In this talk, Marianne Webb presented some findings from her ongoing research into the degradation of Asian lacquers.
She produced 11 different samples of lacquer formulated with Urushiol-based lacquer (sourced from Japan, China, and Korea), Laccol-based lacquers (from Vietnam or Taiwan), oil, and pigment. The sample boards were all prepared in the same manner, coated on both sides with raw lacquer, a layer with inert clay filler applied to both sides, and then ground smooth after drying in a wet box. The different lacquer formulations were then applied. The samples were artificially aged in a weather-ometer, exposing a new section of the sample each week for four weeks.
The degradation was evaluated using five factors: color, gloss, surface pH, autofluorescence, and microcracking.
Color – Fading was not a reliable assessment of degradation of black lacquer, but worked well for red lacquers. Microfadometer tests have placed red lacquers between Blue Wool 2 & 3, indicating that lacquer has similar light sensitivity to paper or textiles.
Gloss – Each sample had a different original gloss, with laccol on its own having less gloss than urushi, but becoming very glossy with the addition of oil. Transparent urushi retained its gloss well, but formulations with added oil lost a significant amount of gloss after aging.
Autofluorescence – chemical changes in lacquer seemed immediate once exposed to light (after 12 hours, the differences were obvious) but there is a maximum point at 1 week, then the autofluorescence decreases.
Surface pH – The pH of lacquer doesn’t necessarily drop as it ages. Transparent lacquer seems to have the lowest pH after aging because the light can penetrate it and therefore it gets more light damage. There was not direct correlation between the amount of oil and pH after aging, but they generally had higher pH. After approximately 1 week the pH reaches a plateau and then after 3-4 weeks it goes back up. Clearly, lacquer’s pH does not have a linear relationship to aging. Disclaimer: All pH tests damage the surface of lacquer since they require water to solubilize degradation products, leaving a void.
Microcracking – All of the samples showed changes after exposure to light. The patterns were distinct and all different.
The level of detail in the investigations and results of the study was incredibly impressive. The study emphasized the number of factors that one should consider when approaching each individual piece of lacquer. The author also indicated a desire to investigate formulations with Thitsi lacquer (from Burma and Thailand) and the sudden fogging phenomenon sometimes observed when lacquer is exposed to heat and moisture in the future.