The following article from Approfondimento Sindone is
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The results of the work of three radiocarbon laboratories to date samples from the Shroud of Turin by the radiocarbon method1, while accepted by many, have not convinced everyone. In this Note I describe how the principles and techniques of the method have been brought to bear on this particular problem, and give a scientific perspective, as an expert, of the result.
I consider it beyond my scientific perspective to comment authoritatively on non-scientific aspects of the dating operation. For example, it has been asserted that the sub-samples of the shroud cloth (but presumably not of the control samples) were deceptively substituted by ones not from the Shroud of Turin. Having witnessed the sampling operation, I find this assertion incredible, but I can produce no scientific evidence to refute it (although the characteristic weave of the shroud cloth is certainly unusual and easily recognised). Another assertion, that the sample was from a repair of the cloth, is also contrary to my own perception, but there are others involved in the sampling, more expert than I, who are better able to refute this argument. Here I am concerned with explaining how the measurements obtained leave no room for doubt that the textile sample, shared between the participating laboratories, was made from plants growing in the early part of the second millennium after Christ.
The basic principle was formulated, and tested on known age materials, by Libby (who obtained the Nobel prize in 1960 for this work) fifty years ago. Two isotopes of carbon, 13C and 12C, are known to be stable and were formed when the other atoms of this planet were also formed, and they constitute almost all the carbon component in organic and inorganic material on earth. A third isotope, 14C or «radiocarbon» is radioactively unstable, so that any formed with the planet will have long since disappeared. However, 14C is being continuously formed in small quantities as cosmic rays impact the earth's upper atmosphere, and the amount found on earth corresponds to the balance between cosmic ray formation and the natural disappearance of the isotope due to its instability. This balance is extremely small, 14C being approximately one part in a million million of the overall carbon content. The important point, for dating purposes, is that the 14C formed in the atmosphere is able to be taken up, along with 12C and 13C, in atmospheric carbon dioxide by photosynthesizing plants, and thereby spreads throughout the biosphere, so that all living things in fact have a similar ratio of 14C/12C. This ratio is maintained throughout because the carbon isotopes all have the same chemical behaviour. However, once living organisms die, their carbon content is no longer maintained by their metabolism, and, the level of 14C falls according to its radioactive half-life. Therefore a measure of the 14C/12C ratio provides a measure of the time elapsed since the organism died. This principle, as simply given here, requires a number of qualifications if accurate dating is to be sought, and a great deal of work, in many radiocarbon dating laboratories, has been extended over decades in order to show exactly what modifications are required.
The main modification has been to show that the «balance» level of radiocarbon in the atmosphere has fluctuated. This can be seen by dating known age material (such as tree rings) and comparing the result with the known age. There is a small difference at 0 AD and 1000 AD of about 60 years, but the difference can be quite large (e.g. 500 years at 3000 BC). It is, however, universal for (almost) all living systems and in all regions, at least to an accuracy of better than 30 years. The qualification above, of almost all living systems, is due to a second modification, which recognises that not all life obtains its carbon directly from the atmosphere. Actually atmospheric carbon dioxide is in equilibrium with a far larger quantity dissolved in the oceans - this huge reservoir is in fact buffering the atmosphere against the excessive carbon dioxide released from industrial fossil fuel burning. The ocean reservoir lags behind the atmosphere, and radiocarbon dates from, for example, sea shells, tend to be some centuries older than terrestrial animals and plants, and so need their own calibration curve. A third qualification is essentially an elaboration of this point, namely that the origins of all the sources of carbon in a sample need to be considered and separately accounted. For example, in archaeological potsherds, carbon may be present from carbonaceous material in the original clay, from the addition of animal or vegetable temper (e.g. chopped straw), from fire smoke, from surviving fats from cooked food, or from mobile carbonaceous material in the soil in which the potsherd was deposited after use. All these sources can have quite different times since their carbon atoms were part of living systems, and so the dating of the pottery will depend on which source is selected and measured. I have described these qualifications in some detail to show that there is a very rigorous attitude to understanding the ways in which radiocarbon dating requires a more detailed approach than the simple basic principles imply.
There are additional further considerations, but usually with only a restricted application (such as effects of local volcanic gaseous emissions, or of hardwater chemistry - i.e. containing dissolved limestone), but I do not have the space to cover them all.
The very small relative amount of radiocarbon present (a maximum of only fifty million atoms per milligram of carbon) means special techniques are needed to measure it. Furthermore, to be useful, the measurement must be rather precise (an error of forty years corresponds to a measurement error of 0.5% in the 14C/12C value). A system of international calibration and checking of blind results has been recently set up, to which more than 50 radiocarbon labs submit results2. This enables, at least for the samples measured and the techniques and routines used, individual laboratories to make an unbiassed comparison of their abilities, and to apply quality control methods to their work. All three laboratories engaged in dating the shroud have performed well in these intercomparisons (which were instituted after the shroud measurements). The fact that the shroud and control results, taken as a whole, compared well between all three laboratories, despite their somewhat different approaches, argues very strongly for the validity of the measurements. It is an essential part of science that results can be replicated by other scientists, and, without question, that was established in this case. The measurements were performed «blind» in order to prevent claims that the results had been biassed by wishful thinking or collusion.
Two aspects for measurement need further mention. They are the assessment of error in the result, and the avoidance of contamination in the sample which is being measured. Essentially, the error term that the laboratory gives with its result is an estimate of the likely reproducibility of that result if it were to be measured repeatedly. There are various terms which go into estimating the error - often the largest is the fact that the number of 14C atoms detected during measurement is subject to a statistical sampling error (as in taking an opinion poll), and this contribution can be accurately estimated. Other errors include various measured characteristics of laboratory processes, such as the subtraction of variable background signals. Whether the error is generally being correctly estimated or not can be checked, over time, by the laboratory measuring known age or replicate material and looking at the statistical distribution of results (for example, checking the extent to which they are Gaussian). The question of sample contamination is more difficult to quantify. One tries to extract a pure chemical compound for dating, but archaeological materials are often complex, with degraded chemistry, and it is rarely possible to be certain that all extraneous material has been removed.
Radiocarbon laboratories have built up a great deal of experience in developing extractive chemistries appropriate to many individual types of material3, and the results can often be compared (e.g. dating bone and charcoal from the same stratum should give the same result, when each has been environmentally contaminated, and subsequently de-contaminated in the laboratory by quite different chemical methods.
This sample was provided, unidentified, together with several other control samples of linen textile whose age was known (though not divulged to the participating laboratories). The main question facing the laboratories was the best way to decontaminate the cloth. Most methods used were similar, although Oxford used an additional step of a lipid extraction in order to remove any grease or candle wax, as well as oxidatively removing lignins from the basic cellulose structure of the cloth. In any case, essentially the same result was obtained by all three laboratories for the whole group of samples, although taking only the shroud result, there was a just statistically significant difference between Oxford's result and the other two laboratories (this is most likely to be due to an underestimate - of 5-10 years - of the errors by the laboratories; in any case, in the context of the question whether the Shroud date could be in error by centuries, the difference is negligible).
The method of measurement used, accelerator mass spectrometry (AMS), was necessary because of the small sample size available. Although at that time AMS was a less well-developed technique than that used for the majority of radiocarbon dates, the participating laboratories had already measured several thousand dates by the AMS method, and its accuracy, both then and subsequently, has been shown to be comparable to the best of the laboratories using conventional methods.
From a scientific point of view, the laboratories were in good agreement overall, and in adequate agreement for the shroud dating itself. The fact that, when calibrated, the most probable date was close to the historical time when the shroud was definitely known to exist, while not strengthening the scientific case for the date, is nevertheless extremely powerful circumstantial support for it.
However, the date has been disputed as being wrong on various counts. Firstly, it is worth noting that small systematic errors cannot be completely ruled out; while unlikely, some contamination which evaded the chemical purification methods is conceivable. However, if the main consideration is whether the shroud is 1st century AD or 13th century AD, the degree of contamination required to shift a 13th century date by 1300 years is very large (such a shift would require the addition of about 50% more material of «modern» carbon), and this quantity, or indeed any amount above a few per cent, can be totally ruled out. A more subtle form of contamination, whereby the carbon atoms of the cellulose are exchanged or carboxylated with those from a hot carbon dioxide atmosphere, has been proposed as possibly having taken place during the fire to which the shroud was historically exposed when kept at Chambéry. The scientific grounds for this proposal are, in my view, quite dubious, but nevertheless have been published in a scientific journal4, together with experimental evidence that such an effect is possible, and so do require careful consideration. At least three experiments have been carried out to test the possibility, with one been published so far5, and none has been able to find any such effect. Experiments we carried out at Oxford put an upper limit to any possible change of the date of about 5 years. The issue is important, not just for the Shroud dating (although it is most unlikely to affect the authenticity question, since a massive amount of exchange would be required to change the date by 1300 years), but because if such a process did occur during combustion events, it could spoil the accuracy of many other archaeological dates. The care with which radiocarbon laboratories have responded to even the outside chance that some additional process which might feasibly affect the date, demonstrates the level of vigilance which is felt necessary to ensure that radiocarbon dates are accurate.
Most other possibilities are less scientifically testable. For example, whether the shroud samples were irradiated by neutrons (which certainly could make a date appear younger than the true date). This idea is difficult to disprove, but also difficult to take seriously. In any case, to manage an irradiation so finely calculated as to give a very credible historical date is a remarkable coincidence.
Therefore, short of deception in the selection of the sample, or some parapsychological influence, or some unique scientific phenomenon totally unknown and unguessed at by the scientific community of radiocarbon dating at large, the date of the shroud cannot be significantly different from that of the published calibrated radiocarbon results.
NOTES
1 Damon, P. et al., Radiocarbon dating of the Shroud of Turin, in «Nature» 337 (1989), pp. 611-615.
2 Gulliksen, S. and Scott, M., Report of the TIRI workshop, Saturday 13 August 1994. Proceedings of the 15th international 14C conference (eds. Cook, G. T., Harkness, D. D., Miller, B. F. and Scott, E. M.), in «Radiocarbon», 2 (1996), pp. 820-821.
3 Hedges, R. E. M., 1992, «Sample treatment strategies in radiocarbon dating», in Radiocarbon After four decades: an interdisciplinary perspective (eds. Taylor, R. E., Long, A. and Kra, R. S.). New York: Springer-Verlag, 1992, pp. 165-183.
4 Kouznetsov, D. A., Ivanov, A. A. and Veletsky, P. R., Effects of fires and biofractionation of carbon isotopes on results of radiocarbon dating of old textiles: the Shroud of Turin, in «Journal of Archaeological Science», 1 (1996), pp. 109-122.
5 Jull, A. J. T., Donahue, D. J. and Damon, P. E., Factors affecting the apparent radiocarbon age of textiles: a comment on «Effects of fires and biofractionation of carbon isotopes on results of radiocarbon dating of old textiles: the Shroud of Turin» by D. A. Kouznetsov et al., in «Journal of Archaeological Science», 1 (1996), pp. 157-160.