TL dating-frequently asked questions, TL authenticity dating, thermoluminescence dating
HOW DOES THERMOLUMINESCENCE DATING WORK? Although conceptually straightforward, TL has proven to to be far from simple in practice. In all. Thermoluminescence dating is the determination of the date at which materials were formed by measuring the light energy released when heating it. The natural . THERMOLUMINESCENCE DATING OF QUATERNARY . In a simplified scheme, non luminescent 'C for 3 hours showing that thermoluminescence.
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.
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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.
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.
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! 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 purposebut 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.
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: 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.Absolute dating methods (ANT)
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 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.
Thermoluminescence dating - Wikipedia
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. 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 formasserting that the sample is indeed from the object depicted in the photographs.
The sample-taker must sign both photos.
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. These imperfections lead to local humps and dips in the crystalline material's electric potential. Where there is a dip a so-called " electron trap"a free electron may be attracted and trapped. The flux of ionizing radiation—both from cosmic radiation and from natural radioactivity —excites electrons from atoms in the crystal lattice into the conduction band where they can move freely.
Most excited electrons will soon recombine with lattice ions, but some will be trapped, storing part of the energy of the radiation in the form of trapped electric charge Figure 1. Depending on the depth of the traps the energy required to free an electron from them the storage time of trapped electrons will vary as some traps are sufficiently deep to store charge for hundreds of thousands of years.
In practical use[ edit ] Another important technique in testing samples from a historic or archaeological site is a process known as Thermoluminescence testing. Which involves a principle that all objects absorb radiation from the environment. This process frees electrons within elements or minerals that remain caught within the item. Thermoluminescence testing involves heating a sample until it releases a type of light.
This light is then measured to determine the last time the item was heated. When irradiated crystalline material is again heated or exposed to strong light, the trapped electrons are given sufficient energy to escape. In the process of recombining with a lattice ion, they lose energy and emit photons light quantadetectable in the laboratory. The amount of light produced is proportional to the number of trapped electrons that have been freed which is in turn proportional to the radiation dose accumulated.
In order to relate the signal the thermoluminescence—light produced when the material is heated to the radiation dose that caused it, it is necessary to calibrate the material with known doses of radiation since the density of traps is highly variable. Thermoluminescence dating presupposes a "zeroing" event in the history of the material, either heating in the case of pottery or lava or exposure to sunlight in the case of sedimentsthat removes the pre-existing trapped electrons.