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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.
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 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 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 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 (Cu
2CO
3(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:
- 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.
- 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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