This is Oxford Authentication’s frequently asked questions page. FAQs are listed questions and answers pertaining to our services and antiquity authentication. If you don’t find the answer to your query here please do not hesitate to contact us and we will be in touch.
What can you test?
Can you test pieces from any culture?
Do you need a database?
Why do you quote such a wide date range?
What is spurious TL?
What is the meaning of “unreproducible results”
What is the youngest piece we can submit for testing?
What is the oldest piece you have tested?
What do you mean by “resin”?
What is an organics test?
What is a pigment test?
What is the difference between an authenticity test and dating a piece?
X-rays and artificial irradiation.
Why do you drill a hole in porcelain rather than cutting a fragment?
What else can TL tell us?
Is TL sufficient for authenticity?
We can test fired clay such as pottery, earthenware and terracotta, as well as porcelain, stoneware and the casting cores of bronzes.
Yes, as long as they have been fired to above 500°C and the clay contains a sufficient quantity of TL minerals (quartz and/or feldspars) as well as radioactive inclusions (normally uranium, thorium and a radioactive form of potassium). More recent pieces can be harder to test.
No, we do not require a database. TL is an absolute method which relies on measurement to calculate an age. Radiocarbon dating is the only other absolute method on an archaeological timescale.
We quote an age range of ±20% for authenticity testing. Under most circumstances this is sufficient to distinguish an 18th century piece from a modern copy or a 17th century piece from a 19th century copy.
Our measurements are very precise but accuracy is limited. It is due to:
• The loss of information regarding the burial site and surroundings when the object is excavated, so we cannot know the extent of irradiation from its environment. The need to take a small drilled sample from the object rather than crushing a fragment. This limits the number of measurements which can be made.
• All authenticity tests from all laboratories are subject to these limitations, regardless of what limits are actually quoted. An authenticity test is not the same as dating an archaeological site where the object is excavated under controlled conditions and the theoretical limit of accuracy is ±6%. A much more comprehensive set of measurements is needed to give the more accurate date. This is rarely justified for authentication, since our normal tests are usually fully sufficient to identify modern imitations.Analysis of our results shows that 92% of our tests either give ages consistent with those expected within the 20% limits or the piece is modern. The results of another small percentage of the tests lie just outside the 20% range, and there is little doubt whether these pieces are ancient or modern. Of the remainder, some cannot be dated because of problems with the material sampled: there may be a spurious signal (see below), or there is contamination, or the samples are too insensitive to radiation. Other pieces will have been mis-attributed. Occasionally, pieces have to be re-tested.
The datable TL signal comes from quartz and feldspar crystals in the clay, the so-called TL minerals. For them, the TL signal grows linearly with age. However, if there are no TL minerals, or if there are other minerals which decompose on heating and produce TL, there will be a spurious signal that swamps the archaeological signal, so the piece cannot be dated.
We make several measurements on each sample. Sometimes we find that we get two different, inconsistent results. This is usually due to problems with the clay. We do not issue any report and ask for more sample so that we can try to get a definite result.
In authenticity testing, usually we want to distinguish between modern copies and original pieces. So one important question is when the modern copies might have been made. If there were no modern copies until the 20th century then, providing the sample is sensitive and we do not obtain a spurious signal, the practical limit for a trustworthy test is the 18th century. We regularly test Kangxi porcelain, and Staffordshire ware pottery. Italian Renaissance pieces are more difficult. Here the pieces were frequently copied in the 19th century and so that even these 19th century copies have a TL age. The latest pieces in this case would be the 17th century for reliable testing.
The oldest pottery comes from the site of Hacilar in Turkey. The pottery fertility idols from the site are 7000-8000 years old.
Resin is an organic substance which can be used to mould or bind clay. We use the term rather loosely when we mean that we have detected some organic material in the clay which modifies the shape of the glow-curve. See organics test.
What is an organics test?
If we suspect that a sample contains some organic material, we subcontract for FTIR (Fourier Transform Infra Red) spectroscopy measurements to be carried out on a small portion of the sample. If there is organic material present, the spectrum will consist of a series of peaks and troughs corresponding to the different chemical compounds. Pure clay has no peaks or troughs in the spectrum.
We can test to see if a pigment contains a modern element or not, or uses some pigment not available at the time the object was supposed to be made. This test, which is subcontracted, will tell us if the object has been repainted.
Authenticity aims to distinguish between an original piece and a modern copy. Dating aims to give a precise historical date to an object. We can only date an object if it has been excavated under controlled conditions and if we can monitor the burial site in situ. If a large number of fragments in each location is dated, a mean value for that location can be computed. The quoted accuracy is ±6%. So much information has been lost by the time an object reaches the art market that the quoted age limits cannot realistically be better ±20%. In most cases this is sufficient to determine the piece’s authenticity.
Irradiation will modify the TL response. No appreciable affect on the ultimate conclusions is to be expected from low doses of diagnostic x-rays or from airport or postal x-rays. However, in some cases, pots are exposed to high medical radiation doses intentionally, so as to increase the apparent radiation dose and hence to mislead as to the apparent age of the piece. This is not an easy or cheap procedure, but we know this is taking place, principally on porcelain. We have now developed a method to detect this through its effects on the TL signal.
Porcelain is very hard and has to be cut using a diamond drill under running water to keep the sample cool. Any excessive heating would interfere with the TL. A hollow core drill is the safest way of doing this. The core is cut into thin slices using a fine, water-cooled diamond blade and the slices used for TL (pre-dose) measurements. Some other laboratories crush the sample after cutting, but this not only causes overheating but the crushing puts in extra TL signals, unrelated to the archaeological age, which give an erroneous result. We have seen glow-curves from such laboratories which indicate a huge radiation-induced signal, wholly unrelated to the age of the piece.
TL can indicate:
Whether the clay is fired or unfired.
Differences in clay types.
Treatment with organic material (can be supported by an organics test).
Restoration using plaster or other material which is not clay.
TL is only one tool in the investigation for authenticity. It cannot give the complete picture although it can do many things. It gives an absolute, objective measurement of the time since the clay at the sampling site was fired. In addition it can detect unfired clay and restoration material. If samples are taken from several locations on the piece, TL can indicate if sections are made from a similar clay. As an independent dating tool, it is invaluable. Several other methods will be needed to get the complete picture. For example,
* X-rays will show joins and restoration which may not be visible under the outer layers.
* Various methods can be used to detect the elemental composition of a pigment.
* FTIR (see above) will find organic materials in the clay, which may have been used for consolidation or even re-modelling.