No abstract

In this paper, we address the behaviour of lacquer coatings similar to that found on the Mazarin Chest, an important Japanese lacquerware artefact currently held by the Victoria and Albert Museum (V&A) in London. The response of Japanese lacquer (urushi) to changes in environmental conditions was investigated by examining the deflection of a glass substrate coated with a thin film of urushi subjected to changes of humidity. This deflection, measured using phase-shifting interferometry, was then used to determine the two in-plane hygral stress components. Results were compared for two sample conditioning regimes-subjected to intense UV ageing and no ageing-each at a range of relative humidity (RH) steps. The changes in humidity were found to cause rapid stress changes in the lacquers, which then relax over much longer time scales. A simple one-dimensional model of the moisture transport and the stress development is shown to be effective in describing the response of the material to changes in environmental RH.

A hygromechanical model has been developed to simulate the in-service behaviour of the natural lacquer urushi, using a phenomenological description of viscoelasticity. The material and mechanical properties were determined as a function of the relative humidity (RH), and the relationship between RH and moisture content was determined. These properties served as inputs to a finite-element-based model that was then tested against experimental measurements of the depth-averaged stresses in a thin layer of urushi deposited on a substrate and exposed to changes in the environmental conditions. Good agreement was seen between the predicted and measured behaviour. The validated model was used to investigate the spatial and temperature variation of stress in urushi films subjected to cyclic environmental conditioning.