“Chemists have found a way to stop the paint from peeling off the famous terracotta warrior statues unearthed in China.
The life-sized statues, discovered in 1974, guard the tomb of Qin Shihuangdi, the first emperor of China who died in 210 BC. Over 1500 warriors have been unearthed to date, many with bright red uniforms.
But all lost their paint almost as soon as they were removed from the moist soil where they had lain undisturbed for 2200 years. “If you excavate them, they dry out instantly and in five minutes, the paint peels off,” explains Heinz Langhals, from the University of Munich in Germany.
Thousands more of the warriors remain buried at the mausoleum in Lindong, awaiting excavation. Now, Langhals and his colleague, Daniela Bathelt, have discovered why the paint peels and developed a way to stop it happening.
The cause of the problem was the natural lacquer applied to the terracotta surface as an undercoat for the paint pigments. Changes over time in the chemical structure of the lacquer mean that when it dries out, it curls off the terracotta and takes the overlying paint layer with it.
Super glue
The chemists solved the problem by finding a way to moisturise the lacquer layer and glue it irreversibly to the underlying terracotta.
To avoid instant desiccation, they pack all newly excavated items into containers which match the moisture levels of the soil. Next, they dab aqueous solutions of hydroxyethylmethacrylate (HEMA) onto the paint surface. HEMA is a monomer building block used to create some modern plastics.
When the HEMA has diffused through to the lacquer, the researchers apply a beam of high-energy electrons to the surface. This causes the HEMA to polymerise and harden, forming a glue which fixes the lacquer permanently to the terracotta.
Langhals says damage to the mausoleum caused by rebellions and fires resulted in most of the buried warriors being smashed into fragments, typically 15 centimetres square and two to three centimetres thick. These are easier to treat than whole soldiers, and can be reassembled into full figures once they have undergone the curing treatment.”
“Once they’re treated, you can handle them without any precautions,” he told New Scientist. “We hope we can see the first full soldier at the end of 2004.”
Journal reference: Angewandte Chemie International Edition (vol 42, p 5676)]
Super Glue Corporation’s Blogger’s Note: Hydroxyethylmethacrylate is a type of super glue with similar chemistry to one of our super cool products recommended for plastics! Plastic Fusion is polymethyl methacrylate chemistry and has a very high bonding strength. Recently checked in with our senior chemist about this topic and he told me that HEMA is a chemical from the family of methacrylate monomers (like our CAs) and is normally used as an additive. Super Glue Corporation’s super glues contain the polymerized form of methyl methacrylate called PMMA which is an additive to “flexibilize” the hard cyanoacrylate (CA). Actual HEMA is slightly yellow and would not be desireable for our purposes – but the chemistry is from the same family. We’re glad to know this chemistry was used to restore these amazing terracotta warriors so that they can be appreciated by future generations for a long time!!
