Testing the hydrophobicity of wood tar adhesives and coatings using contact angle measurements

Abstract

Pine wood tar has long been used as a protective coating for wooden structures in the Nordic countries and has recently been identified as an adhesive in stone buildings. However, the conservation of structures historically reliant on wood tar is increasingly threatened by declining expert knowledge, reduced access to high-quality forest resources, and warmer, wetter climates that accelerate decay of both tar and wood. Positive progress is being made in pine tar research and conservation, including efforts to preserve the remaining expertise among craftspeople, yet the cost of regularly reapplying traditionally made pine tar can be prohibitive in many cases. This study presents a pilot investigation comparing the hydrophobicity of wood tar coatings using water sessile drop contact angle measurements. Traditional Finnish pine was compared to a spruce tar byproduct from industrial biochar production, both with and without powdered charcoal filler. Results show that spruce tar exhibits superior hydrophobicity, and the addition of charcoal significantly improves the hydrophobicity of both tars. These findings highlight the importance of preserving traditional material knowledge for the conservation of cultural heritage and for the development of sustainable, biobased materials. Further research on diverse tar types and substrate materials is needed to optimise both traditional and modern wood tar products.