A new understanding of the aging characteristics of Asian lacquer

Marianne Webb


Recent studies at the Getty Conservation Institute have demonstrated, that Asian lacquer coatings are not simply the processed tree sap of urushi, thitsi or laccol but contain many other ingredients from common linseed oil to the unusual tofu. This study looks at how these additives are affecting the physical behavior and the aging characteristic of lacquer objects. Eleven samples boards were made that represented five different formulas based on laccol and six based on urushi. The samples contained transparent lacquers, some with oil added and other with pigments as well. Similar ground layers were used for all the samples so the characteristics of the surface layers could be compared. The Atlas weatherometer with a water cooled xenon arc lamp was chosen for the artificial aging of the samples boards. A sodium borosilicate glass inner filter and soda lime glass with a CIRA coating outer filter were used to replicate the spectra of natural light through window glass. Following light aging the sample boards were exposed to four cycles of changing relative humidity consisting of 1 week at 22% RH followed by one week at 80%. After artificial aging, the physical characteristics were compared by four methods of assessment that conservators have previously used on Asian lacquer: measurement of gloss, measurement of surface pH, a comparison of micro cracking using a scanning electron microscope and observation of auto-fluorescence. In all methods of evaluating the surface the initial lacquer formulas made a difference in the results and patterns of behavior are beginning to emerge. Conservators have used low pH measurements to evaluate the severity of surface degradation; however, when the 11 different Asian lacquer formulas were compared the transparent lacquers consistently measured lower that any samples containing oil or pigments. Although they had the lowest level of pH they retained the highest gloss. Gloss varied according to formula with greatest loss occurring on the sample colored by iron oxide. Under ultraviolet light the transparent lacquers showed more intense auto-fluorescence that those with oils or pigments with laccol being brighter than urushi. Micro-cracking after exposure to RH cycling occurred on most samples, however, many different patterns occurred. Transparent urushi had the least micro cracking which is consistent with the gloss measurements. Asian lacquer can longer be considered a single type of coating. It must be looked at more like a painting where variations in the medium, additives and pigments differ according to time, place and the individual artist.

2016 | Montreal | Volume 23