Barrier effect to water vapour of early European painting materials on wood panels

This paper reports results of experimental measurements of diffusive water vapour flux through spruce wood specimens coated with 16 combinations of paint layers replicated according to materials and techniques used in the period from the late Middle Ages to the Renaissance period. Layers represented two base preparations (ground), four polychrome paint layers and presence or absence of varnish. The flux of water vapour was measured from sorption tests in unsteady-state conditions. The internal resistance to the transport of water in wood and the external resistance due to the coating layers was calculated with a numerical method. The results show that all painting layers reduce the flux and that there are significant differences among paint layer combinations. The calculated mass transfer coefficient is an important physical characteristic of the material and it allows improved models for the understanding and prediction of the influence of coating and painting layers on the hygroscopic and deformative behaviour of wooden objects subjected to changes in environmental climatic conditions.