New methods to characterise and to consolidate the polychrome qi-lacquer of the terracotta army

Abstract

Describes the experimental use of electron beam radiation to polymerize 2-hydroxyethyl-methacrylate (HEMA) and consolidate qi-lacquer layers on fragments from the Qin terracotta army. The terracotta army, entombed 2,200 years ago with emperor Qin Shihuangdi in Lintong, China, has paint layers bound to the surface with an intermediate layer of qi-lacquer. When relative humidity (RH) drops below 84% following excavation from the wet clay site, this layer delaminates, presenting a particularly difficult problem of consolidation. In this investigation, methacrylate monomers were chosen for their permanence and optical qualities. Surfaces were impregnated by applying cotton wool compresses containing HEMA. Doses of electron beam radiation with an electron energy of 1.0 MeV were administered in three steps of 20 kGy using an ELV-2, INP Nowosibirsk electron accelerator. The radiation is also able to destroy microorganisms and mold. Evaluation using laser desorption mass spectroscopy (LD-MS) detected the formation of 4-6 monomer units of HEMA polymers. The three fragments treated demonstrated good consolidation and no shiny surface. Laser video holography was used to determine whether drastic changes in humidity would affect the consolidated surface, and infrared spectroscopy was used to detect the degree of polymerization. No damage occurred after four humidity cycles of 35-83% RH. This technique is a promising application for treating the terracotta army and other objects presenting similar problems. The treatments' advantages are listed.