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Terracotta Army

August 29, 2014

There was a lot of human history before the advent of the systematic practice of chemistry. In this pre-chemistry period, humans used natural materials for most of the things for which we use synthetic materials. One ancient example of such a material is Tyrian purple, also known as royal purple. This natural dye, which is an extract from the sea snail, Bolinus brandaris, was expensive; thus, the "royal" appellation.

Even today, some natural materials have properties that are difficult to replicate. I was reminded of that when I was doing research on superconductivity early in my career. In those days before high temperature superconductors, all superconductivity experiments were done using liquid helium.

We used so much liquid helium that we had our own machine for helium liquefaction. The technician who was responsible for running the machine showed me some of the washers used in the machine. These were made from natural leather, since the material functioned well at low temperatures.

One common human activity is gluing things together. In the days before polyvinyl acetate-based Elmer's Glue and my favorite, five-minute epoxy, animal glue was used. Animal glue is produced by boiling animal connective tissue, such as skins, bones, and hides, in water to induce hydrolysis of their contained collagen.

Off to the glue factory (WWI poster)Old bones off to the glue factory.

This is a World War I era poster about wartime recycling in England.

(Via Wikimedia Commons.)

Since older horses were in good supply in the days before motorized vehicles, the appropriate parts of horses were mostly used; thus, the expression, "off to the glue factory." Although the principal use of the best quality glue, hide glue, was in woodworking, it was also used as a binder for paints and inks.

Hide glue has quite a few excellent properties. It can be stored as dried flakes, or sheets, that are turned into the glue by dissolving in 140°F (60°C) water. It sets rather quickly, within a few minutes,a glued joint can be heated for release, and it adheres to itself, which is something that a polyvinyl acetate glue won't do. Its surface tension allows an automatic alignment of parts, which is useful in the construction of precision woodworks, such as violins.

Structure of hydroxyprolineStructure of hydroxyproline (4-hydroxypyrrolidine- 2-carboxylic acid).

Hydroxyproline is a major component of collagen.

(Via Wikimedia Commons.)

The Chinese World Heritage Site, The Mausoleum of the First Qin Emperor, is familiar to most people by its collection of sculptures known as the Terracotta Army, discovered in 1974. There are a myriad of human figures, mostly soldiers, buried there, along with figures of horses, chariots, and a small number of other things. As can be expected, the government of China has devoted considerable resources to the Army's preservation, and to research on the figures.

Terracotta ArmyTerracotta Army of Emperor Qin Shi Huangdi near Xian, as seen in May, 2007.

(Photo by Eikenhein, via Wikimedia Commons.)

Qin Shihuang, who was China's First Emperor, had this underground palace complex created as a duplicate of his palace in Xianyang in 221 BC.[1-2] His imperial guard was replicated in full armor and in fine detail. The sculptured figures, along with their horses, chariots and weapons, are all different. They document this period in history, and also the craft and techniques of potters and bronze-workers of the time.[2]

As can be seen in the photographs, this is an impressive work of art, but it was more impressive at the time of its conception, since the statuary was actually painted to closely represent the subjects. The burial in damp soil has eradicated nearly all traces of the pigments. Now, Chinese scholars from the Key Scientific Research Base of Ancient Polychrome Pottery Conservation of the State Administration for Cultural Heritage have done an analysis of the pigment traces and found that the sculptures were coated first with a non-pigmented lacquer that was overcoated with polychrome layers. Animal glue was used as the binding medium of the polychrome layers.[1-2]

Terracotta horse and two soldiers from the Terracotta ArmyTerracotta horse and two soldiers from the Terracotta Army.

(Via Wikimedia Commons.)

The base coat for the polychrome layers was one, or two, layers of an East Asian lacquer obtained from lacquer trees. The polychrome pigments included cinnabar (HgS), apatite (Ca5(PO4)3OH), azurite (Cu3(CO3)2(OH)2) and malachite (Cu2CO3(OH)2).[2]

Although extremely low levels the proteinaceous binding medium survived immersion in water-saturated loess for more than two millennia, the research team was able to identify the medium using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS).[1] This technique provides high sensitivity with only minimal sample pretreatment.[2] Extracted proteins were complexed with ethylenediaminetetraacetic acid (EDTA), subjected to dialysis, then hydrolyzed with high purity trypsin to generate peptide fragments.[2] The peptide fragments identified the binder as an animal glue.

Scientists commonly double-check their findings to ensure that they're not somehow fooling themselves. The Chinese research team prepared their own version of terracotta coated using the materials discovered in the analysis. The glues tested included an adhesive formulated from the eggs of free-range chickens. These model samples were buried in one meter of loess soil for one year, and the model analysis gave the same results as the Terracotta Army colorants.[2] This research was funded by the National Key Technology R&D Program, China, grant no. 2010BAK67B12.[2]

References:

  1. Hongtao Yan, Jingjing An, Tie Zhou, Yin Xia, and Bo Rong, "Identification of proteinaceous binding media for the polychrome terracotta army of Emperor Qin Shihuang by MALDI-TOF-MS," Chinese Science Bulletin, vol. 59, no. 21 (July, 2014), pp. 2574-2581.
  2. Scientists solve 2000-year-old mystery of the binding media in China's polychrome Terracotta Army, Press Release from Science China Press, August 1, 2014.

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