Studying the Composition of Historical Relics with Raman Spectroscopy

The Terracotta Warriors and Horses of the First Qin Emperor.

The Terracotta Warriors and Horses site, known as the ‘Eighth Wonder of the World’, was discovered in 1974. It is made up of three figure pits covering an area over 20,000 square meters and forms part of Emperor Qin Shihuang’s Mausoleum Site Museum, considered to be the largest imperial tomb in China.

The pits comprise of approximately 8,000 terracotta horses and warriors, and also more than 100 chariots. In 1987, they were included in the ‘World Heritage List’ by UNESCO.

The sculptures are situated in Lintong District, Xi’an, Shaanxi Province, China, which sits 1 km to the east of Emperor Qin Shihuang’s mausoleum. Each life-sized figure is exceptional, with complex details, including individual costumes and facial expressions. Originally, the terracotta warriors were painted in bright pigments but, after more than 2000 years, the colors have faded and just small traces of pigment can be detected on their faces, hands, shoes and costumes.

The conservation, analysis and study of these precious ancient cultural relics are extremely complex. Dr. Yin Xia, the Deputy Director of the Relics Conservation and Restoration Department at the Emperor Qin Shihuang’s Mausoleum Site Museum, has employed Raman microscopy for studying historical relics since 2006. In 2008, the museum bought a Renishaw inVia confocal Raman microscope to enable identifying pigments on the terracotta figures, besides determining vital information needed for their conservation and restoration.

In addition to his work with Emperor Qin Shihuang’s Mausoleum Site Museum, Dr. Xia is the Deputy Director of the Key Ceramic Painted Cultural Relics Research Site, under the State Administration of Cultural Heritage, and also a part-time tutor of North-West University, China. At the university, his research largely focuses on the analysis and identification of painted relics.

Dr. Yin Xia,

Dr. Yin Xia, Deputy Director of the Cultural Relics Conservation and Restoration Department of Qin Terracotta Warriors and Horses Museum / Emperor Qin Shihuang’s Mausoleum Site.

Dr. Xia began his analysis of the ancient pigments of cultural relics in 2003 and has made several landmark discoveries. For instance, under the polarizing microscope, Chinese Purple (BaCuSi2O6), Chinese Blue (BaCuSi4O10) and dark blue particles of a new pigment (BaCu2Si2O7) frequently co-exist in a sample. As they appear in tiny amounts — sometimes there are just one or two particles—it is difficult to separate them individually with consistent analytical means. Dr. Xia first discovered the new dark blue pigment with the inVia Raman microscope1.

Dr. Xia has taken part in a number of conservation programs on the historical relics of Emperor Qin Shihuang’s mausoleum and Qin terracotta figures. Since 2004, his research concentrated on painted terracotta figures and pigments on historical relics. He has completed the analysis of painted relics and murals at nearly one thousand sites spread across 14 provinces and autonomous regions in China.

Non-Destructive Testing is Important in the Study of Historical Relics

Dr. Xia and his team have also analyzed the composition of colored pigments on ceramics unearthed from West Han Dynasty tombs in Weishan, Shandong province, and discovered Chinese Purple on these cultural relics for the very first time. The pigment of Chinese Purple was seldom used on ancient colored painted artifacts. In the past, it has been discovered in just a few provinces such as Gansu, Shaanxi, Jiangsu and Henan. The discovery of this pigment in relics within the Shandong province has expanded the geographical area in which it was used, which has vital archeological significance2.

Fine art sculpture of infantry

Fine art sculpture of infantry

A number of different scientific techniques can be used for studying historical relics, including: elemental analysis, using laser induced breakdown spectroscopy (LIBS); X-ray fluorescence spectroscopy (XRF); chemical and structural analysis methods, such as laser Raman spectrometry; X-ray diffraction (XRD); and morphological analysis, with scanning electron microscopes (SEM) and powder polarizing microscopes (PLM).

Each method has its pros and cons. For instance, XRF is a mature elemental analytical technology but cannot be employed for chemical structure analysis; XRD is extensively used for the qualitative analysis of cultural relic materials but needs huge amounts of sample; PLM can examine samples by crystal morphology, colors and impurities of particles, but cannot identify the structure of samples. All these analytical methods are vital but, given the delicate and precious nature of historical relics, non-destructive testing and analysis has indeed become increasingly vital in recent years.

Raman spectroscopy is considered to be non-destructive and non-contacting, making it perfect for the analysis of historical relics. It offers information about origin, authenticity and age, while preserving the whole sample. It also has the flexibility to take measurements in situ, hence there is no need to remove fragments for analysis.

According to Dr. Xia, small particles left on the gloves of the museum staff after they have moved the terracotta figures are gathered for Raman analysis. These samples can be analyzed repeatedly using Raman spectroscopy, without damage, allowing subsequent analysis with other techniques, such as electron microscope analysis.

Prior to using a Raman Microscope, a polarizing microscope was a key tool in Dr. Xia’s laboratory.

We are unable to determine the molecular structure of a sample with the polarizing microscope. For example, if we observe atacamite under a polarizing microscope, we cannot distinguish it from its isomers such as botallackite and paratacamite, but with a Raman microscope, we can get the results immediately.

Dr. Yin Xia, Deputy Director of the Cultural Relics Conservation and Restoration Department, Qin Terracotta Warriors and Horses Museum

inVia Confocal Raman Microscope: Highly Flexible and Easy to Use

The inVia Raman microscope was selected by Emperor Qin Shihuang’s Mausoleum Site Museum for its great flexibility.

The Renishaw inVia Raman microscope and spectrometer are discrete, so I can configure what I want on the microscope without affecting the whole system. Products from other vendors adopt embedded or integral microscope designs, which do not meet our requirements.

Dr. Yin Xia

Understanding colored pigments is vital for scientific archeology and cultural relic conservation. It can offer valuable information for exploring the evolution of ancient pigment technology, which assists in the development of relevant conservation programs. Additionally, research on the structure and composition of ancient colored pigments has indeed become a vital basis for determining the origin and the age of raw materials, as well as restoration and conservation programs for cultural relics. “We are using Raman microscopy for colored painting analysis in this intense area of detailed research” Dr. Xia commented.

References

1. Development of Chinese barium copper silicate pigments during the Qin Empire based on Raman and polarized light microscopy studies. Journal of Archaeological Science 49 (2014) 500-509

2. Xia Yin, Wu Shuang Cheng, Cui Sheng Yuan, et al. Study on Coloured Pigments on Ancient Ceramics Unearthed from West Han Dynasty Tombs in Weishan, Shandong Province. Sciences of Heritage Conservation and Archaeology, 2008, 20 (2): 13-18

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This information has been sourced, reviewed and adapted from materials provided by Renishaw plc.

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