Characterization of Natural Resin Films and Identification of Ancient Coating

Typical Japanese natural resin films, i.e., oriental lacquer films, cashew resin films, and drying oil polymers were characterized, and also an ancient coating was identified using two-stage pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). Urushiol, laccol, and thitsiol components, which are the main constituents of the respective oriental lacquers, were detected by the pyrolysis at 400°C. Additionally, the pyrolysis products, which reflect the structure of the urushiol, laccol, and thitsiol polymers, were detected by pyrolysis at 500°C after pyrolysis at 400°C. All the results have shown a significant progress occurring in the side chain-side chain C-C coupling and aromatic nucleus-side chain C-O coupling of urushiol, laccol, and thitsiol concurrently with the formation of the respective lacquer films. The cashew resin film was found to be terminated with saturated cardols and cardanols by the pyrolysis at 400°C. Furthermore, pyrolysis at 500°C revealed that the autoxidative side chain-side chain C-C coupling accelerated the cross-linking of trienyl- and dienylcardanols during the polymerization. The C4-C18 free fatty acids were detected by the pyrolysis of the drying oil polymers. These pyrolysis products are significant markers for a drying oil polymer. The drying oil polymer is terminated with saturated fatty acids, especially with hexadecanoic acid and octadecanoic acid. Based on these results, the ancient coating sample obtained from the surface of a wooden dish excavated at Tobarikyu, Kyoto prefecture, Japan was identified as Rhus vernicifera lacquer combined with an drying oil, presumably added as the solvent. Py-GC/MS was proved to be effective for the characterization of natural resin films and identification of organic coatings of historic artifacts.