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Scientific Examination of Art: Modern Techniques in Conservation and Analysis A Review of Some Recent Research on Early Chinese Jades Janet G. Douglas Department of Conservation and Scientific Research Freer Gallery of Art/Arthur M. Sackler Gallery Washington, D.C. ABSTRACT Chinese jades produced in the earliest periods of China, during the Neolithic period (5000 to 1700 BCE) to the Han dynasty (206 BCE to 220 CE), were typically fashioned by abrasive techniques using fine mineral powders without the advantage of metal tools. Most of these jades are composed of nephrite, a fine-grained variety of the tremolite-actinolite series of amphiboles, although other stone materials were used as well. The study of early Chinese jades using scientific techniques is a relatively narrow field aimed at developing the cultural and archaeological contexts of these materials. The primary areas of investigation include mineralogical identification, geological source of jade, early jade working methods, detection of heating in jade, burial alteration, and surface accretions. Research in this field is particularly exciting given the large number of excavations in China during the past few decades. INTRODUCTION In early China most jade manufacturing involved abrasion, using fine mineral powders, without the advantage of metal tools. The material of choice was nephrite, a fine-grained variety of the tremolite-actinolite series of amphiboles, although other stone materials were used as well. Nephrite is a calcium magnesium hydroxyl silicate that occurs in a massive form consisting of interlocking fibrous crystals (Hurlbut and Switzer, 1979). Another jade material, jadeite, was not
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Scientific Examination of Art: Modern Techniques in Conservation and Analysis known in China until the eighteenth century, when it was imported from Myanmar (Burma) for working by Chinese artisans. Scientific study of Chinese jades produced in the earliest periods of China, during the Neolithic period (5000 to 1700 BCE) to the Han dynasty (206 BCE to 220 CE), is leading to a richer understanding of these early cultures and their use of jade. Study of well-documented, preferably excavated Chinese jades is helping to place into context those jades of uncertain origin and address issues of authentication. MINERALOGICAL IDENTIFICATION During the last few decades, analysis of early Chinese jades has focused on the identification of the mineral content of jade materials. In China over 500 excavated jades from a wide variety of sites dating from the Neolithic period to the Han dynasty have been analyzed for their mineral content at the Chinese Academy of Geological Sciences (Wen, 1996, 1997, 1998; Wen and Jing, 1992). Many of the over 800 early jades at the Freer and Sackler galleries have been analyzed for mineral content, thus making this collection one of the most extensively studied in the West. In the last decade minimally invasive analytical methods such as X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) have been routinely used for identification. Most of these early Chinese jades have been found to be composed of nephrite, a fine grained, massive variety of tremolite-actinolite. Other materials identified include serpentine, marble, olivine, and corundum. Examples of FTIR spectra from three of these jades are given in Figure 1. Similar findings appear throughout a wide range of Chinese archaeological reports. In addition to the study of individual jades, some composite works have been studied in detail, such as the Freer Gallery’s jade and gold pectoral from the Jincun site in Henan province, dating to about the third century BCE (Douglas and Chase, 2001). The pectoral consists of 10 jades attached to a gold chain, and was examined to determine whether its configuration was correct. The jades were found to be similar in material and workmanship and consistent with other jades from the site. The pectoral, however, was found to be a pastiche where the jades were attached to the gold chain with modern gold wires and cut links from the chain. GEOLOGICAL SOURCE OF JADE Both nephrite and jadeite are known to occur in geological environments through metasomatic processes in a variety of worldwide localities (Harlow and Sorensen, 2001). Two major types of geologic occurrences of Chinese nephrite are known: nephrite associated with metamorphosed dolomitic marbles and nephrite associated with serpentinites.
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Scientific Examination of Art: Modern Techniques in Conservation and Analysis FIGURE 1 FTIR spectra of some early Chinese jades. The geological sources of nephrite used by early cultures in China are not currently known. Such sources may have been depleted in antiquity, as nephrite can occur in small localized deposits. Research involving scientific methods on early Chinese jades has been addressing issues related to the geological origin of nephrite in early China, as well as jade production and use (Douglas, 2003). Analysis of early Chinese jades at the Freer and Sackler galleries using X-ray fluorescence spectroscopy (XRF) indicates that the geological sources of the material used for these jades are most likely associated with dolomitic marbles. The 145 jades analyzed by XRF were found to be consistently low in Cr2O3 (< 0.08 percent by wt.) and NiO (< 0.01 percent by wt.), characteristic of nephrites associated with dolomitic marbles. Future work on geological sourcing of nephrite should concentrate on these types of deposits in China. Of particular interest are the FeO and MnO contents, which have been determined by XRF to point to simple source patterns for the nephrite used by the Neolithic cultures of Hongshan, Liangzhu, and Longshan, possibly involving one or more related geologic sources for each culture. Longshan jades were found to have unusually high FeO (0.35-17.95 percent by wt.) and MnO (0.02-0.89 percent by wt.) contents, suggesting a source particularly rich in iron and manganese. Jades of the Shang and Western Zhou dynasties show a wide range of compositions, suggesting multiple nephrite sources for these objects. XRF is a simple noninvasive tool for determining minor elemental oxide concentrations, but further work on jade sources will need to involve an expanded suite of analytical methods on a wider range of jade objects and geological samples from China.
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Scientific Examination of Art: Modern Techniques in Conservation and Analysis EARLY JADE WORKING METHODS Jade working methods have been investigated by a variety of researchers, and we are beginning to understand how early jades were worked. This type of study can include examination of tool marks on finished and unfinished jades as well as remains from jade working (Wu, 1994). It is particularly important to understand the working methods used on jades of the Neolithic Hongshan culture because of the large numbers of forgeries that have been produced (So and Douglas, 1998; Forsythe, 1990). The remains of one jade workshop were discovered in 1997 north of Dingshadi near Nanjing in the proximity of the remains of the Neolithic cultures of Majiaban, Songze, and Liangzhu (Lu and Tao, 2001). The Nanjing Museum Institute of Archaeology and the Institute of Geological Research of Huadong are currently excavating this area. To date, the workshop has yielded a variety of stone tools that may have been used to work jade through cutting, drilling, surface abrading, polishing, and incising. Raw jade pebbles can still be found along a nearby river’s bank that may have been a source of jade for craftsmen during the Neolithic period. The Lingjiatan site in Anhui province was discovered in 1987, and jades yielded from the site are being studied with the aid of stereomicroscopy (Zhang et al., 2002). The Lingjiatan site is thought to be the location of the earliest agriculture-based city in China, dating to 4000 BCE or earlier. A proficient jade-producing culture inhabited the area as evidenced by the approximately 1,200 jades that have been unearthed there. This work is showing the presence of highly developed working methods, and evidence of the earliest use of the “tuo,” a small rotary disk tool to create fine incised decoration. A cutting edge of the tuo would be similar to the flat head of a nail, although other shapes could have been used for different purposes. In most cases the technique for drilling holes through jade was typically done from both sides of the object. The high level of craftsmanship is exemplified by the glossy polish on these jades, which has left little or no surface striation visible under the stereomicroscope. Tool marks preserved on jades from the collection at the British Museum have begun to be studied using detailed impressions with silicone dental resin (Michaelson and Sax, 2003), which follows from previous work on Mesopotamian seals composed of several quartz varieties. Impressions of small tool marks from jades are imaged using scanning electron microscopy (SEM), which greatly facilitates examination and documentation of these marks for comparison among jades. One Eastern Zhou jade plaque mentioned in this work was worked with several different handheld tools. Polishing techniques used in early China has been a largely unexplored area of research, but quartz sand and related fine-grained materials are generally accepted as the abrasives employed. After a Liangzhu corundum-rich axe was studied at the Freer Gallery of Art in 1998, a fragment from a similar axe was studied
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Scientific Examination of Art: Modern Techniques in Conservation and Analysis in a polishing replication experiment using commercially available abrasives that approximate natural diamond and corundum (Lu et al., 2005). The resulting polished surfaces were compared with the original surface polish produced by Liangzhu jade workers in antiquity. The polished surfaces were examined using optical and electron microscopes and characterized using atomic force microscopy. Three abrasives were used to polish the corundum-rich stone, and the diamond-polished surface most closely matched the surface polished in antiquity. The data suggest that extremely hard mineral abrasives composed of diamond may have been used to polish jades to the high gloss observed on these jades today. Likewise, corundum would have been another likely hard abrasive used. Ornamental jade rings from the Spring and Autumn period (771 to 475 BCE) are decorated with spiral grooves that were created through a mechanical method involving the use of a precision compound machine (Lu, 2004). Such rings must have had their spiral design drafted or directly carved through precisely linked rotational and linear motion of the type that has been demonstrated in recent experiments. These findings imply greater mechanical sophistication than has previously been assumed for this period in ancient China. DETECTION OF HEATING Some physical and chemical changes that occur with the heating of nephrite are known from studies of the amphibole group minerals, tremolite-actinolite (Whittels, 1951; Vermaas, 1952). The dehydration of actinolite occurs in three stages, including the loss of adsorbed water, the loss of structural water, and a very small quantity of absorbed water. Studies using differential thermal analysis (DTA) show that an exothermic reaction takes place between 815°C and 824°C, and is associated with the oxidation of the small amounts of ferrous iron present in the mineral. This oxidation is not associated with any structural change in the crystal structure. Structural water is liberated at temperatures between 930°C and 988°C, and at lower temperatures with increasing iron in the mineral structure. This change occurs through a solid-state reaction:  Detection of heating in jades using minimally invasive analytical methods is of interest because some jades may have been heated in antiquity prior to working or during burial rituals involving burning. Heat treatment may also be used in the production of modern-day forgeries to make jade appear older due to natural weathering or alteration. At the Freer and Sacker galleries, XRD and FTIR have been used to detect heating in jade, but these techniques have been found to be successful only if the object has been heated to at least 900°C (Douglas, 2001). In this study a nephrite pebble was sliced and heated in 100°C increments
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Scientific Examination of Art: Modern Techniques in Conservation and Analysis FIGURE 2 (a) Heating series of low-iron (tremolitic) nephrite heating from Hetian (Khotan), Xinjiang province (nephrite slice length approximately 3 cm). (b) Heated bracelet (F1917.43) dating to the Neolithic period or Shang dynasty (bracelet diameter 6.0 cm). from 500°C to 1100°C to observe visual changes and to investigate XRD and FTIR as methods to identify heating in jade. This heating series is shown in Figure 2, along with an example of a heated jade bracelet dating to the Neolithic or Shang dynasty. The heating series samples became more white and opaque with heating. Vickers hardness measurements on the heated samples showed that nephrite becomes slightly harder, rather than softer up to 800°C. After this temperature the material becomes brittle and tends to fracture more easily. In addition, black areas developed in the nephrite upon heating, which then became brown at 900°C. This black coloration may be due either to carbonization of small amounts of organic material trapped in crevices or oxidation of iron in the nephrite. The applicability of noninvasive Raman spectroscopy to the detection of heating in jade is also being investigated on the same nephrite heating series (unpublished information from P. P. Knops-Gerrits). Some of the XRD, FTIR, and microRaman data that can be used to identify heating in jade composed of nephrite are summarized in Table 1. BURIAL ALTERATION AND SURFACE ACCRETIONS Burial alteration is a particular type of alteration known to occur on early Chinese jades composed of nephrite. Such alteration usually appears as opaque, white, chalky areas on otherwise translucent, polished jades. In many cases these patchy
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Scientific Examination of Art: Modern Techniques in Conservation and Analysis TABLE 1 Some XRD, FTIR, and MicroRaman Data That Can Be Used to Identify Heating in Jade Based on a Nephrite Heating Experiment Heating Series Sample XRDa FTIRb Peaks MicroRamanc Peaks Diffuse d-spacings 8.38 Å d-spacing 3670 cm−1 1075 cm−1 Rel % T (475 and 512 cm1) Unheated _ + + _ 475 cm−1 393, 368, 348, 1058, 1029, 930 cm−1 500°C _ + + _ 475 cm−1 393, 369, 349, 1058, 1028, 930 cm−1 600°C _ + + _ 475 cm−1 391, 365, 349, 1058, 1027, 930 cm−1 700°C _ + + _ 475 cm−1 391, 365, 1058, 1026, 929 cm−1 800°C _ + + _ 475 cm−1 391, 367, 1058, 1025, 927 cm−1 900°C + + + + 512 cm−1 391, 323, 1340, 1010 cm−1 1000°C + _ _ + 512 cm−1 392, 327, 1341, 1011 cm−1 1100°C + _ _ + 512 cm−1 392, 327, 1341, 1013 cm−1 NOTE: _ absent; + present. aPhilips RG-2600 X-ray diffractometer with Gandofi camera. bMattson Alpha Centauri Fourier transform infrared spectrometer. cRenishaw R1000 microRaman spectrometer, 785 nm excitation.
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Scientific Examination of Art: Modern Techniques in Conservation and Analysis areas of alteration are softer than the unaltered areas of the jade. This type of alteration consists of a selective dissolution or leaching on a microscopic scale along mineral grain boundaries by solutions of high pH (pH > 9) rather than a mineralogical change (Gaines and Handy, 1975). This type of high-pH environment can occur during decay of the corpse(s) with which the jades were buried. Experiments to produce burial alteration on jade indicate that it is likely that this type of alteration occurs during the months immediately after the burial when a corpse decomposes (Aerts et al., 1995). Optical coherence tomography (OCT) is a noninvasive technique that is being used to study the subsurface morphologies of jade objects to determine whether surface whitening is due to burning or natural alteration (Yang et al., 2004). Tomography images are used to show the refractive index or dielectric constant variations in jades, which reflect their internal structures. To date, OTC has been applied to a relatively small number of early jades but may prove to be useful in the future to answer questions relating to the authenticity of jade objects. Surface accretions remain an unexplored area of focused research, probably because it can be difficult to determine the significance and relative age of such deposits. Many jades are heavily cleaned and waxed, which often obliterates any accretions on their surfaces. Other accretions may unintentionally find their way to the surface of a jade but are typically not related to its early history. Earthy encrustations typically include calcareous deposits and soil. Occasionally lacquer and other organic remains can be seen. CONCLUSIONS AND FUTURE DIRECTIONS Research using scientific techniques is helping us to understand the mineral composition and early history of well-documented and excavated jades. Similar work on unknown jades is helping to solve questions of authenticity (Douglas, 2000). Such study is most fruitful if it can include thorough visual examination using a stereomicroscope along with comparison to similar, preferably excavated jades. No direct methods of dating jade materials exist. Some future areas for research include identification and distribution of surface accretions, weathering, and alteration; continued study of jade working methods, with particular emphasis on the study of large groups of related jades from individual sites and cultural areas; study of jade working remains, including tools and jade debris; analytical and technical methods of dating jade workmanship; and study of early jades from areas neighboring China, such as Korea, Taiwan, Siberia, and Southeast Asia, as all of these areas had jade-producing cultures.
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Scientific Examination of Art: Modern Techniques in Conservation and Analysis REFERENCES Aerts, A., K. Janssens, and F. Adams. 1995. Orientations Nov.:79-80. Douglas, J. G. 2000. Orientations Feb.:86. Douglas, J. G. 2001. Proceedings of the Conference on Archaic Jades across the Taiwan Straits. Taipei: Guo li Taiwan da xue li xue yuan di zhi ke xue xi yin xing and Guo li Taiwan da xue chu ban wei yuan hui. pp. 543-554. Douglas, J. G. 2003. In Scientific Research in the Field of Asian Art, Proceedings of the first Forbes Symposium at the Freer Gallery of Art, ed. P. Jett, with J. G. Douglas, B. McCarthy, and J. Winter, pp. 192-199. London: Archetype Publications in association with the Freer Gallery of Art, Smithsonian Institution, Washington, D.C. Douglas, J. G., and W. T. Chase. 2001. Studies in Conservation 46:35-48. Forsythe, A. 1990. Orientations May:54-63. Gaines, A. M., and J. L. Handy. 1975. Nature 253:433-434. Harlow, G., and S. Sorensen. 2001. Australian Gemmologist 21:7-10. Hurlbut, C. S., and G. S. Switzer. 1979. Gemology. 243 pp., Canada: John Wiley. Lu, J., and H.Tao. 2001. In Enduring Art of Jade Age China, vol. 2, ed. E. Childs-Johnson, pp. 31-42. New York: Throckmorton Fine Art. Lu, P. 2004. Science 304:38. Lu, P. J., N. Yao, J. F. So, G. E. Harlow, J. F. Lu, G. F. Wang, and P. M. Chaikin. February 2005. Archaeometry 47:1-12. Michaelson, C., and M. Sax. 2003. APOLLO Nov.:3-8. So, J. F., and J. G. Douglas. 1998. In East Asian Jades: Symbol of Excellence, vol. 1, ed. C. Tang, pp.148-163: Hong Kong: Chinese University of Hong Kong. Vermaas, F. H. S. 1952. Transactions of the Geological Society of South Africa 55:1. Wen, G. 1996. Acta Geological Taiwanica 32:55-83. Wen, G. 1997. Chinese Jades-Colloquies on Art & Archaeology in Asia 18:105-122. Wen, G. 1998. In East Asian Jades: Symbol of Excellence, vol. 2, ed. C. Tang, pp. 217-221. Hong Kong: Chinese University of Hong Kong. Wen, G., and Z. Jing. 1992. Geoarchaeology 7:251-255. Whittels, M. 1951. American Mineralogist 36:851. Wu, T. 1994. Renshi Guyu: Gudai yuqi zhizuo yu xingzhi. Taiwan: Zhonghua ziran wenhua xuehui. Yang, M. L., C. W. Lu, I. J. Hsu, and C. C. Yang. 2004. Archaeometry 46(2):171-182. Zhang J., Z. Yang, and Q. Cheng. 2002. Dong nan wen hua 5:17-27.
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