Chinese jade artifacts are not only pieces of artworks but also the physical evidences of the records of Chinese history. The previous statement is true only if these artifacts are genuine. Therefore, the value of jade artifacts depends on its genuineness in addition to other factors such as quality of jade and mastery of workmanship etc.. As a result, jade authentication is of paramount importance in the Chinese jade studies.
There were many writings on the jade authentication theory and practice since the Song and Yuan dynasties (906 CE -1379CE) in the Chinese history. Among them, the most popular two works were published in the late Qing and early Republic periods. Today, although there are many jade authentication methodologies that appeared in the publications, there are a lot of disagreements on the accuracy of the various methodologies and the theories behind them. Many of the methods used by professional researchers and collectors in the authentication of Chinese antique jades tend to be very subjective, often rely heavily on appearance, feeling, etc…
In the 1995 jade’s authentication conference held in Taipei, Prof. Tsien, H. H. proposed a scientific approach on jade’s authentication in the article of Mineralogical Studies of Archaic Jades. Since then, the scientific method for jade’s authentication has gained wide acceptance from both academia and professionals in Taiwan and other parts of the world. According to Prof. Tsien, the steps for authentication of jade antiques consist of medium authentication, jade antiquity authentication and period verification. The first two steps of jade’s authentication method are fully explained in the following sections. For the discussion of the period verification, please refer to the information on motif and carving style for a particular period shown in the A Comprehensive Handbook of Chinese Archaic Jades or go to timeline of chinese jade-neolithic period and timeline of chinese jade-the dynasties in the front page.
1.1 Medium authentication
The first step in jade authentication is to identify the kind of material of the object under investigation. According to E. S. Dana 1898’s A Textbook of Mineralogy, jade may mean nephrite or jadeite. Nephrite is a variety of the calcium and magnesium-rich amphibole mineral actinolite. The chemical formula for nephrite is Ca2 (Mg, Fe) 5Si8O22 (OH) 2. And jadeite is a pyroxene mineral with composition formulas presented by NaAlSi2O6. Several major physical characteristics differences between nephrite and jadeite are shown in Table 1.
Table 1 Major Physical Characteristics of Nephrite and Jadeite
|Characteristics Nephrite Jadeite|
|Mohs scale hardness: 6-6.5 6.5-7|
|Specific gravity: 2.95 (+.15, -.05) 3.24-3.43|
|Polish luster: vitreous to greasy vitreous to greasy|
|Refractive index: 1.606-1.632 1.667-1.693|
Material identification of jade can be achieved by looking at the spectrum from a Raman spectrometer or by the refractive index reading via the gemological refractometer such as the GIA Duplex Refractometer. When using the Raman spectrometer, there is a prominent fluorescence peak between 650-700 cm-1 region for Nephrite and Jadeite. For Nephrite, there is an additional water peak around 3700 cm-1 which is absent in Jadeite. This feature allows the used of Raman spectrometer in differentiating between Nephrite and Jadeite. The tool can also be used for identifying jade that have been artificially treated by bleaching, dyeing and polymer impregnation. The common chemical used in the artificial treatment would often add additional peaks to the Raman spectrum.
When a refractometer is used, the refractive index of nephrite is 1.606 to 1.632 via refractometer while the refractive index of jadeite is 1.667 to 1.693.
1.2 Jade Antiquity Authentication
Jade antiquity authentication may be verified through evidences of certain physical and chemical characteristics. These evidences consist of dissolved pits, cleaving veins, differential weathering, diffusive markings, exposure crystals, deterioration marking and secondary additive crystal planes. In addition, mineral attachments such as gold or silver placer may be served as an evidence that the sample jade has been buried under the ground. These physical and chemical phenomena are formed by nature through ages and cannot be made by human beings. The authentication of these antiquity evidences is of paramount importance in the whole process of the scientific method of antiquity jade authentication. The microscopic images provided here will be helpful to identify these antiquity phenomena. In the following, each of the antiquity phenomena is explained.
Additive crystals: Additive crystals are the crystals added to the jade artifacts due to exterior materials. Needle like flat crystals is fabricated into the jade itself as a new material. These crystals are raised above the jade plane and scatter from the center into different directions. The distribution of the additive crystals has a natural and regular arrangement.
Cleaving veins: The cleaving veins seen inside the jade artifact are caused by the interaction of physical forces from heat and pressure on the jade. They are different from the cracks caused by percussion. Exterior cracks are common phenomena for cracks caused by percussion. Usually there are no exterior cracks for cleaving veins.
Diffusive markings: Diffusive markings are the phenomena caused by the incremental penetration of exterior chemicals after the impact of heat and pressure conditions of the underground environment. Different levels of condensate of colors are usually seen in the jade artifacts. The diffusive markings should not be mistaken as raw jade skin which is saved in the carved object on purpose. The practice of leaving raw skin of jade for the jade carved object has a special name called skillful carving. This practice has a long history in the Chinese jade carving since the Shang Dynasty
Differential weathering: By the definition from the Random House Unabridged Dictionary, differential weathering is the difference in degree of discoloration, disintegration, etc., of rocks of different kinds exposed to the same environment. Here we define it as the different deterioration of the surface of the traces of carving on jade artifact due to humidity, heat and chemicals in the air after a long period of time. Wrinkles are seen in the traces of carving due to differential weathering.
Dissolved Pits: These are the holes formed by natural physical and chemical causes. Mineral crystals are usually seen inside the holes which have shape showing natural irregularity. The shape of the pit and the the mineral crystals insides the pit are impossible to imitate by artificial methods. Holes made by artificial methods using chemicals are seen round shapes and showing white spots.
Exposed crystals: Exposed crystals are crystals transformed from the jade itself due to physical factors such as heat and pressure. They are sometimes called micas. Almost all the antique jades buried underground have shown exposed crystals. One postulate of the exposed crystals of antique jades is that the inner force of jade elements of jade carving has been changed from the inner force of jade elements of raw jade stone. Exposed crystal is caused by the changing shape and arrangement in the whole structure of jade due to the heat and pressure forces.
Deterioration caused by underground plant root: Channel marks and traces shown on the surface of antique jades may be caused by interaction of chemicals surrounding plant roots nearby the jades buried underground through ages. The shape and traces of deterioration shown on the surface of jade are in consistency with the spreading area of plant roots. The original color and texture of the jade can be seen from the channel marks and traces.
Deterioration marks: These are the areas inside antique jades and having shapes like the legs of the ant or cow’s hair. Therefore, deterioration marking sometimes is called aging spots, ant’s leg or cow’s hair. This phenomenon of physical or chemical transformation of jade substances may be caused by underground heat, pressure and chemicals through time. Some sources claimed that the length of time to cause these deterioration marks are at least one thousand years. If it is true, this fact may be served as an important criterion to date the age of antique jade.
The phenomenon of calcification is believed to be caused by decomposing the structural water of the jade. The cause of decomposition may be from the interaction of heat and the pressure of the environment where the jade is buried. Another explanation is that calcification is caused by the interaction of the jade and the surrounding calcium where the antique jade buried. Most people tend to in favor of this kind explanation of jade calcification before any scientific proof has been found. It should be noted that fake calcification jades are often found to fake Liangzhu Culture jades. The faking technique is to form a coating layer on inferior jade or fabricated jade to imitate the calcification appearance looks like the Liangzhu Culture jades.