The degradation of Burmese lacquer (thitsi) as observed in samples from two cultural artefacts

Two cultural artefacts produced in Southeast Asia were analysed by Py-GC–MS with in situ silylation using HMDS. The page of a Burmese book (Kammavaca manuscript) from the early 20th century and a Cambodian wooden gilded sculpture representing a Buddha from the 16th century were investigated for diagnostic purposes. Burmese lacquer (thitsi) and a drying oil were identified. Benzoic acid and alkylphenylketones were detected, indicating that oxidation at the benzylic position of thitsi monomers occurred. Alkylphenylketones resulted from the pyrolysis of the polymeric network, and we believe they were the result of the oxidising environment generated by the drying oil during the curing. In the case of the Cambodian Buddha, the detection of alkylphenylketones was crucial for identifying the lacquer, as the marker pyrolysis products of thitsi were present at the trace level. Alkylphenylcarboxylic acids, alkyl-oxo-phenylcarboxylic acids and alkylphenols with a carboxylic group on the side chain were also detected. These compounds were identified as trimethylsilyl derivatives on the basis of their mass spectra interpretation, and were present as free acids or esters. Although their molecular structure might be ascribed to the oxidation of the original components of thitsi, the length of the alkyl chains did not make the association straightforward. Therefore, we propose that the oxidative cleavage of the aromatic ring may be responsible for the formation of these compounds.