Newsletter No. 82

CUHK Newsletter No. 82 19th January 1996 3 All the Heat that Emanates f rom the Dat ing Process A n d A l l the Light that Is Shed on Age F ine imitations of antique ceramics often defy even the experienced art student, who can only rely upon typological or stylistic considerations for authentication and dating. How do we ascertain that what is being bought is 'genuine'? That it is what it is purported to be? That it is of the period that we are told it belongs to? The University's Yeung Shui Sang Laboratory for Thermoluminescence of Ancient Ceramics offers a solution. It uses the thermoluminescence (TL) method to give absolute dating. TL means the emission of light from a substance (usually an insulator or a semiconductor) when it is heated. The light represents the release of energy that has been stored in the substance. Stored energy exists in the form of electrons that have been absorbed by the substance when exposed to radiation. In the case of ceramics, the radiation it receives comes from (1) natural radioisotopes (uranium, thorium and potassium 40) that exist in the clay from which it is made, (2) gama rays from the surrounding burial soil, and (3) cosmic rays. Heating a ceramic sample to 500°C or above will remove its geologically accumulated energy. Thus the initial 'baking' of raw clay into an artifact sets its 'TL clock' to zero. Thereafter, the energy in the artifact grows with time from zero to the present amount observed. It follows that if we can measure the total accumulated radiation (energy) absorbed by the artifact over the years and divide it by the annual amount of radiation it absorbs, we can get its age. TL dating is based on this theory and the entire process in five steps is illustrated separately (see right). The TL method depends only on laboratory measurements without any need of known-age samples for comparison. However, tests on known-age samples for establishing the validity of the methods is essential. These tests indicate the marginal error for pottery dating to be around ±10 per cent. The procedure of taking a sample from the ceramic piece without in any way harming the piece or detracting from its aesthetic or monetary value, is a task that requires a great deal of expertise. At the University, this responsibility falls on Mr. Lee Chung-kay, assistant director of the laboratory. Mr. Lee, a physics graduate from the University, did his M.Phil. thesis on TL dating. He has been with the laboratory since its inception in 1986, and is practically involved in the sampling, testing and dating procedures. Before 1986, collectors and archaeologists in Hong Kong and its neighbouring regions had to rely on TL dating services provided mainly by laboratories in UK. In 1986, a well-known collector of Chinese ceramics, Mr. Yeung Wing-tak, and his brothers donated HK$1,000,000 to the University for the establishment of Hong Kong's own laboratory for thermoluminescence. They named it the Yeung Shui Sang Laboratory for Thermoluminescence of Ancient Ceramics after their father. According to Dr. K.P. Chik, honorary director of the laboratory, the laboratory aims at researching on the fundamental TL properties and refining the methods of TL dating of ancient ceramics. It also provides research facilities for the physics graduates of the University who are working on theses related to T L dating. The laboratory also gives authentication and dating service to the public. In the course of this, it collaborates with other scientific institutes in Israel, Australia, and USA. Today, the laboratory tests and dates approximately 14 ceramic pieces a month on the average, and provides reports stating the results — the estimated time the sample was last fired, and its consistency with the suggested date of manufacture. The laboratory has been asked to determine the age of some important ceramic pieces, including pots, vases, human and animal figurines, dishes, and ladles belonging to the neolithic times, the Han, Sui, Tang, and Song/Jin periods. The laboratory is now a self-sup- porting unit under the Institute of Chinese Studies, the Art Museum, and the Physics Department. Income generated by its services is used to maintain its activities and research. Clients range from private ollectors (who constitute about 80 per cent of the total number) to art houses and institutions. O S h a l i n i B a h a d u r The Process of TL Dating Step 1: Sampling A small sample (about 30mg.) of the artifact is carefully extracted with a drill and put onto a small disc. Step 2: Heating The disc is put into a temperature control unit and heated. The amount of energy released in the form of light is indicated on the computer screen as a peak along a graph. Heating the sample A peak appears along the graph Step 3: Irradiating the sample byartificialradiation doses (to measure total energy stored) Standard artificial doses of radiation are applied to the sample to induce a peak similar in size to that obtained in step 2. The total amount of artificial radiation required should thus be equivalent to the total amount of natural radiation the sample has absorbed since it was made. It is called the 'equivalent radiation dose'. Step 5: Applying the age calculation formula In a hypothetical test where we assume the 'equivalent radiation dose' to be 9.5 units and the 'dose per year' 0.0045 unit, the age of the sample/ artifact can be obtained by a simple division of 9.5 by 0.0045: Age=EauivalentRadiationdose/Doseperyear=9.5/0.0045=2,111years Step 4: Measuring the natural radiation content of the sample The uranium, thorium, and potassium 40 content in the clay taken from the sample is then measured. From such measurement the annual dose of natural radiation absorbed by the sample per year can be derived. (The amount of daily radiation absorbed from cosmic rays and burial soil is either insignificant or difficult to measure and thus not directly calculated.) It is called the 'dose per year'.