New Methods to Characterise and to Consolidate the Polychrome Qi-lacquer of the Terracotta Army

Abstract

A method for the conservation of aged qi-lacquer layers on silicate material such as terracotta is described. Detachment of the layers would result in the loss of the paint layer. The terracotta army of the Chinese emperor Qin Shihuangdi in Lintong / China is a prominent example of this problem because the paint layer is bound to the surface by an intermediate qi-lacquer layer. This layer has aged 2200 years, buried in wet clay and will detach from the surface if relative humidity drops below 84 % after the excavation. Mcthacrylic monomers were of special interest as solidifying materials because of their long lifetime and their excellent transparency. Lacquer samples were treated with watersolublc 2-hydroxyethyl-methacrylatc (HEMA) which was polymerised by electron-beam radiation with an electron energy of 1.0 McV In the experiments the dose was administered in three steps of 20 kGy. Micro-organisms and mould are destroyed by the electron-beam radiation. Infra-redand mass spectroscopical evaluation shows no damaging effect of electron-beam radiation (300 kGy) on the qi-lacquer. Original qi-lacquer and lacquer consolidated with the method described above were characterised by laser desorption mass spectroscopy (LD-MS). The formation of HEMA polymers with 4-6 monomer units within the lacquer was proved by laser desorption MS. Infrared spectroscopy reveals that the degree of polymerisation is proportional to the applied dose. Three original polychrome fragments were successfully treated by electron beam curing. The qi-lacquer is bound to the terracotta, the fragments can be dried, a natural look (not shiny) of the polychrome surface is obtained. Laser video holography was employed to investigate if drastic changes in humidity will affect the consolidated polychrome layer. No damage could be detected after four humidity cycles (35-83 % r. h.). The long term stability will have to be evaluated. The application of electron-beam polymerisation seems to be a promising method for the conservation of the terracotta army of Qin Shihuangdi and other works of art.