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Thermo luminescence Dating
The thermo luminescence technique is the only physical means of
determining the absolute age of pottery presently available. It is
an absolute dating method, and does not depend on comparison with
similar objects (as does obsidian hydration dating, for example).
Most mineral materials, including the constituents of pottery, have
the property of thermo luminescence (TL), where part of the energy
from radioactive decay in and around the mineral is stored (in the
form of trapped electrons) and later released as light upon strong
heating (as the electrons are de-trapped and combine with lattice
ions). By comparing this light output with that produced by known
doses of radiation, the amount of radiation absorbed by the material
may be found.
When pottery is fired, it loses all its previously acquired TL, and
on cooling the TL begins again to build up. Thus, when one measures
dose in pottery, it is the dose accumulated since it was fired,
unless there was a subsequent reheating. If the radioactivity of the
pottery itself, and its surroundings, is measured, the dose rate, or
annual increment of dose, may be computed. The age of the pottery,
in principle, may then be determined by the relation: Age =
Accumulated dose / Dose per year
Although conceptually straightforward, TL has proven to to be far
from simple in practice. In all, close to two dozen physical
quantities must be accurately measured to establish the relationship
between doses of different kinds of radiation and light output, and
to compute dose rate. A leaflet from Daybreak describing the TL
technique in more detail and giving a bibliography will be provided
to interested persons.
The phenomenon of thermo luminescence was first described by the
English chemist Robert Boyle in 1663. It was employed in the 1950’s
as a method for radiation dose measurement, and soon was proposed
for archaeological dating. By the mid-1960’s, its validity as an
absolute dating technique was established by workers at Oxford and
Birmingham in England, Riso in Denmark, and at the University of
Pennsylvania in the U.S.. The Research Laboratory for Archaeology at
Oxford, in particular, has played a major role in TL research.
While not so accurate as radiocarbon dating, which cannot date
pottery (except from soot deposits on cooking pots), TL has found
considerable usefulness in the authenticity of ceramic art objects
where high precision is not necessary.
What is the accuracy of TL Dating?
Studies at Oxford back in the 70’s on Romano-British pottery
indicated that when all quantities entering the age equation are
measured, the TL date of a single potsherd will typically fall
within 15 per cent of the known date. When dates of a number of
shards associated together are averaged, the error is reduced
typically to 7-10 per cent. This is for well-behaved samples only.
The succeeding 30 years, and increased understanding of the
dosimetry, have not brought much improvement.
Unfortunately, it is not possible to achieve this precision for the
majority of art objects. Among the reasons for this is the small
amount of material that may be taken for testing. Drilling, the
usual method of sampling, introduces some uncertainty. It is also
rare that any information about the radiation from the burial soil
can be obtained, as art objects are usually thoroughly cleaned.
This radiation may in some cases contribute over half the total
dose. Finally, one has to make the measurements regardless of
whether the TL of the clay is well-behaved or not. Some clays are
hardly thermo luminescent at all; some may not have a straight-line
relationship between dose and TL; spurious luminescence due to
chemical or pressure effects may mask the radiation-induced TL;
occasionally, a condition called “anomalous fading”, where part of
the TL is unstable, may lessen the accuracy of the dose measurement.
Generally speaking, when a sample is drilled and there is no
information available about the burial environment, one may expect
up to 40 per cent uncertainty. This is adequate for the purposes of
authentication where the question is whether the piece was fired in
antiquity or recently; it will not differentiate, say, between a
classic Greek terra cotta and a Roman copy. In some categories of
objects, from China, for example, the actual age is quite precisely
known for short-lived styles, and it is possible to work “backwards”
to get information about the environment in many parts of the world,
and some other parameters not usually measurable for art objects.
Using this information often reduces the uncertainty to 15-25
percent.
What Materials can be Dated by TL?
Nearly any mineral material which has been heated above 500C at a
time one wishes to know isa candidate for TL dating. This includes
all forms of pottery. Porcelains, being nearly vitrified, are a
special case requiring a fairly large solid core sample, and TL
dating of intact objects is not recommended because of the damage
caused by sampling. Most porcelain dating is done for insurance
purposes on broken objects. Much stoneware is not so hard as
porcelain and may be sampled by drilling. The clay cores from lost
wax metal castings may readily be tested. Heated stone material,
such as hearths, pot boilers, and burnt flints, has been dated as
well. Some regions known to present problems for TL include
Indonesia and West Mexico; objects from these areas usually do not
successfully yield TL dates.
Visit Oxford Authentication Ltd. for
more info on TL Testing
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