Dating Ancient Pottery: Thermoluminescence

There are many ways of establishing the date range of a given artefact, through the context the artefact was found in, by comparing its style to similar artefacts with a well established date range, or by using scientific methods.

Through Comparative Style

With regards to Cypriot pottery, most date ranges are well established, if you come across a white slip Tankard you immediately know it's likely late bronze age, from around 1600 to 1200 BC. You can then try to figure out what kind of white slip Tankard you have based on the style of pattern, if for example it only has simple parallel lines that seem rushed in application, it's likely a White Slip III (also referred to as II Late) and therefore from around 1300 to 1200 BC.

Things are not always so easy, with some styles having been produced over several hundred years, such as specific Cypro-Geometric or Archaic styles, or sometimes you are dealing with sherds or heavily damaged pottery where the style is not clear.

Through Context

When it comes to artefacts that are dug up by professional Archeologists, the context surrounding the location of this artefact are more important than the artefact itself. For example if some small non-descript sherds are found, but these happen to be in an undisturbed tomb next to some White Slip III, the sherds are likely from the same time period.

This comparative style is also used to date things that are harder to scientifically test than pottery. For example if a White Slip III is found in a tomb with a specific type of bronze dagger, the dagger might be harder to date than the pottery, but the pottery can be used to date the dagger.

Why Use Scientific Methods?

There are two primary reasons to use scientific methods to date pottery - either all above methods have failed, or you suspect that the artefact may be a modern forgery or replica. So what scientific methods should one use to date pottery?

What is Thermoluminescence Dating and How does it Work?

Thermoluminescence dating is a scientific technique used to determine the approximate date when certain ceramics or rocks were last heated.

Let's try to break down the science without needing a Physics degree to understand it.

Crystals and Electrons

The main ingredient in pottery and ceramics is clay, which is composed of tiny crystal particles.

Imagine a crystal as a neat and orderly arrangement of atoms, like a grid. However, in reality, these crystals are not perfect. They have some defects or impurities that disrupt the regular pattern of the electric field that holds the atoms together.

These imperfections create tiny "hills" and "valleys" in the crystal's electric landscape. The "valleys" are called "electron traps" because they can catch and hold onto free electrons.

Radiation from outer space and natural radioactivity can give some electrons in the crystal enough energy to break free from their atoms and move around. Most of these free electrons will quickly find a new place to settle down, but some might fall into the "electron traps" and get stuck.

Depending on how deep these "traps" are, how much energy an electron needs to escape, the trapped electrons can stay there for a very long time, sometimes even hundreds of thousands of years. This way, the crystal stores a part of the radiation energy in the form of trapped electrons.

Freeing the Electrons

Heat is energy. When you heat the crystal, you give the trapped electrons a boost of energy. If the heat is strong enough, it can help the electrons escape from their traps and return to their original positions in the crystal!

As the electrons return to their original positions, they release the extra energy they had in the form of light. This is the phenomenon of thermoluminescence. The amount of light emitted is proportional to the number of electrons that were trapped, which in turn depends on the amount of radiation the crystal was exposed to over time.

How this Helps Measure Time

When the potter from thousands of years ago placed the ceramic in their Kiln and fired it, they unknowingly freed all of the trapped electrons in the crystal. Once the ceramic cooled down, it started trapping electrons again, but from scratch!

For example, let's arbitrarily say that a piece of ceramic traps 1 electron per year. The clay or ceramic may have been around for many years and trapped hundreds of thousands since it was last heated. The ancient potter, let's say in 1000 BC, then took it, made it into a shape, and fired it in a kiln resetting the count to zero.

Every year thereafter, it collected 1 electron per year, until 2024 when it had collected over 3000 of them. So if you now heat the ceramic again, it would release these 3000.

How does that help us measure time? Well what if you could count these electrons? You could then count the number released upon heating, and know how many years have passed since the ceramic was last fired!

Measuring the Freed Electrons

When these electrons are freed, a small amount of light (luminescence) is emitted. The more electrons are freed, the more light. Scientists can use very sensitive instruments to measure this light, with more light being emitted indicating more electrons were freed, and therefore more time has passed since the ceramic was last fired.

How it Works in Practice

So do scientists in a lab take all this ancient pottery and place it in an oven? Of course not! That would cause an extreme amount of damage.

Instead what is usually done is that several small samples are drilled out of the pot in inconspicuous places, such as the inside of the rim or the base.

Several samples are usually taken to avoid the risk of a single sample being an outlier and to avoid forgeries, you'll learn why this helps in the later section discussing forgeries.

These samples are then crushed into a fine powder, which is then heated in a highly controlled fashion under laboratory conditions. The light emitted is then measured by a sensitive instrument, and the number of electrons released is counted.

Limitations and Flaws in Thermoluminescence Dating

Thermoluminescence dating, as with many scientific tests, is often seen as a flawless method of irrefutably proving the age of an artefact. However, there are several limitations and flaws in the method that must be considered when interpreting the results.

Thermoluminescence tests measure the rough date when the pottery was last fired. They do NOT measure if an artefact is ancient! Their only task is to tell you when the pottery last reached a certain temperature.

Accuracy

Commercially available Thermoluminescence tests will provide a confidence interval for the date range. Rather than stating that an object is from 1000 BC, they may say that it is from 1000 BC +/- 300 years, meaning from 1300 BC to 700 BC.

This is very important to take into account, because using our White Slip example above, this means that TL tests would be useless to figure out if something was produced at the very end of the Bronze Age or several hundred years earlier!

Composite Forgeries

Recall that the entire ceramic is not tested, but rather small samples that are drilled out from various spots in the pottery. While this is done to avoid damaging the pottery, it also opens up the possibility of a composite forgery.

Let's say I am a forger, and I have the base of a genuine ancient terracotta statue. It would be far more valuable if it also had a head, sadly it is missing. So through my skill I forge a separate head, and glue it onto the body.

To make my forgery appear genuine I order a TL test, however I am careful that samples are only taken from the base, which as we know is genuine. The TL test comes back and says that my terracotta statue is thousands of years old! When we very well know that the head is just from last month.

Composites have been used in fraudulent ways, however most well established TL testing labs are aware of this and take steps to avoid it by sampling from around the pottery.

Purposeful or Accidental Reheating

So what happens if one takes a sherd that was fired in 1000 BC, and puts it in a kiln in 2024 for a long period of time? The test will show that the pottery was last fired in 2024, not 1000 BC. Does that mean the peace is not ancient? No!

Reheating can reset the "clock" - If an artifact was heated enough after its original firing, this can drain all the stored energy, resetting its thermoluminescence signature to that reheating event's date instead of the original manufacture date.

Real Ancient Pottery can Still be a Forgery?

Let's again imagine that I am a skilled forger and got my hands on the very real Bichrome jug seen in Artefact 1.

It's a nice jug but it's not very valuable, I managed to buy it for only $500 and I'm looking to make a very large profit. What I can do is take the real ancient jug, and simply draw on some patterns that would massively increase its value, for example an animal as in Artefact 2.

I can then order a TL test to make it appear more genuine. No matter where the lab drills out samples, they'll find that they are all genuinely ancient.

This is the risk in misunderstanding scientific tests. a TL test does not tell you whether something is genuine, only when the ceramic was last fired.

Faking Thermoluminescence through Artificial Radiation

Let's say I am an even more skilled forger, and want to create an ancient looking jug from scratch in order to fully control its appearance and quality. Once the jug is made and fired however, even with my fancy use of acids, chemicals and mechanical aging process, a simple TL test would reveal that it is in fact modern.

Recall how the TL test works, it measures the amount of trapped electrons in the crystal. If I can artificially add electrons to the crystal, I can make it appear as if the ceramic is far older.

TL tests assume that the ceramic was exposed to background radiation, which as we mentioned is quite small. I could speed this process up by dosing my ceramic with a massive amount of radiation, for example by using an X-Ray machine. If done very carefully, I could control the process and make it appear to be any age I want.

In 2012, a group of 7 forgers were arrested in Italy for forging Greek and Etruscan artifacts. One member of this group was a Nurse at a local hospital, and used the hospital's X-Ray machine to artificially age the ceramics. They were only caught due to the suspicious amounts of high quality ancient artefacts they were producing in a short period of time.

Commercial and Private Thermoluminescence Testing

There are many labs that offer TL testing services at various price ranges and qualities. If you are interested in such a test, it is important to first know what you are trying to find out.

Are you trying to get a TL test to prove to others that it is genuinely ancient? In this case it's important to use a reputable well known lab that has experience in dealing with forgeries, and is known to deliver accurate results.

Some private labs that are rather well known would be Artemis Labs, but do note they are also an auction house/dealer of antiquities so supporting them may be a conflict of interest and Oxford Authentication.

TL tests cost between $200 and $500 at most labs. This sometimes but not always includes the sampling process.

Shipping and moving antiquities is a risky endeavor, many labs offer at home testing where a sampler will come over to your institution or home and take samples there. This is usually more expensive but also less risky.

Conclusion

Thermoluminescence dating is a powerful scientific technique that can provide valuable insights into when ceramic objects were last fired, helping to establish their age and authenticity. By measuring the light emitted when crystalline materials are heated, TL testing estimates the time elapsed since the electrons in the crystal lattice were last freed by high temperatures.

However, it's crucial to understand the limitations and potential pitfalls of TL dating. The technique only determines when a ceramic was last fired, not necessarily when it was originally made. Accuracy is limited to a range of hundreds of years. Purposeful reheating, whether accidental or fraudulent, can reset the TL "clock." Composite forgeries can yield deceptive results if only genuinely ancient portions are tested. And artificial irradiation can be used to fake greater antiquity.

Therefore, thermoluminescence should be seen as one important tool in the larger context of authentication, alongside comparative stylistic analysis, archeological provenance research, and expert assessment. Understanding both the strengths and limitations of TL testing is key to properly leveraging this fascinating application of physics to archeology. Wise collectors rely on multiple lines of evidence, not just a single scientific date, when evaluating the likely age of antiquities. By appreciating both the power and limitations of the technique, we can most effectively harness thermoluminescence to shed light on the ancient past.