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LUMINESCENCE ANALYSIS OF FIRED CLAY FROM VIRGINIA 16 January 2017 James K.
Feathers Luminescence
Dating Laboratory University
of Washington Seattle, WA
98195-3412 Email: jimf@u.washington.edu This report presents the results
of luminescence analysis on some
fired clay from an apparent smelting site in northern Shenandoah
Valley, Frederick County, Virginia.
The sample, UW3151, is a
portion of furnace wall submitted by Adam Arkfield, the owner of the
property. Luminescence was evaluated
using fine-grain procedures. Coarse
quartz and feldspar grains were prepared, but both had poor luminescence
sensitivity. Work proceeded only with
the fine grains. Laboratory
procedures are given in the appendix.
Dose rate The dose rate was measured on the
sample and associated sediment. Dose
rates were mainly determined using alpha counting and flame photometry. The beta dose rate calculated from these
measurements on the clay sample was compared with the beta dose rate measured
directly by beta counting. These were
within 1-sigma error terms. Moisture
content was estimated as 80 ± 20 % of saturated value for the clay sample,
which was 7%. Cosmic dose radiation
was calculated as explained in the appendix.
Table 1 gives the radioactivity data and comparison of the beta dose
rate calculated in the two ways mentioned.
Table 2 gives the total dose rate. Table
1. Radionuclide concentrations
Table
2. Dose rates (Gy/ka)*
* Dose
rates for fine grains are calculated for
OSL. They will be higher for
TL due to higher b-values. Also the
beta dose rate is lower than that given in Table 2 due to moisture
correction. Equivalent Dose Equivalent dose on 1-8µm grains
was measured for TL, OSL and IRSL as described in the appendix. The TL plateau was broad (290-390°C. There was no sensitivity change with heating. Measured TL anomalous fading was
insignificant. OSL/IRSL was measured on 6 aliquots. Scatter was low
with over-dispersion less than 12%.
An IRSL signal was not detected on UW3151. IRSL stems from feldspars, which are prone to anomalous
fading. No IRSL suggests the OSL is
dominated by quartz. Moreover, the
OSL b-value, which is a measure of the efficiency of alpha radiation in
producing luminescence as compared to beta and gamma radiation, is in the
typical range of quartz. It is likely
the OSL signal stems mainly from quartz and does not fade. As a test of the SAR procedures, a dose
recovery test was performed and the derived dose was within two sigma of the
given dose. Equivalent dose values
and b-value are given in Table 3. Table 3.
Equivalent dose and b-value – fine grains
Ages For UW3151 the TL age
and the OSL age were in statistical agreement. The age is given in Table 4.
Prehistoric smelting has previously been unknown in the United States,
although it was present in South America about this time. Table
4. Ages
Procedures for Thermoluminescence
Analysis of Ceramics Sample preparation -- fine grain The fired clay is broken to expose a fresh
profile. Material is drilled from the
center of the cross-section, more than 2 mm from either surface, using a
tungsten carbide drill tip. The
material retrieved is ground gently by an agate mortar and pestle, treated
with HCl, and then settled in acetone for 2 and 20 minutes to separate the
1-8 µm fraction. This is settled onto
a maximum of 72 stainless steel discs.. Glow-outs Thermoluminescence is measured by a Daybreak reader
using a 9635Q photomultiplier with a Corning 7-59 blue filter, in N2
atmosphere at 1°C/s to 450°C. A
preheat of 240°C with no hold time precedes each measurement. Artificial irradiation is given with a 241Am
alpha source and a 90Sr beta source, the latter calibrated against
a 137Cs gamma source.
Discs are stored at room temperature for at least one week after
irradiation before glow out. Data are
processed by Daybreak TLApplic software.
Fading test Several discs are used to test for anomalous
fading. The natural luminescence is
first measured by heating to 450°C.
The discs are then given an equal alpha irradiation and stored at room
temperature for varied times: 10 min, 2 hours, 1 day, 1 week and 8
weeks. The irradiations are staggered
in time so that all of the second glows are performed on the same day. The second glows are normalized by the
natural signal and then compared to determine any loss of signal with time
(on a log scale). If the sample shows
fading and the signal versus time values can be reasonably fit to a logarithmic
function, an attempt is made to correct the age following procedures
recommended by Huntley and Lamothe (2001).
The fading rate is calculated as the g-value, which is given in
percent per decade, where decade represents a power of 10. Equivalent dose The equivalent dose is determined by a combination
additive dose and regeneration (Aitken 1985). Additive dose involves administering incremental doses to
natural material. A growth curve
plotting dose against luminescence can be extrapolated to the dose axis to
estimate an equivalent dose, but for pottery this estimate is usually
inaccurate because of errors in extrapolation due to nonlinearity. Regeneration involves zeroing natural
material by heating to 450°C and then rebuilding a growth curve with incremental
doses. The problem here is
sensitivity change caused by the heating.
By constructing both curves, the regeneration curve can be used to
define the extrapolated area and can be corrected for sensitivity change by
comparing it with the additive dose curve.
This works where the shapes of the curves differ only in scale (i.e.,
the sensitivity change is independent of dose). The curves are combined using the “Australian slide” method in
a program developed by David Huntley of Simon Fraser University (Prescott et
al. 1993). The equivalent dose is
taken as the horizontal distance between the two curves after a scale
adjustment for sensitivity change.
Where the growth curves are not linear, they are fit to quadratic
functions. Dose increments (usually
five) are determined so that the maximum additive dose results in a signal
about three times that of the natural and the maximum regeneration dose about
five times the natural. A plateau region is determined by calculating the
equivalent dose at temperature increments between 240° and 450°C and
determining over which temperature range the values do not differ
significantly. This plateau region is
compared with a similar one constructed for the b-value (alpha efficiency),
and the overlap defines the integrated range for final analysis. Alpha effectiveness Alpha efficiency is determined by comparing additive
dose curves using alpha and beta irradiations. The slide program is also used in this regard, taking the scale
factor (which is the ratio of the two slopes) as the b-value (Aitken 1985). Radioactivity Radioactivity is measured by alpha counting in
conjunction with atomic emission for 40K. Samples for alpha counting are crushed in
a mill to flour consistency, packed into plexiglass containers with ZnS:Ag
screens, and sealed for one month before counting. The pairs technique is used to separate the U and Th decay
series. For atomic emission measurements, samples are dissolved in HF and
other acids and analyzed by a Jenway flame photometer. K concentrations for each sample are
determined by bracketing between standards of known concentration. Conversion to 40K is by natural
atomic abundance. Radioactivity is
also measured, as a check, by beta counting, using a Risø low level beta GM
multicounter system. About 0.5 g of
crushed sample is placed on each of four plastic sample holders. All are counted for 24 hours. The average is converted to dose rate
following Bøtter-Jensen and Mejdahl (1988) and compared with the beta dose
rate calculated from the alpha counting and flame photometer results. Both the ceramic and an associated soil sample are
measured for radioactivity.
Additional soil samples are analyzed where the environment is complex,
and gamma contributions determined by gradients (after Aitken 1985: appendix
H). Cosmic radiation is determined
after Prescott and Hutton (1994).
Radioactivity concentrations are translated into dose rates following
Guérin et al. (2011). Moisture Contents Water absorption values for the ceramics are determined
by comparing the saturated and dried weights. For temperate climates, moisture in the pottery is taken to be
80 ± 20 percent of total absorption, unless otherwise indicated by the
archaeologist. Again for temperate
climates, soil moisture contents are taken from typical moisture retention
quantities for different textured soils (Brady 1974: 196), unless otherwise
measured. For drier climates,
moisture values are determined in consultation with the archaeologist. Procedures for Optically Stimulated or
Infrared Stimulated Luminescence of Fine-grained ceramics. Optically stimulated luminescence
(OSL) and infrared stimulated luminescence (IRSL) on fine-grain (1-8µm)
samples are carried out on single aliquots following procedures adapted from
Banerjee et al. (2001) and Roberts and Wintle (2001. Equivalent dose is determined by the
single-aliquot regenerative dose (SAR) method (Murray and Wintle 2000). The SAR method measures the
natural signal and the signal from a series of regeneration doses on a single
aliquot. The method uses a small test
dose to monitor and correct for sensitivity changes brought about by
preheating, irradiation or light stimulation. SAR consists of the following steps: 1) preheat, 2) measurement
of natural signal (OSL or IRSL), L(1), 3) test dose, 4) cut heat, 5)
measurement of test dose signal, T(1), 6) regeneration dose, 7) preheat, 8)
measurement of signal from regeneration, L(2), 9) test dose, 10) cut heat,
11) measurement of test dose signal, T(2), 12) repeat of steps 6 through 11
for various regeneration doses. A
growth curve is constructed from the L(i)/T(i) ratios and the equivalent dose
is found by interpolation of L(1)/T(1).
Usually a zero regeneration dose and a repeated regeneration dose are
employed to insure the procedure is working properly. For fine-grained ceramics, a preheat of
240°C for 10s, a test dose of 3.1 Gy, and a cut heat of 200°C are currently
being used, although these parameters may be modified from sample to sample. The luminescence, L(i) and T(i),
is measured on a Risø TL-DA-15
automated reader by a succession of two stimulations: first 100 s at 60°C of
IRSL (880nm diodes), and then 100s at 125°C of OSL (470nm diodes). Detection is through 7.5mm of Hoya U340
(ultra-violet) filters. The two
stimulations are used to construct IRSL and OSL growth curves, so that two
estimations of equivalent dose are available. Anomalous fading usually involves feldspars and only feldspars
are sensitive to IRSL stimulation.
The rationale for the IRSL stimulation is to remove most of the
feldspar signal, so that the subsequent OSL (post IR blue) signal is free
from anomalous fading. However,
feldspar is also sensitive to blue light (470nm), and it is possible that IRSL
does not remove all the feldspar signal.
Some preliminary tests in our laboratory have suggested that the OSL
signal does not suffer from fading, but this may be sample specific. The procedure is still undergoing study. A dose recovery test is performed by first zeroing the
sample by exposure to light and then administering a known dose. The SAR protocol is then applied to see if
the known dose can be obtained. Alpha efficiency will surely
differ among IRSL, OSL and TL on fine-grained materials. It does differ between coarse-grained
feldspar and quartz (Aitken 1985).
Research is currently underway in the laboratory to determine how much
b-value varies according to stimulation method. Results from several samples from different geographic
locations show that OSL b-value is less variable and centers around 0.5. IRSL b-value is more variable and is
higher than that for OSL. TL b-value
tends to fall between the OSL and IRSL values. We currently are measuring the b-value for IRSL and OSL by
giving an alpha dose to aliquots whose luminescence have been drained by
exposure to light. An equivalent dose
is determined by SAR using beta irradiation, and the beta/alpha equivalent
dose ratio is taken as the b-value. A
high OSL b-value is indicative that feldspars might be contributing to the
signal and thus subject to anomalous fading. Age
and error terms The age and error for both OSL
and TL are calculated by a laboratory constructed spreadsheet, based on
Aitken (1985). All error terms are
reported at 1-sigma. Ka is thousand
years before 2016. References Adamiec, G., and Aitken, M. J., 1998, Dose rate
conversion factors: update. Ancient TL 16:37-50. Aitken, M. J.,
1985, Thermoluminescence Dating,
Academic Press, London. Auclair, M., et al., 2003. Measurement of anomalous fading for feldspar IRSL using SAR. Radiation Measurements, 37: 487-492. Banerjee, D., Murray, A. S., Bøtter-Jensen, L., and
Lang, A., 2001, Equivalent dose estimation using a single aliquot of
polymineral fine grains. Radiation Measurements 33:73-93. Bøtter-Jensen, L, and Mejdahl, V., 1988, Assessment of
beta dose-rate using a GM multi-counter system. Nuclear Tracks and Radiation
Measurements 14:187-191. Brady, N. C., 1974,
The Nature and Properties of Soils,
Macmillan, New York. Galbraith, R. F., and Roberts, R. G., 2012. Statistical aspects of equivalent dose and
error calculation and display in OSL dating: an overview and some
recommendations. Quaternary Geochronology 11:1-27. Guérin, G., Mercier, N., and Adamiec, G., 2011,
Dose-rate converstion factors: update.
Ancient TL 29:5-8. Huntley, D. J., and Lamothe, M., 2001, Ubiquity of
anomalous fading in K-feldspars, and measurement and correction for it in
optical dating. Canadian Journal of Earth Sciences
38:1093-1106. Mejdahl, V., 1983, Feldspar inclusion dating of
ceramics and burnt stones. PACT 9:351-364. Murray, A. S., and Wintle, A. G., 2000, Luminescence
dating of quartz using an improved single-aliquot regenerative-dose
protocol. Radiation Measurements 32:57-73. Prescott, J. R., Huntley, D. J., and Hutton, J. T.,
1993, Estimation of equivalent dose in thermoluminescence dating – the Australian slide method. Ancient
TL 11:1-5. Prescott, J. R., and Hutton, J. T., 1994, Cosmic ray
contributions to dose rates for luminescence and ESR dating: large depths and
long time durations. Radiation Measurements 23:497-500. Roberts, H. M., and Wintle, A. G., 2001, Equivalent
dose determinations for polymineralic fine-grains using the SAR protocol:
application to a Holocene sequence of the Chinese Loess Plateau. Quaternary
Science Reviews 20:859-863. |
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