The following information has been kindly supplied by Oxford
Authentication Ltd, the Internationally recognised leader in the
authentication of antique ceramics. The company is headed by a qualified
physicist with almost 40 years experience in this field and is the only
company recognised by major international auction houses such as
Christie's and Sotheby's, as well as museums and other international
Oxford Authentication is the only TL testing laboratory in the world
working to full International quality assurance standards, with Lloyds
Quality Assurance, ISO 2001.
What is a TL test?
When a sample of pottery is heated to a sufficiently high temperature it
emits a faint blue light known as thermoluminescence, or TL. The amount
of TL can be measured and its intensity is proportional to the time
which has elapsed since the object was last heated, normally during kiln
firing, and can be used to date when the object was made.
Is the TL test still a valuable tool in authenticating antique
Yes, very much so as long as you know the strengths and
limitations. Several samples should be taken from each object at
TL analysis can then tell us the following:
Whether the clay is ancient or modern.
Whether the clay contains restoration material.
Whether the clay contains organic material, such as PVA,
a bonding material used to consolidate archaeological material. This may
be due to restoration or combined with powdered ancient clay to create
new artifacts. If a sample contains organic material, a distinctive TL
signal is obtained.
TL can detect the "marriage" of sections from unrelated pieces by
comparison of the TL signals from the different
If a modern copy has been artificially radiated in an attempt to fool
analysis, can the TL test detect this?
It is highly unlikely that artificial irradiation is used on ordinary
pottery as it is expensive and time consuming. However it is used on
porcelain, which in general will fetch a higher price than pottery on
the art market. Exposure to irradiation will create an artificially high
'TL age'. Oxford has devised a test for irradiated porcelain which looks
at four indicators to determine the likelihood that a piece has been
On a genuine Tang dynasty object (618 - 906AD) why is the quoted age
range so wide, i.e. 900 - 1500 years ago?
Excavated pottery cannot be dated to great precision as too much
information regarding the environmental radiation contribution to the TL
is lost when the pieces are removed from the ground. Very precise TL and
internal radiation measurements are made but these do not compensate for
the unknown factors. An estimate of these, based on experience, is
substituted in the equation to calculate the age.
The quoted age limits are +/- 20% of the calculated TL age. This is
based on the statistical analysis of hundreds of known age pieces.
Although quite wide, this distinguishes between ancient and modern
pieces, and hence determines authenticity.
TL remains a powerful weapon in the authentication of ancient ceramics
and Oxford Authentication is constantly evolving to combat the faker.
From a logical standpoint it is far better for a piece to have been TL
tested showing that the clay is ancient, rather than relying on the word
of the unqualified "expert" or vendor saying that the piece is genuine.
For more information see also:
Oxford Authentication Ltd
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