Notes on Thermoluminescent Testing

In deciding if thermoluminescent testing (TL) is the way to go, it's important to know it's not just a cut-and-dried process. First, you have to have a piece that is large enough to allow the drilling and taking of a sample, and in some cases several samples. And secondly, you should have an understanding of the material used during the Yuan / Early Ming period versus that used during later reigns. If the piece in question is suspect of possibly being late Yuan / Early Ming, taking an additional sample might be a good idea.  My most recent experience with Oxford is a good example of this. In arranging the taking of the sample, I told the owner that probably 4 or 5 samples would be taken. I even provided an image showing such as an example. I made this statement based on the strong probability of the piece belonging to the 14th century. I was surprised that only one small sample was taken.  After about 5 weeks Oxford said they were unable to get a valid reading and requested 2 additional samples for testing. I almost expected this. A second session had to be scheduled for the drilling and taking of additional samples.  This second session could have been avoided.

The following information will explain how fourteenth century material differed from fifteenth century, and the impact it has when it comes to TL testing.

Kaolin (China-clay) and baidunzi¹ (China-stone) are the two main ingredients used in porcelain. Knowing the quality of the primary clay kaolin is important to having a full understanding of 14th and early 15th century porcelain.

During much of the 14th century, the quality (purity) of the kaolin itself depended largely on its geographical source. Northern and Southern China are the result of two tectonic plates that merged. This explains the many geological / mineral differences between Northern and Southern China and, of specific interest, that of the primary clay kaolin and its feldspathic properties.

Though glaze² offers a very high content of quartz / feldspar, it can not be used in TL dating since its exposure to surface light for hundreds of years would affect the TL results, giving a false reading. It's because of this, as pointed out by Doreen Stoneham of Oxford Authentication, that the glazed portions are removed from all core samples before testing.

The kaolin of Northern China was mixed at an approximate 50/50 with baidunzi. Kaolin for plasticity and strength, baidunzi for translucence. This 50/50 mix provides a high content of quartz and other crystalline feldspathic minerals required for TL testing.

The kaolin of Southern China was sometimes mixed with baidunzi. Sometimes baidunzi was not added. And the kaolin stone itself was just partially kaolinized, originating from partially decomposed granite, often containing mica and flourite. In addition (or should I say subtraction) to this, when baidunzi was used, it was not added 50/50, but just 10-15% for translucence. Though true proto-porcelain dates to the Shang period, pieces made using this coarse-grained, more porous southern variety of China-clay are also sometimes referred to as proto-porcelain.

Note that SiO₂, silicon dioxide (quartz) and Al₂O₃, aluminum oxide (alumina) make up most of the crystalline feldspathic property in both. Traces [bold] of calcium, sodium, potassium, and barium (also feldspathic) contribute as well. Though the kaolinite breakdown shown is from a northern China-clay sample, it allows us to estimate what a degraded southern China-clay property might be. The baidunzi is from a hammer milled and levigated stone formed into a baidunzi brick. It came from a hammer mill on the premises of the Sanbao Ceramics Institute near Jingdezhen.

Taking all the above into consideration, worst-case³ pattern with a partial decomposition factor of 75%, one can estimate that an early piece from a southern kiln will have about 27% less silica and feldspathic content. 

 The properties needed for a valid TL reading.

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¹ [Baidunzi] - Baidunzi is the now accepted Pinyin transliteration for what was previously known by the Wade-Giles term petuntse (or petunse). Like cobalt, baidunzi was used sparingly on proto-type porcelain until methods were proven. After the 14th century, baidunzi was mixed in with southern variety kaolin at approx 50/50, matching the method already in use in Northern China. Refinement methods also improved.

² [Glaze] - Only the whitest baidunzi rocks containing the greenest specks were selected and further refined to create the glaze. A small amount of oxide of manganese was also added, as well as wood and fern ash during the process.

³ [Worst-case] - Degraded southern kaolin (85.12 count minus 21.28) = 63.84% versus 90.6% northern kaolin / baidunzi mix.

Kaolin [Al₂ Si₂ O₅ (OH)₄]

Note: Feb 2nd, 2007

Just about every site on the Internet that mentions TL testing concludes that Oxford Authentication of England is the most recognized and widely acceptable source. Many sites also mention Daybreak Archaeometric Laboratory of Connecticut as also being widely recognized and accepted. Just today I had a nice conversation with Victor Bortolot, owner / director of Daybreak. He has informed me that Daybreak Archeometric Laboratory has discontinued TL testing of porcelain for the time being. He is presently working on a project in China and does not know when / if he will resume TL services for porcelain.