What not to do in dating
What not to do in dating
The thermoluminescence technique is the only physical means of determining the absolute age of pottery presently available. It is an absolute dating method, and does not depend on comparison with similar objects (as does obsidian hydration dating, for example).
Most mineral materials, including the constituents of pottery, have the property of thermoluminescence (TL), where part of the energy from radioactive decay in and around the mineral is stored (in the form of trapped electrons) and later released as light upon strong heating (as the electrons are detrapped and combine with lattice ions). By comparing this light output with that produced by known doses of radiation, the amount of radiation absorbed by the material may be found.
When pottery is fired, it loses all its previously acquired TL, and on cooling the TL begins again to build up. Thus, when one measures dose in pottery, it is the dose accumulated since it was fired, unless there was a subsequent reheating. If the radioactivity of the pottery itself, and its surroundings, is measured, the dose rate, or annual increment of dose, may be computed. The age of the pottery, in principle, may then be determined by the relation
Although conceptually straightforward, TL has proven to to be far from simple in practice. In all, close to two dozen physical quantities must be accurately measured to establish the relationship between doses of different kinds of radiation and light output, and to compute dose rate. A leaflet from Daybreak describing the TL technique in more detail and giving a bibliography will be provided to interested persons.
The phenomenon of thermoluminescence was first described by the English chemist Robert Boyle in 1663. It was employed in the 1950's as a method for radiation dose measurement, and soon was proposed for archaeological dating. By the mid-1960's, its validity as an absolute dating technique was established by workers at Oxford and Birmingham in England, Riso in Denmark, and at the University of Pennsylvania in the U.S.. The Research Laboratory for Archaeology at Oxford, in particular, has played a major role in TL research.
While not so accurate as radiocarbon dating, which cannot date pottery (except from soot deposits on cooking pots), TL has found considerable usefulness in the authenticity of ceramic art objects where high precision is not necessary.
Since the university laboratories involved with TL are research facilities, they generally will not accept art objects for authentication on a routine basis. The TL laboratory at Daybreak was established in 1977 to make TL available to the art community in general.
Studies at Oxford back in the 70’s on Romano-British pottery indicated that when all quantities entering the age equation are measured, the TL date of a single potsherd will typically fall within 15 per cent of the known date. When dates of a number of sherds associated together are averaged, the error is reduced typically to 7-10 per cent. This is for well-behaved samples only. The succeeding 30 years, and increased understanding of the dosimetry, have not brought much improvement.
Unfortunately, it is not possible to achieve this precision for the majority of art objects. Among the reasons for this is the small amount of material that may be taken for testing. Drilling, the usual method of sampling, introduces some uncertainty. It is also rare that any information about the radiation from the burial soil can be obtained, as art objects are usually thoroughly cleaned. This radiation may in some cases contribute over half the total dose. Finally, one has to make the measurements regardless of whether the TL of the clay is well-behaved or not. Some clays are hardly thermoluminescent at all; some may not have a straight-line relationship between dose and TL; spurious luminescence due to chemical or pressure effects may mask the radiation-induced TL; occasionally, a condition called "anomalous fading", where part of the TL is unstable, may lessen the accuracy of the dose measurement.
Generally speaking, when a sample is drilled and there is no information available about the burial environment, one may expect up to 40 per cent uncertainty. This is adequate for the purposes of authentication where the question is whether the piece was fired in antiquity or recently; it will not differentiate, say, between a classic Greek terra cotta and a Roman copy. In some categories of objects, from China, for example, the actual age is quite precisely known for short-lived styles, and it is possible to work "backwards" to get information about the environment in many parts of the world, and some other parameters not usually measurable for art objects. Using this information often reduces the uncertainty to 15-25 per cent.
Nearly any mineral material which has been heated above 500C at a time one wishes to know is a candidate for TL dating. This includes all forms of pottery. Porcelains, being nearly vitrified, are a special case requiring a fairly large solid core sample, and TL dating of intact objects is not recommended because of the damage caused by sampling. Most porcelain dating is done for insurance purposes on broken objects. Much stoneware is not so hard as porcelain and may be sampled by drilling. The clay cores from lost wax metal castings may readily be tested. Heated stone material, such as hearths, pot boilers, and burnt flints, has been dated as well. Some regions known to present problems for TL include Indonesia and West Mexico; objects from these areas usually do not successfully yield TL dates.
Recently there has been a spate of forgeries devised expressly to attempt circumventing TL dating. These use pottery of the appropriate period to construct objects. Some of these are quite easy to detect; some quite difficult. For example figures, normally modeled, may be carved out of brick or assembled out of fragments. These will give an ‘authentic’ date for a bogus object. It must be realized that TL dating is but one of the criteria for judging authenticity. The expertise of the conservator may be of equal or greater importance in many cases. Some problem areas include Northern Nigerian ceramics, especially Nok, which are becoming quite scarce. Ife ceramics are virtually all fake (or stolen, if genuine!), but some heads are made from old pot fragments (often too old). New Nigerian (and Asian) bronzes may have introduced old cores, so it is imperative that the interface between metal and core be examined very carefully before the assumption can be made that the age of the bronze is the age of the core. Chinese unglazed ceramics constructed from fragments (or carved from brick) are a particular concern. Glazed objects generally cannot be pieced together in this way without re-firing (which would defeat the purpose), but be sure the glaze is glass and not a synthetic resin! Often we recommend radiography of objects to ascertain the state of restoration before proceeding with sampling .
We reserve the right not to sample and date an object based on concerns about tampering.
Since the TL age is proportional to radiation dose, it is logical to be concerned about the effect of airport security x-rays and radiography done to examine the object. In general it is not a problem. Airport security x-rays devices use very high sensitivity detectors so that the x-ray dose is in fact quite small, perhaps adding a week or month to the age, well below the uncertainty of dating. Radiography, if many films are taken, may be more of a problem, so we recommend that samples be taken prior to exposure. It may also be possible to compute an approximate correction, but in almost every case the effect is small. Due to concerns about bioterrorism in the wake of the events of this past autumn, the US Postal Service has begun limited sterilization of mail by electron beam. This will destroy the dose information carried in the pottery and rendered it unsuitable for TL dating. While at present the chance of this occuring is remote, we are recommending that samples be sent by way of FEDEX or UPS.
SHOULD I BE CONCERNED ABOUT ARTIFICIAL IRRADIATION?
There have been rumors circulating lately about recently fired Chinese pottery being artificially irradiated to circumvent TL dating. While this is certainly something we watch for, there is little real cause for concern. There are several reasons why this dose tampering is difficult to impossible to achieve successfully. First, it is difficult to get the dose right without considerable research into the properties of the clay and access to expertise in TL measurements. Second, it is very difficult to get that dose sufficiently uniform over the extent of the entire object. It also and obviously requires a sophisticated means of irradiation, not easily available here, let alone in China. There are many considerations that we will not detail so as not to offer 'aid and comfort to the enemy'. The 'impossible' part is that different size grains in the clay actually have different doses in a naturally irradiated ceramic, but will have the same dose in the artificially irradiated example. This fortunate phenomenon is due to the heterogeneity of pottery clays, which are a mixture of fine grains (silt) and coarser grains (sandy inclusions). The radiation dose we measure in the lab is due to a mix of different kinds of radiation: alpha particles (which are heavy and have a very short range in matter--typically about 25 micrometers or 1 thousandth of an inch), beta particles (which are light and travel up to several millimeters or 1/16 to 1/8 inch), and gamma rays (which can pass through up to 30 cm or one foot of mineral material). The major part of the natural radiation dose is due to alpha particles, and the alpha emitting nuclides--uranium and thorium and their daughters--are primarily found in the fine grains. Because of this, the fine grains have the maximum dose, while the larger sandy grains have that dose only on their surface, and a considerably smaller dose in their interior. If the different size grains are measured, and the dose is found to be the same, there is good evidence of dose tampering, and the converse is true as well. When all these considerations are taken together, it is extremely difficult to get an artificially dosed object past routine TL dating. Given the quantity of older pottery available in China, your concerns should be directed more toward pastiches and assembly of new objects out of old fragments. There is one problem area, however, and that is porcelain. This material is so high fired that it actually becomes a glass with small islands of quartz usually remaining (which makes TL dating of porcelains possible). In theory, there should be a difference in measured dose between small and large quartz grains, but the glass matrix makes it extremely difficult to extract the grains intact. It is unfortunately not practical at this time to do differential dosimetry on porcelain, and it becomes more difficult to tell for certain when irradiation has been attempted.
When the TL test is for routine authentication, a sample of about 100 mg, roughly a third the volume of a pencil-end eraser, is drilled out of an inconspicuous part of the object with a carbide dental burr. If the object is extremely small, the amount of sample may be reduced, but the error margin may increase. It is sometimes preferable to obtain a fragment a half-inch in diameter and a quarter-inch thick, as the precision attainable is greater. This is advisable whenever the age, if genuine, is less than twice the age of the earliest forgeries.
If the object to be tested has been restored, it may be advisable to take more than one sample, as the component parts may differ in age. For heavily restored objects, or those where construction from diverse fragments is suspected, we have taken up to ten samples (there will be a modest increase in the fee for samples in excess of two).
Sampling does not lessen the value of a piece; indeed, confirmation of authenticity by TL generally enhances an object's value and saleability considerably. The site of the samples may easily be restored if desired.
First, you should contact Daybreak to discuss the advisability of testing and to arrange sample-taking. We make occasional trips to New York City for this purpose; travel elsewhere is by arrangement. You may also bring the piece to Daybreak, located just east of New Haven, CT (about 1 3/4 hours from mid-town Manhattan). (link to map) We recommend that you use our Sample Submission form, available on our website, for providing information on the object, and it is necessary to include two photographs. One photo is retained in our files for reference; the other is annotated with the result, signed, and returned with the test result. Polaroid snapshots are adequate; it is necessary only that the object be readily identifiable from its picture. If the sample is taken by anyone other than Daybreak or its representative, it is recommended that the sampling be witnessed (as provided for on the Sample Submission form), asserting that the sample is indeed from the object depicted in the photographs. The sample-taker must sign both photos. Otherwise, the test report must state that the object submitted is "a chip (or powder) sample said to be from a ______ of the ______ culture".
It is extremely important to note whether the object is known or is suspected to have been exposed to x-rays or neutrons, as this could alter the results. Exposure to airport security x-rays does not present any problem. Also, if the piece has been heated for some purpose during restoration, the TL result may be affected. In either case, consultation is required to determine whether the object can be accepted for test.
Please be assured that all information about objects submitted is held in the strictest confidence.
I DON’T LIVE NEAR CONNECTICUT. HOW CAN I GET THE SAMPLE TAKEN?
We also have conservators and restorers located throughout the country who can take samples for us (at an additional charge). (List available) If these alternatives are not practical, you may ship us the object, at your sole risk. We recommend registered mail as a means of shipment, as the cost of insurance is very reasonable. Please consult us about packing methods.
Usually test results are available verbally within three weeks after the sample arrives at Daybreak, with the written report to follow. Rush service (as soon as the next day) may be available at a premium, if our schedule permits. The extra charge is US$ 100 per object for one week service. A large number of samples submitted as a group may take longer.
We charge US$ 310 per object submitted for routine authenticity dating, covering multiple samples from an object if required. There may be a modest extra change for a larger number of samples. We are now charging for sample taking: generally this will be a flat fee of US$ 35 (this is not charged when samples are taken by others). In the rare event that we are unable to obtain a reliable TL date after a good faith effort, the dating portion of the fee is reduced, usually to US$ 160. You will pay return shipping of any object sent to us. Quantity rates for dating are available for objects submitted in groups of five or more at the same time: US$ 285 per object (5-9 objects), and US$ 265 (10 or more). Prices are as of 2/15/03, and subject to change without notice.
When we examine objects or collections at length, we charge US$ 150/hour (US$ 250 when associated with a legal proceeding), and US$ 50 for travel. In case of travel outside the immediate area (i.e. flying anywhere), we must make an 8-hour day minimum charge.
For further information, please contact Dr. Victor Bortolot, our technical director.
We will invoice established accounts, and expect prompt payment. New accounts must pay in advance, and work will commence when the check has cleared. We now take Mastercard, Visa and Discover, so we can start work immediately if provided with a credit card number and billing address (with zip or postal code plus the street number--important for security where we do not have your credit card in hand at the terminal). For a credit authorization form to enclose with your object if you send it to us, or for the sample-taker to include with the sample, download the credit authorization form. We will pre-clear payment, but will not charge your account until the work is completed. We regret the necessity of having such a strict policy, but too many in the past have believed that payment is optional.
We are often asked, since our parent company, Daybreak Nuclear, manufactures measurement systems for TL, how much it costs to set up a laboratory. For a simple single sample reader, as would be suitable for authenticity dating, with the rest of the laboratory apparatus required, it actually would be possible to set up for as little as about US$ 25,000. However, the equipment for dating is only part. The major requirement is an experimental science background and experience. Dating pottery is very tricky, and initially people make many errors in interpretation until they come to know the materials. The problems vary by geographical area. We estimate that one would have to work hard on a worldwide range of materials of known age for a couple of years, or apprentice oneself at a good laboratory for a year or more, before going public with any TL dates. Even people with considerable experience in geological dating make a botch of pottery dating at first when going it alone. The truth is that TL dating of art objects has an element of 'art' itself and one cannot do it from a cookbook. This is a major reason that there are so few TL dating laboratories (and the quality of work at commercial laboratories unfortunately does vary--one has to assess the background and experience of the people doing the work). Another potential problem is that radioactive sources are necessary for calibrations, and this requires licensing by the US Nuclear Regulatory Commission or whatever the competent authority is in your locality, with licensing fees, inspections, and so on. Because the potential for disaster (meaning wrong intrepretations with financial repercussions) is high for neophytes, we very much discourage people without the requisite background and a strong commitment from embarking on such a course. We know this from experience: in the early 80's we set up a fairly large number of museum laboratories. This was at a time when TL dating was considered a 'silver bullet' and a 'must-have' for every museum research laboratory. Virtually all failed to produce anything useful: they eventually sold off their equipment and Daybreak now does their TL dating.
The popular literature on TL dating is slim. The technical literature is extremely broad, but very specialized. We recommend the Aitken book (now in its second edition) for a technical introduction and comprehensive bibliography. There is a link below to a mildly technical paper for those wishing more detail than is provided elsewhere on this site.
Aitken, M.J., Thermoluminescence and the Archaeologist, Antiquity, LI, 11-19 (1977)
Aitken, M.J., Thermoluminescence Dating, Academic Press (London) 359 pages (1985)
Ehlers, E.G., Thermoluminescent Dating of Ceramic Materials, Archaeology, 28, 98-101 (1977)
Fleming, S.J., Themoluminescence Techniques in Archaeology, Clarendon Press (Oxford) 229 pages (1979)
We limit ourselves to work on art objects, as we have insufficient resources to do archaeological work, but some of our instrumentation customers do accept such work. We can recommend
Quaternary TL Surveys in Nottingham England (Dr. Nick Debenham)
and Dr. James Feathers at the University of Washington in Seattle
For radiocarbon dating (accelerator mass spectrometer dating which requires a much smaller sample than the conventional benzene synthesis/liquid scintillation counting or gas counting methods) we have been sending samples and pointing people toward
Geochron Laboratories in Cambridge MA (they do conventional dating in Cambridge and send samples for AMS to the Lawrence Livermore Lab--where swords have been forged into plowshares at least in this case!)
A very useful site for both introductory and detailed information about radiocarbon dating is http://www.c14dating.com/. There are many links to laboratories to be found here.
Anyone who has a radiocarbon date and needs it converted to calendar age may have it done on line at the University of Washington's Radiocarbon Laboratory http://depts.washington.edu/qil/calib/. It is very important that one be aware of the need for calibration. The reservoir of radiocarbon in the atmosphere is not constant (as was initially assumed) and this results in calendar dates being up to several centuries different from the radiocarbon age. This led to improper conclusions about the spread of culture and technologies particularly in the Mediterranean region, with subsequent embarrassing retractions or outraged denunciations of radiocarbon dating by unfortunate archaeologists who relied too much on pre-calibration--1950s--radiocarbon dates. There also is a convention to quote ages according to an early value of the halflife for consistency rather than to use the more precise modern value. The worst, most confusing, case is in the past 350 years, where a given radiocarbon age may have up to three calendar date ranges, separated by hundreds of years. The calibration gives the probability (percentage) of the likelihood of each range. There is also a very large effect from nuclear weapons testing from the 1950s on. At the peak of the effect in the early 1960s, there was about twice the radiocarbon as normal. This actually makes certain things easy, for example answering the question of whether the wood of an African object was growing after 1950.
For a creationist tract on why no scientific dating technique that can give dates prior to 4004 BC is valid, you might take a look at http://www.pathlights.com/ce_encyclopedia/06dat3.htm It raises all the objections, some valid, some spurious. We include this both for the quirkiness of dogma, but also as a cautionary note, because all scientific evidence for age must always be taken with some caution. These are complicated physical systems we are dealing with, and nature plays tricks occasionally, leading to misinterpretation.
Daybreak Nuclear and Medical Systems, Inc., was founded in 1977 to produce laboratory systems for TL dating in archaeology and geology, and to provide dating services to the art community. We are the world's leading manufacturer in this field, with more than 100 systems installed thoughout the world. In 1994 a second company, the Bortolot Daybreak Corporation, was formed to perform Daybreak's dating services. At the beginning of 2001, our client list for TL dating numbered over 900, including many museums throughout the world. We remain the only commercial TL dating laboratory in the Americas.