Degradation of lime wood painting supports: Evaluation of changes in the structure of aged lime wood by different physico-chemical methods

In the present work, aged lime wood from painting supports, differing in terms of the provenance, conservation status, wood ages and of storage conditions (microclimate in churches or rooms) was analysed, applying analytical pyrolysis as the main method. Analytical pyrolysis combined with gas chromatography/mass spectrometry has shown a great potential as a fast, sensitive and good reproducible analytical tool to characterize the structure of a variety of natural polymers. The attraction of the application of analytical pyrolysis for the investigation of objects having the artistic value is connected with the use of minor amounts of samples for analysis. Carbohydrates and lignin compounds released under Py-GC/MS were analysed, and carbohydrate/lignin (C/L) and syringyl/guaiacyl (S/G) ratios were determined. On the basis of the results obtained, it can be concluded that, with ageing of lime wood the carbohydrates moiety decreases significantly as the result of degradation of hemicelluloses, namely, slow hydrolysis of O-acetyl groups of xylan with the release of acetic acid. The increase in the free radical concentration in aged wood in comparison with the reference sample, observed by ESR-spectroscopy, confirms the relative increase in the lignin content and the degradation of lignin–carbohydrate complexes. The S/G ratio did not show any preference in the degradation of S or G lignin of aged lime wood. The main trends in changes in parameters of the porous structure of the objects under investigation are in conformity with the alteration in the composition of wood complex. Summing up the data of Py-GC/MS and ESR-spectroscopy, it may be concluded that, for 180- and 270-year aged wood samples, some tendency for the alteration of the lignin matrix is observed, namely, an increase in condensity, accompanied by the development of polyconjugated systems.