Stuck like glue: Wood tar as a medieval stone adhesive

Abstract

This study outlines a medieval stone repair technique involving wood tar. In the process of condition assessment, an unusual adhesive was identified used for repairs and indents on Stavanger Cathedral, a medieval Norwegian soapstone structure. Based on appearance and context, an initial hypothesis was formulated that tar may have been used as a historic repair method. This hypothesis was subsequently tested and confirmed through analysis. Samples of the adhesive were investigated using light microscopy, scanning electron microscopy with energy-dispersive spectroscopy, Fourier-transform infrared spectroscopy and gas chromatography–mass spectrometry. The material was radiocarbon dated and the date found to coincide with the reconstruction of the choir following a fire in the latter quarter of the 13th century. Several hundred findings of tar repairs have so far been identified on the cathedral, indicating comprehensive use of tar as a medieval adhesive for soapstone. These findings have implications for the development of an alternative conservation adhesive based on traditional materials.