The drawback with both dendrochronology and radiocarbon dating is that they rely in the preservation of organic material. In waterlogged sites like Flag Fen, or in the arid climes of the Sahara, such material is frequently preserved for thousands of years. However many sites preservation of organic material is at best patchy. To dendro-date a log of the right species of tree, usually oak, has to survive more or less intact. To carbon date a fair sized sample usually around 20g has to have survived uncontaminated. It’s not always practical to hope for such finds and so another method of dating was required by archaeologists. The answer came with thermoluminescence, which uses an interaction between background radiation and clay to provide another ‘clock’. The use of clay is a positive boon as, after flint, it is the most durable material to survive in archaeological deposits.
Thermoluminescence relies upon clay, flint or stone having an imperfect crystal lattice structure, which is riddled with flaws. When placed in the soil a pot sherd is bombarded by natural radiation. The radiation hits atoms in the clay and liberates electrons. The electrons get trapped in the lattice’s imperfections and remain as a marker. If we know what dose of radiation the potsherd receives annually, and count the electrons in the traps to find out the total dose, then we can work out how long the sherd has been in the soil. We do however have two problems to consider. How do we know there weren’t already electrons in the traps before the sherd was discarded? If there were already electrons in the structure before the pot was discarded then the total dose we measured would include time before we wanted our count to start. Crucially we also need to know exactly how we can hope to measure the electrons in the traps.
The solutions are linked. As the term thermoluminescence suggests, heat is applied to the sample. The relationship between the heat of a body and the intensity of light that it emits is well known. The principle of black body radiation shows a direct link between the wavelength and intensity of light and its temperature. This is how astronomers can deduce the temperature of star surfaces. The colder stars glow a sullen red, and the vigorous stars emit a piercing blue. The scale of colour to temperature is due to a uniform physical process. Similarly the glowing of clay under heat is also uniform. A pot heated to 500°c will always glow the same way.
The exception is that a sherd will glow with greater light if there are trapped electrons. The heat gives the electrons the energy to escape the lattice. Once free they lose energy by emitting light, which adds to the pot glow. If a pot is held at a steady 500°c for a while the extra glow can be used to measure the number of electrons trapped. Once the traps are emptied the excess glow disappears.
The use of temperature is also necessary for firing the pot in the in the first place. The temperatures are many hundreds of degrees Celsius and are held for several hours. This will have the effect of firmly emptying any traps in the lattice of electrons and setting the clock to zero. Because of this we can be certain that any electrons realised during thermoluminescence tests have entered the pot after it was last fired.
Authenticity is a problem for art. As familiarity with the past grows so do our abilities to fake its appearance. However, short of dangerous mucking about with radiation, it is hard to fake the ageing effects that can be measured by thermoluminescence. It is through this method that Fleming, Jucker and Riederer were able to identify several Etruscan wall paintings as modem fakes (Fleming, Jucker & Riederer 1971:143-167). The thermoluminescence results were able to add objective weight to subjective judgements about the quality of the art that had already caused concern.
These four methods are perhaps the most common forms of dating, but each (bar calendrical dating) has been developed with relatively recent years to cope with arising situations. Further understanding in natural radiation has expanded facilities to date material. Potassium-Argon dating allows archaeologists to peer further back into the past with inorganic material. New forms studying electron traps and their formation have created new techniques for gaining more data. The established techniques are being further refined.
“How old is it?” If we don’t already know the right answer, then we’re getting closer.
For other methods of dating see the contents page of the series Dating for Archaeologists.
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