Characterisation of naturally and artificially weathered pine tar coatings by visual assessment and gas chromatography-mass spectrometry

Abstract

Tarring experiments with pine tar from Scots pine ( Pinus sylvestris) obtained from a traditionally accomplished kiln production have been carried out, in order to investigate potentials of improvement concerning tarring of the preserved Norwegian medieval stave churches. Pine tar coated test panels of pine wood were exposed to three different natural climates in Norway and on a regular basis characterised by visual assessment and gas chromatography–mass spectrometry (GC–MS) during 30 months of exposure. Moreover, test panels that were tar coated in the same way, however, on variable substrata, were exposed in a weatherometer. Tar obtained from different stages in the kiln production varies significantly according to chemical composition as well as coating ability. The weather resistance of tar from an early stage in the production proved superior compared to tar from a final stage. Boiling or seething of the pine tar at temperatures below 200 °C prior to application increased weather resistance and durability of the coating, without accelerating the decarboxylation process in the tar sample. Even after exposure and weathering of the coated surfaces the initial characteristics of the liquid tar, in accordance with manufacturing temperature were detectable by GC–MS. Weatherometer experiments confirmed the results of the outdoor experiments and moreover showed that the quality of the substrata interacted with the tar coat and significantly affected the weather resistance. Despite different wood qualities of the weatherometer panels, the comparison of tar treatments, which were visually assessed on a macro level, was sufficiently evident to constitute a basis for renewed guidelines for tarring of stave churches.