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ISSUE 174 WINTER 2012

C A R P E T S • T E X T I L E S • I S L A M I C A R T

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24 HALI ISSUE 174

WHAT IS RADIOCARBON DATING?There are two important considerations whenplanning to submit a carpet, textile, or otherobject for radiocarbon dating. First, the samplematerial has to be organic; it must be fromsomething that was once alive. Then you haveto think about where the carbon is comingfrom. In other words, you need to think aboutthe connection between the sample and theevent to be dated.

All living things contain the element carbonand there are three naturally occurring forms(‘isotopes’): 12C, 13C and 14C. Carbon-12 andcarbon-13 are the stable forms, while carbon-14is radioactive. This means it spontaneouslydisintegrates to form another element whilegiving off a small amount of energy. Carbon-14is continually produced in the upper atmospherewhen cosmic rays strike nitrogen. The newly-formed 14C atom combines with oxygen toform carbon dioxide, which is mixed through -out the atmosphere.

By breathing, eating, or photosynthesising,a living organism continually ingests carbonand maintains a constant amount of the threeisotopes. As soon as an organism dies, it is no

longer taking in any new carbon and the 14Cbegins to decay. If we know how much 14C wasin an organism while it was alive and we canmeasure how much 14C is left in a sample, wecan calculate how much time has passed sincethe organism died.

Radiocarbon determines how long ago thesample material was alive, not the time ofmanufacture. For most textiles, this connectionis pretty obvious. Fibres will be woven into atextile soon after a cotton or flax plant isharvested, and wool or silk is made soon aftersheep-shearing or the collection of cocoons.Making the association between the age of atree and the manufacture of a table or asculpture can be more problematic in caseswhere species may live for hundreds of years.Old wood can be reused when large structuresare demolished or ancient trees are reclaimedfrom swamps. It is difficult for a 14C laboratoryto ‘date’ a painting or document, as the sampleto be analysed is usually the parchment, paper,or wood backing. The radiocarbon age willrelate to the animal or plant that provided thesupport material, not to the time the paint orink was applied.

WHEN IS A DATE NOT A DATE? A ‘radiocarbon date’ is not really a date, and‘radiocarbon years’ are not the same as calendaryears. What we are really measuring is how muchless 14C is in the sample than we would haveexpected to find in similar modern material. Wedo this by compar ing the amount of 14C to theamounts of 13C and 12C (‘isotope ratios’) andthen using the known rate of decay (‘half-life’)of carbon-14 to infer the amount of time thathas pas sed. But these lab or atory measurementsare sub ject to instrumental and measurementimpre cis ion; radioactive decay is a slowprocess taking many thousands of years, andwe can measure only a fraction of the wholeprocess. Therefore every radiocarbon agemeasurement is reported with an assessment ofthis uncertainty: the plus-or-minus error. If theradiocarbon age is expressed as a normal distri-bution, the ± denotes one standard deviation.As seen in 2 there is a 68% prob ability that theage of the sample falls within one standarddeviation and the probability rises to 95% if theerror is inc reased to two standard deviations.In modern radiocarbon laboratories usingAccelerator Mass Spectro metry (AMS) as the

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A CARBON-14 PRIMERCarbon-14 dating has become the subject of intense debate in the antiquesworld, but how many people really know how it works or what it can andcannot determine? Dr Christine Prior offers a concise guide

1 Red tapis with human figure, Lampung, south Sumatra:

Calibrated age range for NZA-33801, 364 ± 25 radiocarbon

years BP. Because of a wiggle in the calibration curve,

the 95% confidence interval intersects the calibration

curve at two points. While we know that the true age of

the tapis falls somewhere between 1452–1633 AD, in

the absence of other information we cannot tell whether

or not the textile was made in the first period,

1452–1526 AD or in the second, 1557–1633 AD.

See also ‘Red Tapis’, HALI 171, 2012, pp.72–9.

Courtesy Thomas Murray

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measurement technique, reported errors aretypically 20–30 years. Thus a radiocarbon ageis a statement of statistical probability of approx-imately how much time has passed since thesample was alive.

Calibration is a means of determining howclose a radiocarbon ‘year’ is to a true calendaryear. Calibration curves for radiocarbon havebeen developed by measuring the 14C in known-age tree rings. Most trees grow by adding a ringevery year. By analysing the patterns of thesegrowth rings and counting backwards, scientistshave developed tree rings sequences extendingback more than 10,000 years. A calibrationcurve can be produced by graphing the treering years on the X-axis and the measuredradiocarbon age on the Y-axis. If there were aone-to-one correspondence between 14C andcalendar years, this would be a straight line.

However, researchers quickly discoveredthat the calibration line has numerous wiggles.What they learned was that the 14C content ofthe atmosphere has varied over thousands ofyears in response to solar activity and variationsin cosmic rays. These wiggles have been recordedwith great precision through thousands of radio -

carbon measurements on known-year tree ringsover the past three decades. The data havebeen combined into the internationalradiocarbon calibration curves produced andupdated period ically. Numerous computerprogrammes have been written to represent thecalibration curves in a graphical format. Inorder to convert a radiocarbon age to calendaryears, it is entered into a calibrationprogramme and matched to the points where itintercepts the calibration curve.

It is commonly stated that ‘radiocarbon can -not date materials less than 200 years old’. Thisis not true. Carbon-14 laboratories can providevery precise ages for materials up to the present;it is the calibrated calendar ages that can be veryimprecise. For radiocarbon ages older than about200 years we are able to determine calendar yearages for a textile with a fair degree of precision,even if the 14C age intersects the calibration curveat more than one location. In the examples seen

here, a textile that yields a radiocarbon age of500 ± 25 years would calibrate to a very pre cisecalendar year age range of 1408–1444 3a. In this example, because of the steepness of thecalibration curve at that time, the ± 50 yearsof the radiocarbon age 95% probability convertsto a narrower 36-calendar-year age range. Unfor -tunately, some portions of the calibration curveare wigglier than others, so a radiocarbon ageof 900 ± 25 years crosses the calibration curveat more than one point and converts to acalendar year range of 1042–1107 or 1117–1210 3b.

The problem with calendar year ranges forradiocarbon dates of less than 200 years is thatafter 1700 the calibration curve becomes excep -tionally wiggly. Radiocarbon age calcula tionsare based on the measurement of how much14C is in a sample compared with how much ofthe other two isotopes of carbon are there, andit is this isotope ratio that is converted to aradiocarbon age. Because of the burning ofcoal and oil starting in the 17th century, largeamounts of 14C-depleted carbon dioxide (CO2)were released into the atmosphere and the‘normal’ isotope ratios were disrupted. Theeffect was similar to the normal wiggles in the

4 Two-band red tapis, Lampung, south Sumatra:

Calibrated age range for NZA-35105, 221 ± 15

radiocarbon years BP. Because of a wiggle in the

calibration curve, the 95% confidence interval intersects

the calibration curve at two periods, 1650–70 AD or

1781–99 AD. The small possibility that the textile was

made in the period 1945–50 AD has been discounted

on the basis of provenance information.

Courtesy Thomas Murray

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2 Normal distribution graph

illustrating 68% and 95%

probabilities for one and

two sigma

3a–3b Comparison of the

calibration graphs

for 500 ± 25 and

900 ± 25 years

3a 3b

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calibration curve produced by variation incosmic radiation, but were much morepronounced. So the radiocarbon content ofcotton harvested in 1850 is indistinguishablefrom that of cotton harvested in 1695, 1730, 1815or 1920. In the graph illustrating the calibrationof the 14C-age for the ‘big squid tapis’ 5, the fairlyprecise radio carbon age of 136 ± 25 crosses thecalibra tion curve at so many points that it isimpossible to determine on the basis of a singleanalysis when within a 300-year span the textilewas made. It is only through provenanceinformation that part of the calibrated age rangecan be excluded.

All is not bleak. Humans disrupted naturalcarbon isotope ratios before 1950 to thedetriment of radiocarbon dating, but furtheraltered the natural ratio after 1950 in a way thathas proved most beneficial: we can use 14C todiscern forgeries. Radiocarbon analysis candetermine with great precision whether or notwool or cotton was grown within the past 60years by analysing it for ‘bomb carbon’ – that is,whether or not the object is made of materialsthat grew after atomic bombs were detonated.

Atmospheric nuclear weapons testing after1950 released huge quantities of 14C into theatmosphere. We know how much excess bomb-14C was in the atmosphere for any year sinceabout 1950. If this is plotted with calendar yearson one axis and a measure of the excess 14C onthe other, it produces a graph we call the‘bomb curve’. Organisms alive after 1950 containmore 14C than they would have if atomic bombshad not been detonated.

One of our main services for museums,antiquities dealers and art conservators is toidentify counterfeits. We can’t tell if a carpetwas made in exactly 1800, but we can unques-tionably tell if it is a fake made in 1970.

INTERPRETING A RADIOCARBON DATEWhen a sample is submitted for radiocarbonanalysis, the client will receive a detailed reportthat gives the radio carbon age expressed in yearsBP (before present). The age will have a plus orminus error that reflects the uncertainty to onestandard deviation. If the calculated age comesout younger than 1950, most laboratories willreport the age as ‘modern’. There will also bea calibration report converting the radiocarbonage into calendar years.

The question everyone asks when confrontedwith a calibration report is: ‘But how old is it?’Perhaps the best way one might think of it isthat the radiocarbon age is the raw data, andthe calib ra tion is the interpretation.

Some calibration programmes display thegraph in other ways, but the most commonshow some thing similar to the illustrations here.In the graph showing the calibration for theradio carbon age 364 ± 25, the ‘tapis withhuman figure’ 1, the radiocarbon age is drawnas a normal distribution on the Y-axis withlines drawn to the one and two stan darddeviations intercepts on the calibration curve.The calendar years corresponding to the radio -carbon age are indicated on the X-axis; thepeaks in the graph are proportional to the degreeof overlap of the radiocarbon age with its errorand the calibration curve with its associatederror. This probability function is like asking:‘What is the probability that each point on thecalendar axis represents the calendar year corres -ponding to the measured radiocarbon age?’

Interpreting the calibration results for aradiocarbon age of 364 ± 25 is fairly straight-forward. There is a 95% probability that thesample dates to somewhere between 1452–1526or 1557–1633. We do not have enough evidencebased on a single radiocarbon analysis to

determine where in those two approximately70-year spans the ‘true’ age of the textile falls; just because the 1452–1526 period reflectsa slightly larger overlap between the radiocarbonage and the calibration curve (52.7%) than the 1557–1633 period (42.3%) does not meanthat the earlier period is more probable. Becausethe downward wiggle of the calibration curvefalls outside the 95% boundary, however, whatis fairly probable is that the age of the tapisdoes not fall within the 29 years between1527–56.

The graph illustrating the calibration for theradiocarbon age of the ‘two-band red tapis’ 4,221 ± 15 years, is even more striking. Thistextile was made either between 1650–70 orbetween 1781–99. Because of the last upwardwiggle in the calibration curve, there is a smallbut not insignificant probability (~6%) that thetextile could have been made between1945–50, but as with the ‘big squid tapis’ 5,provenance information enables us to discountthis possibility.

Some areas of the calibration may be morelikely than others, but in the absence of othersupporting evidence – such as provenancerecords, or the knowledge that a certain dyewas not used after a certain date – no ‘outlier’region of the calibration can be discardedbecause its peak is smaller.

Calibration is an enormously useful tool forthe interpretation of radiocarbon analysis –but it can also be enormously frustrating.

Christine Prior is the team leader for the RadiocarbonLaboratory at the GNS Science National Isotope Centre inWellington, New Zealand. After obtaining her Ph.D. atthe University of California with a specialisation inarchaeometry and radiocarbon dating, she worked in andmanaged radiocarbon laboratories for 25±5 years.

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5 Big squid tapis, Lampung, south Sumatra:

Calibrated age range for NZA-34146, 136 ± 25

radiocarbon years BP. Although the 95% confidence

interval for the radiocarbon age of this tapis intersects

the calibration curve at numerous places between 1673

and 1942 AD, provenance information allows further

interpretation of the radio carbon results. Since the tapis

predates the 1883 Krakatoa eruption, the 1909–42 portion

of the calibrated age range can be excluded. The calendar

age of the textile falls in either of the two periods,

1673–1778 AD or 1799–1891 AD, with equal probability.

Courtesy Thomas Murray

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