An examination of the response of East Asian lacquer films to changes in environmental conditions

2010-12-13T17:17:07Z (GMT) by Adel E. Elmahdy
This work addresses the material properties and behaviour of Japanese lacquers (urushi) coatings similar to those found on the Mazarin Chest, an important Japanese lacquerware artefact currently displayed in the Toshiba Gallery of Japanese Art at the Victoria and Albert Museum (V&A) in London. For almost four centuries, the Mazarin Chest has been displayed in uncontrolled environmental conditions and has been exposed to a range of lighting conditions. As a result, the Mazarin Chest has deteriorated and started to suffer from different kinds of damage. However, the optimal conservation approach to repairing this damage is not known and as a consequence, research on urushi and related materials is of great interest. For the first time, the effect of changing relative humidity (RH) on the response of urushi is investigated by examining the deflection of a glass substrate coated with a thin film of urushi. Phase shifting interferometry was employed to measure this deflection, from which the in-plane stress developed in the system due to the expansion mismatch in the bilayer was calculated. This was performed for aged (exposed to ultra-violet radiation) and non- aged urushi films. The film stress was observed over 66 hrs under 30%, 36% and 42% RH, while the stress response was observed over 7 hrs during exposure to 60%, 54% and 48% RH. During exposure to 30%, 36% and 42% RH and for both non-aged and aged urushi films, tensile in-plane stress was observed. It was seen to reach a peak value then relax over a longer time scale. The stresses develop in the non-aged urushi films were found to be higher than the stresses developed in the aged urushi films. The peak stress values for non-aged and aged urushi films were found to increase when increasing the difference between the storage RH (75%) and the target RH. When the non-aged and aged urushi films were subjected to a step change from low RH (30%, 36% and 42%) to high RH (60%, 54% and 48% RH), a compression inplane stress was observed and in this case, the aged urushi films exhibited higher stresses than those developed in non-aged urushi films. II To predict the behaviour of the urushi films, a simple 1D model of stress has been proposed. It shows reasonable agreement against the experimental stress measurements when the RH changes from 75% to 30%, 36% and 42% respectively, for both nonaged and aged urushi films. Furthermore, the model is extended to predict the stress response as a function of depth for non-aged urushi layers above an aged layer. For both the upper urushi layer (non-aged) and the lower urushi layer (aged), the model suggests that decreasing the layer depth results in an increase in the stress levels. In addition to the stress response measurements and in order to fully characterise the mechanics of urushi, mechanical properties were determined for non-aged and aged urushi films under different RH levels. These including the moisture diffusion coefficients, the elastic modulus, the tensile strength at break, the elongation at break, and the viscoelastic properties. The moisture diffusion coefficients were determined from the sorption and desorption curves when urushi films, non-aged and aged, subjected to step changes in RH. The results showed that the moisture diffusion coefficients were independent of the moisture content for non-aged and aged urushi. A strong dependence of the elastic modulus and the elongation at failure on the RH has been observed for non-aged and aged urushi films. At all strain rates used and as the RH increases, a significant reduction in tensile elastic modulus and an increase in the elongation at failure were observed. As a result of the UV ageing, no plastic deformation was observed in the stress-strain curves at any RH or tensile speeds. Creep recovery behaviours of non-aged and aged urushi under 30%, 50% and 75% RH and different stress levels were studied. The overall deformation levels at 75% RH for non-aged films were found to be higher than that at 30% RH and the aged films exhibited less stiffness. This trend reflects the strong influence of water, as a plasticizer, on urushi films. The variation of the elastic recovery with the applied stress and the RH for non-aged and aged urushi films were determined from the recovery behaviour. The results showed that the amount of elastic recovery at 30% RH for non-aged urushi was higher III than that for aged urushi films at all stress values. At 50% RH and 75% RH, the aged urushi films recovered to a greater degree than non-aged urushi films. The 4-element Burger‟s model was used for quantitative characterisation of the creep recovery curves to determine the viscoelastic properties for urushi films and their dependence on the RH and UV ageing. It was found that the RH has a strong effect on the viscoelastic properties for both non-aged and aged urushi films. A significant decrease in these parameters has been observed when the RH has been increased suggesting that the films tend to become soft as a result of the high mobility of the molecular chains under elevated RH. We have proposed a methodology to identify the effectiveness of the Japanese traditional consolidation processes that are used to consolidate the formation of microcracks on lacquerware surfaces. A rectangular aluminium substrate, covered with a thin film of aged urushi, was mechanically loaded using three point bending device and the in-plane urushi surface displacement profile was obtained through phase shifting digital speckle pattern interferometry (DSPI). A comparison of the displacement profiles before and after creating a ‟v‟ notch along the film showed an anomalous profile around the notch. While after consolidate the notch, no anomaly in the displacement profiles were observed. The potential for DSPI to play a key role in investigating consolidation mechanics has been demonstrated with the detection of the surface displacement around a notch before and after consolidation.