Applications of TL dating
In archaeology TL is mainly used for
pottery analysis. In anthropology the main use of TL is
the dating of flint stone as early tool material for mankind.
In the art and art dealer world TL is mainly
used to identify possible forgeries The Jemaa Head
terracotta; 500BC
Nok culture, Nigeria Neandertaler tools
Age Range for TL dating
TL 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 to saturation effects, filling all the trap configurations, a flattening of the growth curve may occur
around 10000-15000 years. A typical saturation occurs at a total accumulated dose of 100-500Gy. Saturation is
different for different material depending on mineral content and annual dose. Correction for saturation effects therefore need to be applied!
TL
age
Linear growth- curve saturation
Short-lived
contributions
Evolution of Tools
Dating Flint stone
Flint is the name given to chalcedony nodules that occur in chalk.
Chalcedony is a form of quartz and similar TL characteristics as quartz 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 pre-palaeolithic thermo-
luminescence that has not been reset in the firing process.
preparation of flint
Careful 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 γ radiation
After removing outer
material flint stone can
not be exposed to light
to avoid light induced
luminescence effects!
TL glow curve of flint
The maximum of the glow curve of flint is at 370
oC, glow curve
is weak because of relatively low annual dose from γ radiation.
Age range for TL dating with flint
Flint stone has a longer age range than clay based material (pottery, porcelain). Due to the lack of α and β dose the
γ dose is only 10
-3Gy/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 Palaeolothic 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 the manage
to live together?
Neanderthal Man
TL dating of flint as evidence
Dating of flint at the different layers of Kebara and Qafzeh caves (Israel).
At the Kebara site a Neanderthal skeleton was discovered, dating of flint stone in same layer indicated 60 000y of age, at nearby Qafzeh 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 forgeries
Big 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; ~100mg are necessary for TL test.
Thermoluminescence tests revealed that this ceramic horse, supposedly from the T'ang dynasty (617-908 AD), was 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
40 per cent of objects tested by the Thermoluminescence Laboratories are proven to be fakes; half the antiquities brought for sale at Sotheby’s
are fake; 5,000 forgeries of ancient art enter the market each year.
To overcome the handicap of TL dating forgers expose sample to radiation. 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.
What γ dose is necessary for creating a TL signature of a T’ang porcelain horse for a 20 year old forgery?
[y]
[Gy/y] 1200 [Gy]
dose annual
paleodose
Age + ⋅ =
=
= D
t D
P γ
recipe for TL fakes
The annual dose is characteristic for clay material
[Gy/y]
10 18 5
[Gy/y]
10 24 1 10
58 1 10
36 2
3
3 3
3
−
−
−
−
⋅
=
⋅ +
⋅ +
⋅
=
+ +
⋅
= . D
) .
. .
( D
) D D
D k (
D
α β γ[Gy]
6.11 [Gy]
[Gy/y]
10 .18 5 [y]) 20
- [y]
(1200 [Gy]
[Gy/y]
10 .18 5
[Gy]
[y] [Gy]
1200
[y]
[Gy/y] 20 10
.18 5 Age [Gy]
3 - 3
- 3 -
=
⋅
⋅
=
⋅
= +
⋅ =
=
γ γ
γ
P P
P P
P
for 1200 years of age you have to create a γ dose of 6.11 Gy
How long does it take?
Suppose you have a strong γ source with 10
8[Bq] for irradiation:
] [
5 . 2 ]
[ 75 ]
[ 10 54
. ] 6
[ 10
34 . 9
] [
11 . 6
] / [
10 34
. 9 ]
/ [ 10
34 . 9
] 10
6 . 1 1
[
] /
[ 10 84
. 5
] / [ ] 10
[ 25
] [
46 . 1
6 7
7 7
19 6
8
months d
Gy s t Gy
t s Gy t
s g J
P
J eV
t s g MeV
P
t g s
t MeV m
t E D
P
=
=
⋅
⋅ =
=
⋅
⋅
=
⋅
⋅
⋅
=
⋅
=
⋅
⋅
⋅
=
⋅
⋅
=
⋅
=
⋅
=
−
−
−
− γ
γ
γ
γ
γ
Irradiation time would be still 2.5 months to fake proper TL dose!
The African Ram
$275 000 at Sotheby’s
How to Circumvent
Thermoluminescence Tests
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)
How it is done
In 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."
