TL TESTING PROCESS
WHAT IS THERMOLUMINESCENCE?
When a small sample of ancient pottery is heated it glows with a faint blue light, known as thermoluminescence or TL. During its lifetime the pottery absorbs radiation from its environment and it is this which creates thermoluminescence. The older the pottery, the more radiation it has absorbed and the brighter the pottery sample glows. By measuring the TL, we can calculate how much radiation has been absorbed and use this information to calculate the approximate age of the pottery.
Further details of the Hampton Court terracotta roundels project can be found on the Historic Royal Palaces website
When we receive your sample we must first prepare it for measurement.
Powder samples (from pottery and bronze cores) are mixed with acetone and allowed to settle, so that fine grains, approximately 1/100mm. diameter, can be selected. These grains are deposited and dried onto aluminium discs (for fine-grain analysis) or rhodium (for pre-dose analysis). Any remaining powder is dried and used for radioactivity measurements to complete the dating calculation.
Porcelain cores are glued into thin hollow tubes. When the glue is dry, they are cut into slices 1/4mm thick with a fine diamond blade. The blade is water cooled to prevent overheating. Each slice is soaked in acetone after cutting to remove the glue. Slices are then ready for TL measurement. The remaining core is crushed and used for radioactive analysis to complete the dating calculation.
We have 3 fully automated, computer operated Riso Minisys TL readers for measuring the TL. Sample discs are mounted on a wheel and the readers are programmed to run heating and irradiation sequences. The TL is measured using a sensitive detector called a photomultiplier tube. The total amount of radiation the pottery has absorbed during its lifetime can be calculated from the TL. The annual dose of natural radioactivity within the pottery can be measured in the laboratory using counters. From these measurements we can obtain an approximate age for the piece.
Fine Grain method
The samples are heated and the data appears as a graph of TL against temperature, called a glow-curve. The samples are irradiated in the laboratory with a known radiation dose and heated to produce another glow-curve. By comparing the glow-curves we can calculate the dose of radiation absorbed by the piece during its lifetime. Radioactive measurements on the clay tells us how much radiation the piece is receiving each year. This enables us to calculate the approximate age of the piece. Using TL we can see that one of these ‘fat ladies’ is genuine and the other is a modern copy.
For both pieces: curve (a) is the TL emitted by a sample of powder taken from the object, curve (b) is a laboratory induced glow-curve curve (c) is the background. The ancient piece : (a) is way above the background (c), and approximately midway between background and (b) The modern piece : (a) is only just above the background (c) and way below (b)
The Pre-dose Method
Porcelain and certain other types of clay cannot be tested using the fine-grain method. We then have to use the pre-dose method. The TL reader is programmed to measure changes in the 110oC peak of quartz (the pre-dose peak) in the clay. Each time the sample is irradiated and then heated, the pre-dose peak increases. The increase is related to radiation dose. The first increase is due to the natural dose which the piece has absorbed over its life-time. The sample is then given a laboratory irradiation and a second increase is measured. From these measurements we can calculate the age of the piece.
The black curve is the background (base-line) measurement The red and blue curves are the archaeological TL signals The green curve is from the applied laboratory dose For the genuine vase, the archaeological signal(s) are well above the background and close to the signal from the applied laboratory dose. For the modern vase the archaeological signal is barely above background and well below the TL signal from the applied laboratory dose.