3.10. Applications of TL dating

In anthropology the main use of TL is the dating of flint stone as early tool material for mankind.

In archaeology TL is mainly used for pottery analysis.

The Jemaa Head terracotta; 500BC Nok culture, Nigeria

In the art and art dealer world TL is primarily used for the identification of possible forgeries

Neandertaler tools

Age Range for TL datingTL dating on pottery can reach back to earliest pottery samples,which is circa 10000 years ago. The age limit for TL dating mostly depends on the mineral being used and its quartz content. Due tosaturation effects, filling all the trap configurations, a flattening of the growth curve may occur around 10000-15000 years. Typical saturation occurs at a total accumulated dose of100-500 Gy. The saturation level is different for different materials depending on themineral content and annual dose. Corrections for thesesaturation effects thereforemust be applied!

TL

age

Lineargrowth-curve

saturation

Short-lived contributions

Evolution of Tools

Dating Flint stoneFlint is the name given to chalcedony nodules that occur in chalk. Chalcedony is a form of quartz and similar TL characteristics asquartz inclusions in pottery clay are expected. Flint has been used as major tool material in Palaeolithic times and is therefore ideally suited for thermoluminescence dating.

Two handicaps:• external γ-radioactivity dominates the annual dose which is

therefore substantially lower than for pottery material.• middle and lower Palaeolithic flint is often not sufficiently burned.That may cause errors due to possible pre-Paleolithic thermo-luminescence that has not been reset in the firing process.

preparation of flintCareful preparation of the flint stone is necessary, the outer parts have to be removed to only analyze the inner flint sections, which do not have been exposed to external α or β radiation from the soil material,

the annual dose is only based on the internal γradiation

After removing outer material flint stone can not be exposed to light to avoid light induced luminescence effects!

the annual dose is only based on the internal γ-radiation.

The maximum of the glow curve of flint is at 370oC, glow curve is weak because of relatively low annual dose from γ radiation.

TL glow curve of flint

Age range for TL dating with flintFlint stone has a longer age range than clay based material (pottery, porcelain). Due to the lack of α and β dose the γdose is only 10-3 Gy/y. A saturation is reached much later at 100 000-500 000 y. The possibility for reaching 1 000 000 y exist for samples with very low radioactivity.

Because of the increased age range TL dating of flint has been a perfect tool for exploring the early history of mankind in the Paleolithic age (50 000 – 10 000 BC) while TL dating of early ceramics has emerged as an important tool for dating evolution and migration of Neolithic man (10 000 – 3000 BC).

Neanderthal and Cro-Magnon

Did they manageto live together ?

Neanderthal Man

TL dating of flint as evidenceDating of flint at the different layers of Kebara and Qafzehcaves (Israel).

At the Kebara site a Neanderthal skeleton was discovered, dating of flint stone in same layer indicated 60 000 y of age, at nearby Qafzeh a Cro-Magnon skeleton was discovered with flint stone dating of 92 000y of age. This is strong evidence of early presence of Cro-Magnon man.

A quick tool for detecting forgeriesBig demand at Sotheby’s, Christies and other art auction houses for verification of antiquity claim. Mostly Chinese and Italian renaissance porcelain. Samples are taken by small drill technique; ~100 mg are necessary for TL test.

Thermoluminescence tests revealed that this ceramic horse, supposedly from the T'ang dynasty (617-908 AD), is a fake. The base and the underside of the horse are made from genuine pieces of porcelain from the period, but the saddle, neck and tail are more recent.

Forger’s learn physics

To overcome the handicap of TL dating, forgers expose sample to radiation. A difficulty is however the homogeneous distribution of natural radioactivity versus external exposure. Efficient exposure can be done only with γ sources, external β exposure would only increase TL from surface material of sample due to the short range of β radiation. TL tests indeed indicate that several fake porcelain pieces have been artificially β radiated to produce TL signature.

40% of all objects tested by the Thermoluminescence Laboratories are proven to be fakes; half the antiquities brought for sale at Sotheby’s are fake; about 5,000 forgeries of ancient art enter the art market each year.

[y]1200[Gy/y]

[Gy]dose annual

paleodoseAge =⋅+

==D

tPP γ

What γ dose is necessary for creating a TL signatureof a T’ang porcelain horse for a 20 year old forgery?

recipe for TL fakesThe annual dose is characteristic for clay material

[Gy]6.11[Gy]

[Gy/y]10.185[y])20-[y](1200[Gy][Gy/y]10.185

[Gy][Gy][y]1200

[y]20[Gy/y]10.185

[Gy]Age

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333

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for 1200 years of age you have to create a γ dose of 6.11 Gy

How long does it take?

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γγ γ

Suppose you have a strong 40K γ source with 108 [Bq] for irradiation:

Irradiation time would be about 2 hours to fake proper TL dose!

The African Ram

Having partially reconstructed the history of the Kuhn ram -- its export to the United States by antiquities dealer Samba Kamissoko and its acquisition by New Orleans dealer Charles Davis, I wanted to learn how Amadou faked objects that deceived experts in African art. Even if Amadou were capable of reproducing the ancient terra cottas, how was he able to circumvent thermo-luminescence dating tests? (see The Limits of TL)

$275 000 at Sotheby’s

How to Circumvent Thermoluminescence Tests

How it is doneIn addition to joining larger authentic pieces to fabricated parts, Amadou explained that he digs "holes into the clay where I can bury fragments of authentic terra cotta found at the [looted] sites; I do this after firing the new clay. As far as the piece [the Kuhn ram] you showed me is concerned, I put ancient fragments in the two hind legs and other pieces in the stomach.' Amadou's explanation was believable; I had heard about the technique a few years earlier from an Italian restorer, who described it to me as "both risky and infallible -- infallible because TL can't distinguish an inserted part from the rest of the object ... and hazardous because it's necessary that the TL technician choose to test in an area where authentic pieces were inserted."

I was interested to find out if this method worked, so I traveled to Daybreak Nuclear and Medical Systems, Inc., the Guilford, Connecticut, TL lab that had tested the Kuhn Ram. I hoped I'd be able to see the test results. After a good amount of hesitation (client documents are generally confidential), Victor Bortolot, the lab's' director, agreed to search his archives for dossier 2OIA36. Judging the object to be authentic, the technician at the time had taken only a single sample, from under the left front leg, an authentic part of the piece. Hence the favorable test results published in the Sotheby's catalog, hence the high price fetched by the piece. "But don't forget that this file dates to March 1988," Bortolot said. 'At the time our practice was to make only one test if we felt the object was good. But now, with the great number of fakes circulating, it's necessary to make at least two."

New technologiesInstead of drilling a hole for collecting TL sample material, lasers can be used for heating a selected spot for localized TL release.

TL curve

Comparison of a radiation activated sample with a radioactivity free sample, which contained no TL.

Nondestructive in situ thermoluminescence using CO2 laser heating; J. L. Lawless, S. K. Kam, D. Lo; Optics Express 10 (2002) 291