Surface appearance and morphology changes of Asian lacquer due to artificial aging: Impacts of traditional additives

Abstract

Asian lacquer is one of the earliest natural polymers used in human history. Objects made with decorative Asian lacquer are among the most exquisite collections in museums. This is the first systematic study on the influence of historically used additives on the appearance and morphology changes of unaged and aged lacquer coatings. Macro and microanalytical methods were optimized to investigate the surface changes of lacquer caused by artificial aging. These included visual observation, visible photography, optical measurements (gloss, haze, blanching, macro-texture and color) and microscopy. Results showed that additives altered the transparency, color, and surface roughness and waviness of unaged lacquer surfaces. Aging caused changes in surface morphology, particularly an increase in roughness due to cracking and pitting, and waviness. The transparency of transparent coatings also increased with aging. The widest cracks and earliest formation of cracks with aging were observed for thitsi mixed with wood oil. High surface roughness was observed for aged lacquers treated with iron and for vermillion formulations. A significant reduction in gloss and an increase in blanching was observed for aged iron-treated lacquers due to extensive pitting and cracking. Formulations with vermillion exhibited the largest surface pits, which led to the largest reduction in gloss and the most blanching. Aging caused black spots and darkening of the vermillion pigment. Aged formulations with carbon black exhibited large pits on the surfaces. Analyses of the cracking showed that, to a large extent, the width, length and number of cracks were influenced by components in the formulations. The phenomena of crack closure and displacement were identified for the first time in lacquered objects. The causes of blanching are discussed and the extent of blanching was tentatively quantitatively expressed by gloss and haze data obtained by a goniophotometer. In general, changes in appearance and morphology occurred from the beginning of the light aging cycle, except for cracking. Preventive conservation strategies during exhibition and storage should be tailored according to the formulation and preservation conditions of specific lacquer coatings. This study contributes to the knowledge and conservation of Asian lacquered surfaces. The analytical protocols may also be of value for studying and documenting other coating formulations, and the surfaces of works of art.