Radiocarbon Measurement and the Age of the Turin Shroud:
Possibilities and Uncertainties
William Meacham - Archaeologist
From the Proceedings of the Symposium "Turin Shroud - Image of Christ?"
Hong Kong March 1986
Copyright 1986 All Rights Reserved
Reprinted by permission.
Cover of the Symposium Proceedings
In recent discussions on the possible authenticity of the Turin Shroud (Sox 1981: Meacham 1983: Jumper et al 1984), the question of the value of C-14 dating persistently recurs. Virtually all researchers agree that the test should be performed; sufficiently small samples can now be measured so that the appearance of the relic is not altered. Several C-14 dating proposals are now under consideration by the Archbishop of Turin.
In contrast to these positive developments, however, there appears to be an unhealthy consensus approaching the level of dogma among both scientific and lay commentators, that C-14 dating will "settle the issue once and for all time."This attitude sharply contradicts the general perspective of field archaeologists and geologists, who view possible contamination as a very serious problem in interpreting the results of radiocarbon measurement. In this paper I shall examine the issue of the reliability of C-14 testing to produce an "absolute date" on the linen sheet known as the Holy Shroud of Turin and believed by some to be the gravecloth of Christ.
I have previously (Meacham 1983) treated the question of the Shroud's authenticity at length and shall confine my remarks here to the applicability and ultimate reliability of radiocarbon as an "authenticity test" of the relic. Reviewing recent Shroud literature of all persuasions, I find little awareness of the limitations of the C-14 method, an urge to "date first and ask questions later," and a general disregard for the close collaboration between field and laboratory personnel which is the ideal in archaeometric projects. Regarding the Shroud, consultations should take place among archeologists, historians, conservationists, cellulose chemists and of course radiocarbon scientists in order to formulate a specific C-14 sampling and dating procedure. As I shall endeavor to demonstrate below, the radiocarbon measurement of the Shroud is a complex issue, and the inclusion of all relevant expertise is highly important.
In May, 1985, I submitted such a proposal to Cardinal Ballestrero, Archbishop of Turin and official custodian of the relic, in the hope that the ecclesiastical authorities would consider appointing a scientific panel to plan and implement a C14 testing program.
The first proposal to date the Shroud was submitted in 1979 by Gove and Harbottle (published in Sox 1981:161-167). It was, in my opinion, seriously flawed by the lack of consultation with archaeologists and experts from other fields. Although the more recent STURP proposal has not yet been published, there is reason (discussed below) to suspect that it likewise has not been researched to the degree warranted by the object to be dated, and that significant input from a range of scholars is lacking. Because the next round of scientific testing of the Shroud may well be the last of this century, it is imperative that such details as the amount and number of samples and especially the sampling sites be very carefully considered. Possibilities of contamination should be exhaustively investigated, and pretreatment should be devised accordingly.
In 1979, the much vaunted "Gove/Harbottle Proposal on Carbon Dating the Shroud" (Sox 1981:161:167) outlined only standard pretreatment of the samples for carbonates and humic acids. It did not propose scanning electron microscope screening or other types of direct examination to check the state of the samples prior to testing -- an omission which might have been rectified if the vicissitudes of the cloth over the centuries had been stressed, as an archaeologist would have done. Much worse, the 1979 proposal involved a small sample of cloth removed from the Shroud in 1973 for study by Prof. Raes of Belgium. McCrone and Sox had inspected the sample (apparently unstitched by Raes into two pieces) during a visit with Raes in 1976, and found that "the samples were kept in what looked like an old scrapbook for postage stamps" (Sox: 1978:48). Certainly most archaeologists would have rejected the use of samples subjected to a long separation from the object to be dated and held under unknown conditions of storage and handling.
Further, McCrone (1978:440) made his contribution by proposing to rely on "the person authenticating the Shroud samples as the same ones studied by Raes." (The original sample was apparently not even taken from the Shroud in the presence of Raes.)
An art historian would certainly not have been satisfied that such a procedure could establish conclusively that the pieces were indeed from that sample removed from the Shroud in 1973, and that it had not been tampered with in the intervening years. Finally, the original sample was taken at the junction of the side strip (believed by some scholars to be a later addition) and the (selvage?) border (possibly treated to prevent unraveling, and certainly more subject to contamination than the main body of the cloth). It could not be considered as a typical or representative sample of the relic. In sum, the proposal to use the Raes piece for C14 dating was not an academically sound proposition; it was based on expediency (as the pieces had already been removed from the relic and were "available").
There is consensus now that, had the testing been allowed, it would have been the cause of great controversy regardless of the results. Yet Gove, in urging the release of the Raes samples, wrote that "at long last, the Shroud of Turin's true age will be established in the near future."
Before considering the recent proposals for dating the cloth, it is useful to survey the major problems routinely encountered in the field of C-14 dating.
General Considerations of the C-14 Method
Even among social and physical scientists, there are numerous misconceptions about the radiocarbon method of dating; among journalists and the general public there are of course many more. But among specialists who frequently make use of the test, it is not considered as a method which produces an "absolute date" for every sample that can be measured. When I wrote in Current Anthropology (1983:289) that C-14 dating could not be expected to settle the matter of the Shroud's age and authenticity because of the possibilities of contamination, there was a storm of criticism -- virtually all of it motivated by ideal sample considerations and obviously not tempered by experience in using the method. Stuckenrath (1966:277) certainly had it right 20 years ago in his remark. That "C-14 dating is, after all, only another tool for the archaeologist, but it behooves us, before attempting to use it, to know which end has been sharpened."
In both the Current Anthropology comment and in Shroud writing generally, there is exhibited a lack of awareness of the pitfalls and uncertainties inherent in the C-14 method. To quote from the comments, Alcock said it was "sheer whimsy" to raise the question of contamination; McCrone claimed that "the impurities can be readily removed before dating, hence this argument has no validity''; Maloney thought that "the margins of error supplied with each date (give) a measure of accuracy" in the elimination of contaminants; Schafersman claimed that the idea of contamination was "absurd". In the Shroud literature, a similar absolute belief in the method is found among most writers. Wilson, for example, states (1978:264) that a dating accurate to a plus-minus of 100 years is possible thus "enabling the settling, once and for all, of the question of whether or not the Shroud is a 14th century forgery." Sox (1981:132) follows Wilson in thinking that C-14 dating the Shroud could "remove it once and for all from the Middle Ages, or place it squarely there for all time." Some STURP scientists unfortunately display similar beliefs. Jumper et al (1983:176) claimed that the test "if 'negative', i.e. not first century, can prove lack of authenticity" (emphasis added). Dinegar, who heads the STURP C-14 group which claimed to have made a detailed study of the application of C-14 to the Shroud, stated that "sample preparation procedures can insure no error in date due to foreign contamination accreted over the centuries" (1982:6; emphasis added).
All of the above statements quoted from the literature reveal an unwarranted trust in radiocarbon measurement to produce an exact calendar date for any good sample submitted. However, I doubt that anyone with significant experience in the dating of excavated samples would dismiss for one moment the potential danger of contamination and other sources of error. No responsible field archaeologist would trust a single date, or a series of dates on a single feature, to settle a major historical issue, establish a site or cultural chronology, etc. No responsible radiocarbon scientist would claim that it was proven that all contaminants had been removed and that the dating range produced for a sample was without doubt its actual calendar age. The public and many non-specialist academics do seem to share the misconception that C-14 dates are absolute.
Even the most elementary textbooks of archaeology and geology give a very different picture. "Contamination of samples may cause error in determination of reliable dates" (Heizer and Graham 1967: 165); "contamination of the sample may take place . . . and removal of the contaminant from the pores spaces and fissures is almost impossible" (Goude 1977:10). "Carbon from other sources may easily be trapped in porous materials . . . The archaeologist is the only person who is in a position to know of these contaminating potentials'' (Stuckenrath 1965:279). Excavated samples are "liable to absorb humic matter from the solutions that pass through them (resulting in) contamination by carbon compounds of an age younger than its own ... there is also the possibility of exchange of carbon isotopes under such conditions ... That there are other risks of contamination and other pitfalls involved in this method is obvious enough" (Zeuner 1970:341-6). Stuckenrath noted that contamination could not always be detected or eliminated, even with specialized pretreatments. He cited discrepancies in "dates" of wood house posts in Alaska at 1800-1600 B.C. and of charcoal from hearths within the houses at 1000-800 B.C. Summarizing the attempts to date early man in North America, Wormington (1983:191) stated what must be a nearly universal view among archaeologists: "Over the years, we have learned that radiocarbon dating is not quite the alchemist's stone we once hoped it might be ... "
The possibility of contamination is not of course in any way reflected in the margin of error given with each result. And every radiocarbon laboratory stresses the proper handling of excavated samples to avoid contamination additional to what may already have been deposited. The sample should be dried out immediately upon excavation to avoid mold growth, it should not be handled in a cloud of cigarette smoke or taken back from the field in a lunch box, it should not be placed in contact with a paper label, etc.
These handling considerations are not simply for the sake of white-coated tidiness -- contamination is a real danger for any C-14 material.
The usual contaminants in archaeological samples are lignins from rootlets and humic acid from the soil. Dead carbon particulate matter may be deposited. Inorganic carbonates in even small amounts can also have a severe dilution effect on the specific activity of C-14, and may be occluded in the sample in some way whereby they are not completely removed by the acid pretreatment. Hydrocarbons may be deposited in a sample. With specific attention directed to the removal of contaminants (where known or suspected), the possibility of an' inaccurate date can be minimized, but there remains an inevitable and unquantifiable uncertainty. Even when the contaminant is known, identified and specially treated for, its total removal cannot always be insured. In coral samples from the Great Barrier Reef, for example, Polach et a1(1981:2,13) noted that "examination of thin sections of these samples suggests it is unlikely that all contamination was removed" and concluded that the "observed degree of contamination on validity of C-14 ages cannot be quantified." Bronze Age straw matting from a museum collection in Hong Kong was rejected by radiocarbon scientists, owing to its earlier treatment in a 3% solution of soluble nylon preservative. Solvent extraction would not have been able to insure that all residues were removed. As Burleigh (1974:82) remarked: "Archaeology may be better served by no date in such circumstances than by the later embarrassment of dates subsequently demonstrated, or worse suspected, to be unreliable."
For most C-14 samples, the burial history is known or can be reconstructed, and substances possibly affecting the carbon content can usually be identified. For the Shroud, there is a 600-year history in a number of different environments and unknown handling situations, and a possible further 1300-year existence during which the object could have been in contact with virtually any natural or man-made substance in the areas it was held. To measure Shroud samples, one must therefore consider every possible type of contamination and attempt to identify and counter them all, before the measurement is made and a "radiocarbon age" assigned. Clearly, this result can only be considered as a possibility, at best a good probability, but hedged by many uncertainties. It would not be an absolute calendar date, and it would not "prove" the Shroud to be authentic or a forgery. Rather, it would be one further piece of evidence to be evaluated in the light of the total complex of data about the Shroud. As Barnard (1980:34) observes: "No historian would, for instance, point to a radiocarbon date (or even a whole series of C-14 dates) and assert that this type of data . . . provides ultimate proof of the reliability of a certain point of contention."
Such a restricted use of C-14 results is commonplace in archaeology and geology; many "dates" are rejected as anomalous and in conflict either with other C-14 dates or more reliable data. These common limitations in the use of C- 14 results are apparently not known to or appreciated by virtually all Shroud commentators of whatever predisposition and academic training. Surprisingly, only the Jesuit historian Wild (1984:38), in an article bristling with errors and non-sequiturs, made a fairly accurate summation: ".. -- test results showing a late date would be attributed to contamination, a not unreasonable suggestion in the light of everything the Shroud has been through." Codegone, the Turin physicist whose advice was sought by the Cardinal in 1973 and who has been the object of much criticism for his lack of familiarity with the small sample technology coming to the fore at the time, was absolutely right about the problems: "... the sacred linen has undergone vicissitudes which could have altered its composition [and which] give rise to grave uncertainties" (1976:37; translation mine). On the other hand, Foley (1982:26) has raised false issues about sunspot activities, C-14 decay rate, and counting accuracy. But I have not yet discovered in the literature the claim "of some people'' reported by McCrone (1983:298) that "the resurrection so modified the linen that any carbon date is bound to be meaningless." It is, however, a remote possibility that volcanic activity in the region where the Shroud linen was produced, or even atom bomb testing of the last 40 years, may have done so.
Contamination, Fictitious Ages and Anomalous Results
Advances in radiocarbon research and technology have brought about increasing refinement of the means of measurement and especially of the calibration of results. The half-life of C-14 was corrected from 5570 to 5730 years, the fluctuations in atmospheric C-14 led to calibrations based on measurements of bristlecone pine tree rings, research in isotopic fractionation has given a minor correction factor. At the same time, new problems associated with each type of C-14 material have been uncovered: bone and shell carbonate fractions regularly produce fictitious ages off by thousands of years, bone collagen has been erratic in reliability, charcoal and wood samples are especially prone to forward contamination, pottery almost invariably produces dates too old, soils and clays almost always too young. Before considering its application to the Shroud, I shall briefly review the ability of C-14 testing to produce reliable dates for other materials, from the worst to the ideal sample types.
Bone and shell carbonate dates have a reliability of nil to good, depending on ground water exchange of carbonates, recrystallization of the shell structure and other factors. Bone carbonate dates are highly suspect and many laboratories now refuse this type of sample. Shell has fared slightly better, though ancient shells may be contaminated forward and modern shells can have fictitious ages up to several thousands of years (Keith and Anderson 1963). On the other hand, many shell dates do agree with those from other associated materials. It is generally agreed that examination of the shell structure for recrystallization and comparison of stable isotope ratios between the sample and living shells in the region must be done before a shell date can be given any credence.
Bone collagen dates are held in somewhat higher esteem, but remain controversial. The results may be erratic for reasons not fully understood. (Munnich 1957; Tamers and Pearson 1965). Dates of 17,150 BP and 23,000 BP for early human remains in North America are generally given little credence (Standford 1982:205; Wormington 1983:191). Indisputable Neolithic (at least 3000 years BP) fish bone samples from Hong Kong gave ages of less than 280 years by collagen fraction and 2110 BP by carbonate fraction, whereas Neolithic human remains from the same site gave less than 165 years by carbonate and 800 BP by collagen (Meacham 1978:114-115). The marine fish bone collagen had C-13/C-12 ratios that fell within the range of the C3 terrestrial plant food chain.
Soils, peats and clays have been found to give ages too recent. Gilet-Blein et a1(1980) report that "a continuous process of organic turnover occurs in soils... recent carbon is continuously incorporated into the organic matter... the classic pretreatment with dilute alkali is not adequate for eliminating such contamination from soil specimens... Thus most ages from C-14 dating of organic matter of soils must be too recent."
Contamination is frequently proposed to account for anomalous dates on geological samples. Chao and Coleman (1981) report, in a list of over 100 dates obtained, the following instances of suspected contamination: "sample [1440 BP] may have been contaminated by plant rootlets. Date greater than 4300 BP was expected based on nearby dates of related features."the two dates do not approximate the true age . . . Dates probably resulted from overwhelming predominance of 'dead' organic carbon from ... local bedrock.
"younger than expected date. Leached condition of overlying loess suggests that sample was contaminated by humic acids translocated from modern soil.
"younger than expected age may be due to post-burial contamination.
"two dates appear to be too old and probably contain detrital organic material derived from Paleozoic bedrock.
"date is too young for stratigraphic position.
"serious disparity among these dates suggests sample was contaminated with younger carbon.
"radiocarbon dates from this area are problematic [same stratum gave dates of 31,100 BP, 20,500 BP and 4,090 BP].
All samples were of course subjected to standard laboratory pretreatment.
C-14 dating of pottery originally held great promise for the archaeologist (Taylor and Berger 1965), but recent work has shown it to be quite unreliable, partly because of insufficient combustion in the kiln firing of non-cultural organic matter present in the original clay, and partly from post-burial contamination. De Atley (1980:988) concluded that "Carbon from non-cultural sources may... effectively dilute the age or otherwise contaminate the cultural sample." As an example of the latter, CaO particles in the clay may re-combine with carbon dioxide from the environment, thus introducing carbon of unknown origin and age. Pottery from the Philippines and Hong Kong have given dates too old by 1000-2000 years, whereas dates on ceramics from the southwestern USA ranged from older than expected to younger, even into the future (i.e. indicating incorporation of atom bomb carbon).
Plant materials (wood, charcoal, textiles, grain etc.) are generally considered the best sample types for C-14. Apart from contamination, other factors can cause an apparent age. Some species of hardwoods live several centuries, and the C-14 result should represent the date of the rings present in the wood or charcoal sample. Furthermore, dates on plant materials grown in regions of volcanic activity may be older than the true age because of a natural dilution of the local atmospheric C-14 by volcanic gases. Saup et al (1980) investigated the C-14 result of 5730 BP on wood from a mine shaft believed to be Roman or Etruscan. Plants growing in this region of volcanic emissions (Monte Amiata, Tuscany, central Italy) gave fictitious dates of 1805, 2540 and 4350 years BP. Discrepancies of up to 20% between C-14 results and historical data in the Aegean and in Egypt are thought to derive from minor atmospheric variations (Hood 1978).
Divergence of the C-14 age from the historically dateable context is clearly the best, perhaps the only, method of evaluating the effects of contamination. While it is indisputable that C-14 does give results generally harmonious with the expected historical age, the exceptions are manifold, even with optimum sample material and site conditions. In a series from historical sites in Yugoslavia (Srdoc et al 1981), wood samples of the 13th century gave dates of 240 and 580 BP, a 14th-15th century sample gave 940 BP, 1st-2nd century B.C. samples gave 2170, 6030 and 5600 BP, and a Neolithic sample gave 1940 BP. The site of Akrotiri in Greece, destroyed by the Thera eruption of ca. 1550 B.C. and sealed with an ash layer 3m. deep, was thought to be "an ideal source for radiocarbon samples." A recent series of eight samples yielded only four results deemed reliable: two dates "are exceptionally early and do not fit into the archaeological picture," two other dates "are very late and were probably contaminated'' (Weinstein and Michael 1978). All of the samples were grain or charcoal of short-lived wood, and all were found in jars in one room. Regarding the early dates, the authors remarked:Why are they so early? Are there special factors or phenomena surrounding the Akrotiri samples of which the radiocarbon specialists are unaware? At present, the series II dates are certainly of little or no value for Minoan chronology.
Betancourt et a1(1978:202) also discuss several dates that are too early or too late in the expected historical chronology, and conclude:One or two dates should never be used by themselves to establish a site's chronology. So many dates have proven to be useless because of contamination and other causes that one can only establish a radiocarbon chronology with some degree of confidence if several dates from the same site fall into a consistent pattern that agrees with the stratigraphic sequence.
Possible contaminating agents are many and varied, as described above. Unless there are specific conditions which warrant specialized pretreatment, most laboratories process samples with acid and alkali washes. While this standard pretreatment is usually effective in removing modern contaminants, it may not do so for intrusive materials deposited much earlier. The well-known controversy over the earliest date (pre-10,000 B.C.) from Meadowcroft rockshelter in Pennsylvania hinges on an alleged gradual contamination of the charcoal samples through the injection of dead carbon in the form of coal particles or of organic solubles (Haynes 1980; Dincauze 1981), in spite of the fact that the cave is dry and its earliest cultural layer is well sealed. One of the proponents of contamination, C. Vance Haynes, was a pioneer of chemical pretreatment methods to remove plant debris from C-14 samples. He points out that the dated samples from Meadowcroft are not pure charcoal but "mixtures of finely divided carbon and carbonaceous matter with... a significant percentage of soluble organic matter" (1980:583). Humate extractions were dated some 10,000 years earlier than the residual material in one sample. Cook (1964) investigated apparent charcoal samples from archaeological sites using chemical procedures similar to but stronger than those of C-14 pretreatment, and concluded that many were decayed wood with "considerable amounts of organic matter produced by micro-organisms through past centuries." Others were partially burned (carmelized) wood with considerable infiltration of organic matter.
Another famous early man site in North America, the Old Crow site in the Yukon territory of Canada, also yielded very misleading C-14 results according to a recent study by Nelson (1986). Bone tools from the site had given a date of around 27,000 years B.P. These tools were made of caribou ribs, and Nelson found that the outer portions of the bone had exchanged carbon with the air and ground water. A sample taken from the innermost portion of the bone yielded an age of 1,350 years. As in the Meadowcraft samples, the dating of progressive fractions revealed discrepancies not apparent when the samples were subjected to traditional pretreatment and dated.
My own investigation of a "charcoal" sample dated 8500 BP from a geological context in Hong Kong led to uncertainties inherent in the dates on wood samples from certain depositional environments. The wood was taken from a marine clay 18m. Below sea level; it was jet black as if charred. Laboratory examination (Grisack 1985) revealed however that the cellulose structure did not exhibit the morphological changes associated with charring. Scanning electron microscope study revealed that the pore spaces of the cellulose were almost completely filled. The analytical spectrometer showed the main inorganic substances present were sulfur and iron, with lesser amounts of silicon, aluminium, calcium and sodium. Treatment with 50% hydrochloric acid was effective in removing inorganic materials, but under the SEM the pore spaces remained as occluded with debris as before. The sample also showed very little birefringence under polarized light, whereas wood fibers should be brightly birefringent. "The explanation that suggests itself is that some organic type material has slowly, over a long period of time, been filtering into the lumens of the wood and possibly the cell walls as well, displacing the cellulose or carbon" (Grisack 1985:3). In the opinion of F.H. Kendall, Director of the: Radioisotope Unit at the University of Hong Kong, standard C-14 pretreatment of wood and charcoal samples would not succeed in removing organic material translocated into the lumens and cell walls of the cellulose (personal communication 1985).
It is clear that "more research on dating technology needs to be conducted so that the reliability of dates can be assessed" (Stanford 1982:205). MacDonald(1983:100, 108) believes that the absorption of humates from ground water may have seriously contaminated many dates from the northeastern US with its particularly acidic soils:"The critical question that demands immediate attention is that of humic acid contamination of C-14 dates, since there is growing evidence that current lab pretreatments are inadequate and that we are confounded by dates that may in some cases be too old and in other cases too young ..."
In sum, it should be obvious to the non-specialist, as it is to most archaeologists and radiocarbon scientists, that possible contamination always represents an element of uncertainty which no amount of laboratory pretreatment or measurement can totally efface. Clusters of congruent dates on different materials, replicated at different sites, eventually allow for a reliable radio-carbon chronology to be established, but there is, quite simply, no possibility of an absolute date on a single sample or artifact.
Carbon Dating the Shroud: general
The fact that significant discrepancies do often result from contamination in best sample materials from optimum archaeological conditions has major implications for C-14 measurement of the Shroud. First and foremost is the abandonment of any notion that a radiocarbon age of whatever magnitude will settle for all time the question of authenticity. Second, the choice of sampling sites on the relic should be governed by considerations of possible contamination and by the desirability of measuring both typical and atypical samples. Third, an elaborate pretreatment and screening program should be conducted before the samples are measured. Finally, the result should be interpreted to the general public in the light of contamination and other uncertainties inherent in the radiocarbon dating method.
Undeniably, a "bullseye" result with mid-point at 20 or 1320 A.D. would lend strong support to the proponents or opponents of authenticity. But a result of 300 or 700 or 1000 AD would create more controversy than it settled, especially with the necessary margin of error at -t 300 years or more. As flax is extremely short-lived, minor fluctuations in atmospheric C-14 levels may require that an uncertainty of up to + 120 years (Farmer and Baxter 1972) or ~ 150 years (Bruns et al 1980) be added to the normal statistical errors (+ 80 on a good sample). Calibrated and reported at 95% confidence level, the radiocarbon age of the Shroud would thus probably span 500-600 years. It is of course futile to speculate in advance on the interpretation of results, and I shall proceed to a consideration of the types of contamination which may be present on the Shroud, and of other factors which may influence the C-14 result.
One such factor is the environment where the flax used to make the linen was grown. If it was an area such as Monte Amiata or Vesuvius in Italy, with regular volcanic emissions, obviously a radiocarbon measurement would be meaningless. Another factor is the process of water-retting of the flax, during which ion exchange takes place. Betting is traditionally carried out by placing the cut and dried flax in a pond or pool for several days, during which a process of fermentation takes place resulting in a change of the intercellular substance. During retting, the cellulose takes up calcium, strontium, iron and other ions from the water; these elements are distributed evenly throughout the Shroud linen (Heller 1983:174). Depending on the chemical substances in the water or produced during retting, organic solubles and carbonates may have penetrated the cellulosic pores. The possible adsorption of old carbon could, like atmospheric dilution effects, result in a C-14 age older than the true age. Both types of backward contamination are, however, unlikely in view of the rare conditions in which they occur.
The main contamination possibility is that of carbon from organic materials deposited in the porous cellulose structure long after the Shroud was manufactured. One source would be mold, mildew or other fungal growths which are encouraged in linen by high humidity environments. Hydrocarbons could be deposited in the pore spaces and cell walls, gradually displacing the original cellulose of the linen as it degraded. With frequent handling, stretching, creasing, etc. the Shroud may have been more subject to hydrocarbon entrapment than would a buried specimen undergoing natural decay. Bacterial or insect residues and fine particles of carbonates could similarly have become locked in the cellulose structure.
Substances introduced by man over possibly 2000 years constitute another category of contaminants, and some of these may have interacted with the cellulose. Penetration of the pore spaces may have occured and a water-soluble, carbon-bearing solute deposited therein. The bound water of hydration may have been penetrated by other sustances, lipids and proteins may have been deposited among the fibrils, smoke may have left free carbon deposits within the pores. If this catalogue of contamination possibilities seems overly pessimistic, one must bear in mind the various substances recorded to have been in contact with the relic since 1356 -- oils, wax, soap, paints, ointments, open wounds, saliva, sweat and smoke. Preservatives, starch and image-enhancers may also have been applied. Earlier, the cloth may have been sealed in a city wall for several centuries with a votive oil lamp, and the relic may have been attached to a wooden frame for additional centuries, absorbing decay products from the wood cellulose. Some of the penetrating organic substances may through time have degraded to low order residues not detectable as specific contaminants and shielded by the cellulose substrate.
It is quite conceivable therefore that more recently formed organic compounds may have been absorbed by the linen, and that these residues may be in various, relatively active stages of alteration and degradation. Like charcoal, the open cellulose structure of linen, especially aged linen, is highly porous, with large surface areas, and is particularly absorbent of organic substances in solution "which can subsequently only be dislodged with difficulty" (Burleigh 1974:82). Fractions removed from the sample by selective chemical treatment should each be dated. With increasingly intense extractions, the result should be progressively older, approaching that of the final residue. This residue would represent the original cellulose of the linen if there were no contaminants equally or less soluble than the sample itself.
The idea of dating two or more chemical fractions on each sample has not to my knowledge been suggested for Shroud samples, but it would certainly be of greater scientific value than dividing up each sample to provide material for six laboratories each to run a conventional test. Checking of results between two or three labs is a reasonable proposition; to engage six labs to run a similar test on each sample is unnecessary.
The other side of the contamination coin is that many of the intrusive substances would be removed in ordinary pretreatment, analytical methods would indicate the presence or absence of others, and procedures applied for their removal. But there is no doubt that the Shroud has had an enormous exposure to a host of contaminants during its history. Marano observed under the SEM that Shroud fibers had a "filthy" appearance caused by "abundant deposit of extraneous pollutant material intimately connected with the individual fibers of the cloth'' (1978:202, 381; translation mine). Most if not all of the larger particles such as pollen and insect debris would be removed in ultrasonic washing, but unidentified "nodes" on the cellulose fibers require further investigation. In addition, there are numerous intrusive fiber remnants which would need to be painstakingly removed, if possible. Whereas all radiocarbon laboratories advise against placing a paper label in contact with the sample for the few weeks in transit from field to lab, the Shroud has had a backing cloth for 450 years!
Carbon Dating the Shroud: charred areas
Contamination considerations also bear on the question of sampling site. Recent C-14 proposals (Schwalbe and Rogers 1982 :44) have centered on the charred cloth hidden under the patches and thus removable without altering the physical appearance of the relic. While this charred material should certainly constitute one of the samples, it would be the gravest of errors, in my view, to take it as the only or the principal sample. The common claim that most C-14 dating is performed on charcoal may be incorrect, as noted above, and has no particular relevance here in any event. The folded corner of the Shroud was burned suddenly at a very high temperature, in probably a reducing atmosphere, in the presence of volatizing organic materials possibly 1500 years younger in age. There are numerous contamination possibilities in this event, and few if any C-14 dates on comparable samples.
1. Transfer of cellulose pyrolysis products -- The fire in 1532 at the Sainte Chapelle in Chambery had begun to melt the silver casket in which the Shroud was kept, folded, when the relic was rescued. Molten silver (probably at 850-900"C because of alloys) had dropped onto the cloth, burning through one corner and charring the lines of the folds. Apparently, the folded Shroud was kept in a pouch and the casket itself had a velvet cover. According to the inventory of treasures held in the Sainte Chapelle of Chambery in 1483, "the Holy Shroud, wrapped in a cloth of red silk, is kept in a casket covered with crimson velvet . .." (Ferret 1960: 92 from D. Crispino, personal communication 1985). There is every possibility that the pyrolysis products of these organic materials could have been transferred, even if in minute quantities, onto the Shroud. This transfer could have occurred by direct contact or air-borne movement ("smudging" in ceramic technology) whereby the colloidal carbon and tarry products of combustion permeate the pores of the cellulose in the reducing atmosphere inside the casket. These pyrolysis deposits would have resulted from the rapid heating of the organic materials to temperature greater than 700-800"C, during which its carbon would probably have been converted to a denser, less easily oxidized but mobile form.
Outside of the charred areas, the uneven distribution of pyrolysis products has been suggested (Schwalbe and Rogers 1982:20) to account for the diffusely mottled visual appearance of the cloth. It is possible that a portion of these products derive from other cellulose materials and other organic substances younger than the linen itself.
2. Carbonization of contaminant material -- In addition to the possible intrusive carbon introduced through smudging and colloidal deposits, there may well be carbon from combusted, cracked hydrocarbons of intrusive materials present on the Shroud at the time of the fire. "Cracking" occurs when higher order hydrocarbons break down during combustion into carbon and hydrogen. In addition to whatever lipid, protein or carbohydrate residues may have survived on the Shroud from preceeding centuries, there is mention, in a 1503 chronicle, of the relic being boiled in oil as a test of authenticity. Substances from this oil bath could have been present in quantity on the cloth in 1532, though none can be detected today. Carbon from these cracked hydrocarbons may have been incorporated in the carbonized linen as microscopic soot-like particles or as sub-microscopic colloidal carbon amongst the mico-fibrils.
Differential firing of the charred sample might enable the intrusive carbon to be driven off in a short, low-temperature burn, and the linen structural carbon collected during a longer, higher firing, i.e. the reverse of a procedure now being developed to collect the cultural carbon from pottery. But this procedure is still experimental, and with cellulose probably untried. Except for pottery made from organic-rich clay, there are very few if any C-14 dates on samples which have been burnt in a reducing atmosphere full of more recent carbon. There is, therefore, no precedent and no proven technology for dating such samples. Clearly, the charred linen can not be relied upon by itself to provide an accurate date.
3. Isotopic exchange -- Raaen (1968:70) states that exchange reactions involving carbon atoms of the carboxyl group (one of the compounds produced by oxidation reactions in cellulose and present in quantity in the Shroud) can occur with certain substances at temperatures of 300-400"C, as in reaction:
Isotope exchanges such as the above could pose a major problem for C-14 dating, as clearly an introduction of carbon is involved which may not be identified, measured, pretreated or corrected. Fortunately, such exchanges of carbon atoms are rare in nature, and when they may have taken place in samples of archaeological interest the exchange would almost invariably have been with contemporaneous materials. The Shroud constitutes an extremely unusual instance in which much later substances have been in contact with the sample at elevated temperatures.
It should be abundantly clear that neither the Raes pieces nor the charred portions of the Shroud can be relied upon for carbon-dating the relic. The charred linen has since 1532 constituted a different system from the rest of the cloth, with larger surface areas and higher absorbency, thus being more subject to post-fire contamination. The charred areas are also subject to possible contamination by direct transfer of cellulose pyrolysis products contemporary with the 1532 fire, by carbonized substances present in the cloth at the time of the fire and by the introduction of more recent carbon through isotopic exchange during the fire.
These serious reservations on the use of the charred linen as principal samples for carbon-dating the Shroud would, I believe, have emerged from a wide-ranging consultation with experts both in the theory and practice of C-14. No one has yet investigated the issue properly, consulted with the necessary expertise in the field of radiocarbon science (Polach 1968 personal communictation) nor with archaeologists, museum personnel or other scholars who regularly make use of C-14 dating in field situations. As a result, we have seen twice a scientifically faulty research plan put forward for dating the Shroud. The latest proposal should be completely re-designed, as I have suggested in a submission to Cardinal Ballestrero in 1985.
Maloney (personal communication 1985) has questioned my proposal to include historians and art historians among the experts to plan a C-14 dating program. It should be clear from the foregoing discussion that many of the types of contamination depend on the circumstances of handling and storage of the relic. It would seem desirable to obtain as complete a "contamination history'' as possible of both the recorded historical period of the Shroud since 1356, and of its possible earlier existence as a Byzantine icon. The cellulose chemist, archaeologist and radiocarbon scientist would then consider possible sampling sites, pretreatment and screening in this light. Both art historian and historian should be involved in the discussions regarding sampling sites, with a view to avoiding (or at least being aware of) sites on the cloth which may have been subject to special handling or treatment in the past.
Such consultation would certainly make very clear the danger of sampling an anomalous zone to represent the entire object. To consider a comparable situation, let us say that an ancient coffin was found one metre below ground surface. Upon complete excavation, it was found that a corner of the coffin was charred by an intrusive hearth from a more recent period. It would be the utmost folly, I believe, to rely on the charred portion of the coffin for a C-14 date on the burial. The result might much more likely fall somewhere between the age of the burial and the age of the hearth; it is highly unlikely that any archaeologist would contemplate the use of such an anomalous sample.
My own tentative proposal for dating the Shroud is that at least five samples be taken: 1) a single thread from the middle of the cloth, between dorsal and ventral images; 2) a small piece cut just in from the edge next to the site of Raes' piece I; 3) a piece of the charred cloth; 4) a piece cut from the side strip next to the site of Raes' 11; 5) a piece of the backing cloth sewn on in 1534. The principal samples would be 1 and 2, with 3 possibly confirmatory; 4 would hopefully clarify the question of an added side strip: 5 would be a control for modern contamination. All samples would be subjected to elaborate pretreatment, SEM screening and testing (microchemical, mass spectrometry, micro-Raman) for impurities or intrusive substances such as higher order hydrocarbons, inorganic and organic carbonates. Samples 2-5 would be measured by both gas proportional and accelerator counting. Samples of a least 3sq. cm each would be required for intensive pretreatment (likely to sacrifice a portion of the sample), measurement of fractions, and controls for micro-testing. A total of 12 sq. cm. or so of the relic itself would thus be required. Selvage edges would be avoided, as in the British Museum inter-comparison experiment (Burleigh et al 1985:3). In view of the myriad contamination possibilities, at least two fractions of each sample should be measured, by each counting method, if possible.
In the end, with luck, we would have at least two or three radiocarbon ages in good agreement and possibly, quite possibly, indicative of the true calendrical age of the Shroud linen. That is all we would be justified in claiming. I believe that almost all radiocarbon scientists and archaeologists would concur with the remarks of Johnson et al. (1985:6):The existence of significant indeterminant errors can never be excluded from any age determination. No method is immune from giving grossly incorrect datings when there are non-apparent problems with the samples originating in the field. The results illustrated [in this paper] show that this situation occurs frequently. (emphasis added).
Regardless of the C-14 result, evidence from other sources would of course remain of considerable importance in the overall evaluation of the age and origin of the relic.
A C-14 age later than the first century would not of course consititute scientific proof of the inauthenticity of the Shroud, since radiocarbon dating is a based on a number of unverifiable assumptions -- the most important in this context being that the carbon extracted from the sample is indeed identical with the carbon absorbed from the environment when the sample was alive. But of course C-14 measurement does usually provide a reliable indication of true calendrical age. As an ultimate "authenticity test" for the relic, it is a better indicator than boiling in oil. Probably.
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