Time Tested

July 1, 2009

We have been researching the early Chinese ceramics in the Pacific Asia Museum collection. A component of this research involves testing the ceramic material to learn when it was created using thermoluminescence (TL) testing. When a ceramic is fired, the heat of the kiln resets the clay’s age “clock” to zero.

The clay contains minerals that absorb radiation from both from the clay’s internal radioactivity as well as from its burial environment. The radiation damage produced dislodges electrons which are then trapped in the clay. Heating a sample in the laboratory releases these electrons which each emit a photon of light, which is thermoluminescence. The amount of thermoluminescence is proportional to the time which has elapsed since the object was last fired in the kiln.

A small sample of the clay was taken from several spots on the objects by Dr. David Scott. This sampling was done in dark working conditions so that light would not alter the sample. Our testing was made possible with funds generously provided by the Carpenter and Getty Foundations, allowing us to work with Oxford Authentication labs, the largest laboratory in the world dedicated solely to TL testing of ceramic antiquities.

We sampled ten objects from the collection and five came back with results consistent with dates of manufacture in the Han (206 BCE–220 CE) and Tang (618–906 CE) dynasties. The other five objects came back with inconsistent results that point to a number of possibilities: the object might have been damaged and then restored at a later time, it might have been made as a later composite from fragments of earlier pieces, or it might be of recent manufacture. These scientific results, in tandem with art historical research, help us refine our understanding of the objects in the collection. In sharing this information with our visitors, we can illustrate the complications in dating objects and establishing authenticity.

Bridget Bray
Assistant Curator and Collections Manager

Cow, China, Han Dynasty (206 BCE-220 CE), Earthenware and pigment, Pacific Asia Museum, Gift of Mr. and Mrs. Robert M. Snukal. 2001.43.71AB.

Dr. David Scott and Bridget Bray prepare to take samples for the testing.