The terracotta army from the mausoleum of the first Chinese Emperor, Qin Shihuangdi in Lintong, contains over 1500 soldiers, life-size statues of animals, full-size chariots, and 7-8000 individual statues so far in all. When they were uncovered, starting in 1974, the statues were initially polychrome, but they lose their colourful coats irreversibly soon after excavation from the water-saturated soil in which they had lain for over 2200 years. Because none of the materials and techniques used in conservation could be used successfully, new methods had to be developed.

Heinz Langhals and Daniela Bathelt of the Department of Chemistry of the University of Munich in Germany, have recently described how the consolidation of the polychromy can be achieved through polymerisation of a specially adapted combination of monomers (Angew. Chem. 2003, 42, 5676-5681).

The polychromy is made up of a layer structure consisting of a base and a pigment layer. The base is essentially qi-laquer (east Asian lacquer) which is formulated from the sap of the lacquer tree which contains urushiol, probably with additives such as rice starch, a mix which is still used today.

The pigment layer contains over 15 mainly inorganic components, including orpiment (As2S3), cerrusite (PbCo3), and cinnabar (HgS). The water-saturated lacquer, which resembles phenol formaldehyde resins in many respects, is brownish black, insoluble, finely porous, and non-transparent, properties which handicap conservation attempts as traditionally used, large polymer systems cannot penetrate the layer.

Langhals and his colleagues first delayed the degradation of the polychromy by slowly exchanging the water in the terracotta with low molecular weight polyethylene glycol (PEG). They then applied a water-soluble hydrophilic organic monomer, hydroxyethyl methacrylate (HEMA) which penetrates the porous structure and partially replace the water, and the cross-linker, polyethylene glycol dimethacryl ester (PEG-DMA). After absorption of the monomer and initiator into the lacquer layer the polymerisation process was initiated by electron beam irradiation.

Excavations around Lintong carried out at the end of 2002 have uncovered new sites which contain more figures than those so far known. Preserving an archaeological treasure of this magnitude requires the use of conservation methods which give lasting results and can also be applied routinely. Langhals and his colleagues have gone a long way towards saving the emperor's blushes.