Principles of the TL - Method

There in was given evidence that the measuring of thermoluminescence emitted from objects made of fired clay could be effectively used for the dating and verification of such objects. Quarz and feldspar as well as a number of other minerals have the ability to store energy generated by radioactive radiation. Under exposure to great heat such minerals release this energy again in form of light impulses. Radioactive radiation is created by the traces of uranium-, thorium-, kalium-, and rubidiumisotope which can be found almost everywhere in the earth's crust.

Clay, which is used in the production of every day objects as well as objects of art, generally contains such minerals and radioactive isotopes. All radioactive energy accumulated and stored by the unfired clay in geological time is destroyed at the point of firing. After a period of cooling the energy storing process starts anew and a certain amount of stored energy is gained annually. At the re-heating of a material sample taken from the fired object impulses of emitted light can be measured in the laboratory which correspond with the time interval between the present observation and the last firing. These findings have provided us with the basic principles of scientific dating methods.

The TL-test concerns itself with 3 variables:

1. The so-called 'archaeological energy ' or N T L - . the name already indicates that an archaeological, i.e. historical aging has been registered since the last firing of the object.

2. The 'neutral energy ' - ß or NTL + ß (alpha) which is the amount of energy emitted by the same material after renewed exposure to radiation from a gauged radioactive source.

3. The 'annual energy rate ' or J D
which represents the annual increase of accumulatively stored energy.

These variables interrelate in the following ways:

archaeological energy                            archaeological dose
--------------------------------  =  archae.dose    -----------------------------  =  age
neutral energy                                       annual energy rate

Archaeological / historical and neutral energy rates already provide initial clues to the final test result.

a) In a case where archaeological energy can be established the neutral energy curve, in certain areas, runs proportional to the archaeological curve.
b) In the case of a recent copy no proportional run of curves can be observed due to the obvious absence of any archaeological / historical energy. In order to determine the 'annual energy rate and thereby the exact age of an object, the object specific energy output of the three basic radiation types; alpha, beta and gamma rays, has to be defined. At this point which touches at the boundaries of nuclear physics, the exact dating of objects of art often becomes problematic.
A precise evaluation of the gamma radiation energy is only possible when the exact geological conditions at the location of origin of the examined find are known. Since this is very often no longer possible an insecurity factor of +/- 20 to 35% of the calculated age has to be taken into account. Alpha and beta rates can be measured in the material samples taken from the object. The exactness of the result, i.e. the calculated age depends on the grade of precision with which these values can be determined. A relative limitation of such insecurities can be archived with the provision of comparative data from secured excavation sites and other safe' sources. Empiric values which can often serve to fill the gaps left in the purely analytical evidence supplying process. The Laboratory Kotalla, as one of the oldest institutes of its kind, is consequently in the fortunate position of being able to take full advantage of theirextensive archives.