What is thermoluminescence testing for porcelain, ceramics and bronzes?

Risoe-TL-OSL-DA20-OXford-Authentication-TL-testing-1 What is thermoluminescence testing for porcelain, ceramics and bronzes?

Another super quick post but a link worth including here: https://www.oxfordauthentication.com/

One thing to be aware of is that thermoluminescence testing works only with fired items, in other words items like porcelain, earthenware or bronze. If you don’t know what thermoluminescence testing is have a quick read of the snippet below from their website:


Thermoluminescence

image22-150x150-1 What is thermoluminescence testing for porcelain, ceramics and bronzes?

A small sample of ancient pottery will emit a faint blue light when heated to a sufficiently high temperature. This faint blue light is known as thermoluminescence, or TL and is over and above the background red glow that is emitted from all materials. The TL can be measured using a sensitive detector known as a photomultiplier tube. The intensity of the TL signal is proportional to the time which has elapsed since the clay was last heated, normally since the kiln firing, and can be used to date when the object was last fired.

Preparation

When we receive your sample we must first prepare it for measurement.

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Powder samples

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

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.

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Measurements

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.



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Porcelain slices
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Cutting a porcelain slice

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.

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Genuine
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Glow-curve from genuine Fat Lady
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Modern
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Glow-curve from modern Fat Lady

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.

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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.

Reprinted from https://www.oxfordauthentication.com/

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