To TL or not to TL, that is the question… Item Number: 100021 $0.00

DescriptionLately, I’ve been doing, recommending and defending TL-testing more and more... but, wait you say… just what the heck is TL-testing? For those of you who may not have been involved in collecting ancient art for long, let’s start with the basics. ‘TL’ stands for thermo-luminescence and is a testing procedure that’s reasonably effective in accurately dating ancient pottery. All materials absorb radiation as they age and TL-testing determines the amount of stored radiation since an object was last fired at a temperature of over 500 degrees Celsius (932 degrees Fahrenheit). In most cases, pottery was ‘last fired’ in a kiln when it was made. In rare occasions, pottery may have been exposed to high temperatures during a fire, at which case the readings will indicate the time elapsed since that fire. That’s the basic definition, let’s review the testing procedure itself.

The TL procedure is rather simple. Two or three very small holes are drilled into a piece of pottery. The powder from these holes is carefully collected and sent off to a laboratory that measures the amount of radiation released when this powder is heated. The lab will then analyze the results and determine an age for the powder samples (which would be the pottery itself). In most cases, dates can be determined +/- about 20%, which means an item that tests at 2000 years may actually be between 2400 and 1600 years old. In the vast majority of cases, age is less relevant than determining whether or not the pottery is ancient. As discussed previously, we know approximate ages by other means. The TL-test simply confirms the pottery is ancient or recently fired.

There are several labs around the world that offer TL-testing. The one most widely accepted by major auction houses and galleries is Oxford Authentication in The UK. They have several approved sample collectors in the US and throughout Europe, and they charge $400 per item for the test, plus the cost of the individual taking the samples/preparing the background information to the lab (about another $150). Kotalla Lab in Germany is another highly regarded testing lab, and the one we use most frequently. Their prices are a bit more modest (approximately $350 per item) and the fee includes our time to drill the item/prepare documentation – but this is not a sales message…

Now that we know a bit more about the procedure, when should you TL a piece? Obviously, you do not want to spend more to TL an object than it’s worth. In most cases, we recommend that pieces be tested when their value exceeds $3000; however, we certainly do not recommend that every piece over $3000 be tested. The best rule of thumb is test when the value is high enough and when there are reasons to doubt an item’s authenticity. If you obtain an item with great provenance, has been vetted by experts and they all agree the piece is unquestionably ancient, why put holes in it? But, if you have an item that gives you reason to pause, perhaps did not come from such a reputable source, and you paid a good chunk of change for it, then it might make sense to conduct this test.

Caveats? There are always caveats! TL-testing is not always effective. Some types of pottery (West Mexican ceramics for example) simply do not retain radiation as well as others, which means results will sometimes come back inconclusive. Also, good forgers often will often use pieces of old pottery to construct a new fake. Perhaps the best example of world-class fakes might be several Chinese workshops that have been known to recreate ancient figural pottery (horses, dogs, pigs, people, etc) by carving far less valuable ancient paving tiles. They will glue them together to make a solid block and carve away until an ancient animal appears. Very clever… and very difficult to uncover – and TL-tests will only confirm whether the item is indeed ancient. Restoration also plays a role. In several cases we have seen items test ancient from one sample, and modern from another. This means the piece is either restored using new clay, or a “pastiche” pieced together from old and new pieces. And finally, you have to hand it to the forgers. When one test comes around to uncover the fakes, forgers get creative to better hide their fakes. We are hearing of forgers exposing ancient potteries to massive doses of X-rays. While the labs are not sure this works to hide actual ages, it certainly makes one nervous.

If you happen to require TL services, we can either help you directly, or certainly point you in the right direction.

Happy collecting!
Bob Dodge