due to the greater length of the column used in the second dimension. The results demonstrate that the RFF differential flow modulator is an excellent tool for studying heavy petroleum cuts. It demonstrates the best performances and it is the most versatile modulator. In its two-plate version, it gives more flexibility regarding the set of columns, the flow rates and the modulation periods that can be used compared with the others.

Molecular criteria for discriminating museum Asian lacquerware from different vegetal origins by pyrolysis gas chromatography/mass spectrometry.

This paper focuses on the identification of several chemical markers of vegetal species of Oriental lacquers with the aim at providing a methodology consistent with sampling restrictions necessarily applied in the field of cultural heritage. The method proposed is based on rapid and easy single step thermally assisted hydrolysis-methylation (THM) pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) analysis that can be carried out with a minimum amount of matter (typically 10 µg for a sample collected on a museum or an archaeological object). The main contribution of this study is to provide multiple molecular criteria for discriminating the three Asian species used for making lacquers, namely Rhus verniciflua Stokes, Rhus succedanea and Melanorrhoea usitata. Because these trees grow in specific areas, identifying the species involved in ancient lacquer coatings also provides geobotanical data and fruitful information on the exchange networks and trading routes developed by ancient societies. With this purpose, a systematic study of all pyrolysis products of lacquer coatings was carried out on modern dried lacquer films from authentified provenance. It was demonstrated that the whole pyrolysis products play a significant role in identifying the vegetal species. The chemotaxonomic value of homologous series of alkanes, alkenes and benzene derivatives, rarely explored until now, was assessed. It was shown that the combination of data related to five distinct groups of pyrolytic markers (composition and/or distribution of alkanes, alkenes and benzene, alkenyl-, alkylcatechol and phenol derivatives) provided new strong criteria to establish vegetal origin and provenance of Asian artworks, even though they have been largely altered over time. Case studies of archaeological Chinese lacquered artefacts and Japanese Buddhistic altar were thereafter successfully investigated to address informative potential and efficiency of these criteria on ancient and degraded lacquer coatings.