Using GC-MS with X-ray fluorescence spectrometry, X-ray diffraction spectrometry and polarising microscopy, scientists from the School of Cultural Heritage (Northwest University, Xi'an, China) and Emperor Qin Shihuang’s Mausoleum Site Museum (Xi’an, China) determined the composition of a multifunctional material used as a kind of glue and filler in the construction of the Terracotta Army.
Mystery multifunctional material
In 1974, a group of farmers digging a well outside Xi'an City (Shaanxi Province, China) unearthed fragments of clay sculptures. Following their discovery, archaeologists visited the site and, through years of gradual and painstaking work, uncovered over 7000 terracotta statues, the largest collection of buried sculptures ever found. Each terracotta statue is unique, with different hairstyles, faces, clothing, heights, faces and poses. Now a famous UNESCO World Cultural Heritage site, the Terracotta Army surrounds the mausoleum of Emperor Qin, the first emperor of China, protecting him in the afterlife. Emperor Qin united numerous smaller warring states, marking the beginning of what is now China, and started the construction of the Great Wall.
The terracotta statues are truly remarkable, including military figures, such as warriors, weapons and horses, as well as acrobats, strongmen and musicians, presumably to entertain the emperor in the afterlife. A mausoleum and visitor centre have been constructed over the excavated statues and they have been shown in exhibitions around the world. Since their discovery, the relics have received wide scientific and archaeological interest, revealing rich detail about life in China in the days of Emperor Qin.
A multipurpose material has been observed in various places on the statues, described as a homogenous blue-gray powder, similar in colour to grey pottery. On the warrior figures, it has been found between the warrior and its footrest, as well as joining components such as arms and body, neck and head, and sleeve and palm. On equine statues, this material has been found between the head and neck, on the abdomen, and the limbs and hooves, as well as with pieces of fabric. The material appears to have been an important component in the construction of the Qin statues but thus far it had not been investigated in any depth. A team from the School of Cultural Heritage (Northwest University, Xi'an, China) and Emperor Qin Shihuang’s Mausoleum Site Museum (Xi’an, China) applied a range of analytical methods to investigate the composition of the material and shed light on its function. X-ray fluorescence spectrometry (XRF) and X-ray diffraction spectrometry (XRD) were applied to study the inorganic components, polarizing microscopy (PLM) provided details on the mineral appearance and grain size, as well as the temperature used for firing the material. GC-MS was used to identify the organic components.
GC-MS identifies organic binders
Samples of the multipurpose material were taken from a terracotta warrior and a terracotta horse by scraping with carbon steel surgical blades and collecting in medical spoons. The samples were sealed in glass tubes and stored refrigerated until needed for analysis. For comparison, samples of fired soil, simulated pottery and pottery fragments were also prepared.
For the GC-MS analysis, a GC7890-MS5975 (Agilent, USA) was employed, equipped with an EI ion source and a quadrupole mass analyzer. First, the samples were extracted using ammonia and purified over a C4 solid phase, then placed under vacuum with 6 M HCl for hydrolysis. The samples were then heated to 160 °C over 5 min and held for 30 min. The obtained hydrolysate was dried under nitrogen flow and derivatized using MTBSTFA with 1% TMCS. Hexadecane and N-leucine were used as internal standards and the standard curve method was used for quantitation.
XRF and XRD showed that the inorganic components of the material are SiO2, Al2O3 and Fe2O3 with some K2O, MgO and CaO, with quartz, albite and potassium feldspar being the predominant mineral phases, as typically reported for pottery. Thus, the inorganic components are the same as those in the Terracotta Army statues or the clay that was used to make them. PLM and PETROG further suggest the presence of ground pottery in the material.
GC-MS was able to identify the organic components, which play a special role in the adhesive material. Protein was found at low levels of 0.0023% to 0.0212% in all of the samples. Amino acid ratios and PCA were used to identify the sources of the protein, with all samples containing egg and animal glue proteins. Further work is needed to determine whether whole egg, yolk or white was used, or if other organic substances (other proteins, lipids or polysaccharides) may have been present.
The results suggest that the main function of the material is as a kind of bonding agent or glue, which is backed up by observations indicating that parts of the statues were made separately and then joined together. Moreover, the presence of the material between fractured parts strongly suggests its application as a glue. Another proposed use is for smoothing rough surfaces, as seen on a horse statue’s abdomen. The researchers believe that the combination of the inorganic filler and organic binder endows the material with multifunctional properties.
Facilitating future restoration
Using a range of multiple analytical techniques, this study revealed the composition of the multifunctional material used in the construction of the Terracotta Army. The combination of inorganic fillers with organic binders shows the inventiveness and resourcefulness of the craftsmen and will no doubt be very useful for the restoration of the terracotta warriors and horses from the Qin Dynasty.