Let’s start with breaking down the word. Thermoluminescence can be broken into two words: Thermo, meaning head and Luminescence, meaning an emission of light. It essentially means that some materials that have accumulated energy over a long period of time will give off some light when exposed to high heat.
Thermoluminescence dating is used mostly on pottery and other inorganic materials such as burnt flint. It’s a very popular dating method in archaeology because not only can it date pottery, the type of material we find the most when excavating, but it can also date further back than 50,000 years unlike radiocarbon dating. That, and it much cheaper in comparison to other dating methods.
Thermoluminescence is another form of radioactive dating, except this time we’re measuring the amount of radiation gained over time, instead of radiation lost like in Carbon 14 dating. Ceramics are made from geological material, inorganic material, right? They use clay and sand and a bunch of other stuff from the ground to make these pieces. And all these geological things contain radiation. Materials that are used for pottery are crystalline when you look at them under the microscope, and they essentially form this lattice pattern or net when all the atoms are bonded together. When the atoms in this lattice are exposed to nuclear radiation, individual electrons in get all hopped up on this energy and become detached. They then become trapped in lattice defects, which are caused by missing atoms, or from the presence of impurities in the mix. This is why we call them electron traps!
Of course, this isn’t just a one time thing. Over time, more radiation will be absorbed into the object, and more electrons will break free and subsequently, get trapped in the lattice. If the absorption of radiation happens at a constant rate (something we call the annual dose), then the electrons will accumulate uniformly over time, and the size of the population of these electrons can be measured and directly related to the total amount of radiation that the object has been subjected to (which we call the total dose). This of course relates directly to the total time that the object or specimen has been exposed to radiation, and we can calculate it (in theory) with a simple equation:
Age = Total Dose / Annual Dose
The elements that we get the Annual Dose from are uranium, thorium, and the radioactive isotope of Potassium which is potassium 40. These isotopes emit all the things; alpha particles, beta particles and also GAMMA RAYS, the stuff superheroes are made of. Alpha and beta particles don’t penetrate the surface very much, so they can’t get into the whole specimen, but Gamma rays on the other hand can travel up to 20 cm into the object, which is great news for us!
These isotopes have super long half lives, and their emissions are assumed to be constant over time, unlike Carbon. This makes it very easy to measure the annual dose part of the equation by looking today’s emission levels.
Now we need to get the total dose by measuring the trapped electrons inside the artefact. This is where thermoluminescence comes in. When pottery is made, it needs to be heated at very high temperatures to become hard, strong, and durable. This heating also provides energy to the electrons that are trapped in the lattice, and they’re able to escape their prison. This means that the electron count gets reset to zero, and the accumulation process begins all over again. So if the pot was fired, and just used for cold storage, or other activities that don’t bring it in contact with high heat, we can measure the accumulated electrons and get the date for when this object was made. But, if the pot was used for cooking, or another activity that requires high heat, the electron count resets each time it’s being heated. This means that when we date a cooking vessel or something like that, we get the date for the last time it had been used.
This method of heating and releasing the electrons from their traps is how thermoluminescence works. We reset the clock to find out how long is has been running for. First, you shave away the top few millimetres from the pottery to get rid of the alpha and beta particles, we don’t want those to emit light because it won’t give us an accurate reading. We want the much deeper gamma rays that have penetrated the entire fabric of the ceramic.
Then the material is heated to 500 degrees Celsius or above in a lab. The energy lost by the electrons as they make their great escape is actually emitted as light radiation. The luminescence is measured and is directly proportional to the number of trapped electrons which gives us the total radiation dose! Then you plug all those numbers into the equation, and you get your age.
Now of course, you can’t just sample one piece and get a date for a whole site, it’s also vital to usually measure the radiation accumulation in the soil that it was found in, and you need to measure multiple samples to make sure the analysis was correct. The precision of this technique is +/- 10 percent of the age that was calculated, so older pottery of course has a larger margin of error. Let’s not forget that once you heat a thing, the clock gets set back to zero, and it can’t be dated properly anymore. This can cause lots of problems for checking authenticity of objects. I’ve actually heard stories of art forgers in china using CAT scan machines and X-rays to add radiation to an artefact so if it is tested, it’ll be thought of as original. So beware, friends! As a conservator, I also have to note that this method is destructive to the artefact, as samples need to be taken from the object, so it’s also an ethical question as to whether interference with the object is needed in order to find a proper date.
That being said, pottery is the most abundant thing we find on site, so I’m sure there will be a sample or two without any diagnostic features that can be used for this dating method.
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Historica Canada: Dating in Archaeology by Michel Gagné
Forman SL. 1989. Applications and limitations of thermoluminescence to date quaternary sediments. Quaternary International 1:47-59.
Liritzis I, Singhvi AK, Feathers JK, Wagner GA, Kadereit A, Zacharais N, and Li S-H. 2013. Luminescence Dating in Archaeology, Anthropology, and Geoarchaeology: An Overview. Cham: Springer.
Seeley M-A. 1975. Thermoluminescent dating in its application to archaeology: A review. Journal of Archaeological Science 2(1):17-43.
“Dating In Exposed and Surface Contexts”, ed.: Beck, Charlotte. University of New Mexico Press: Albuquerque, NM, 1994.
Michels, Joseph. “Dating Methods in Archaeology”. Seminar Press, New York: NY, 1973