Shroud of Turin Center

Carboxylation and radiocarbon enhancement...

In June 1997, the Groningen Center for Isotopic Research hosted the 16th International Radiocarbon Conference. There were 216 abstracts submitted to the Conference that dealt with a wide range of radiocarbon research developments. The Conference Proceedings were published in 1998 as an issue of Radiocarbon and contain 111 of the papers eventually accepted. Two of the published papers dealt with the effects of heat-induced carboxylation of cellulose and apparent radiocarbon age. One of these papers was not presented at Groningen but included afterwards by the Conference Editors – Willem G. Mook & Johannes van der Plicht,  (See Radiocarbon, 40, No.1, 1998, 57-60.). Full references are mentioned at the end of the text that follows. Bryan Walsh, Executive Director of the Shroud of Turin Center, provided this review.

The first of these papers reported the results of experiments conducted in 1986 in anticipation of the dating of the Shroud of Turin. Titled Attempt to Affect the Apparent 14C Age of Cotton by Scorching in a CO2 Environment, the paper written by Austin Long documents experiments conducted to determine if CO2 can be incorporated in scorched textile thus increasing the 14C level and reducing its apparent age. To accomplish this, the researchers used cotton grown in 1984 in an atmosphere rich in ‘dead’ CO2 and then treated with xylene to remove oils. The 5 grams of treated cotton were subsequently divided into 8 samples. A smaller portion (0.35g) of these samples was placed into a Pyrex tube that was evacuated at high vacuum and then replenished with ‘post-bomb’ CO2 at a pressure of 300mm. The 30cm long tube was then sealed and heated. Different sections of the tube were heated until the enclosed cotton reached different levels of darkness. The total heating time was 15 minutes and the temperature ranged up to 3500 C. The tube was then broken and its contents separated into four fractions and analyzed for 14C. The author concluded that the heating test was unable to add measurable 14C to the cotton cellulose.

Unfortunately, the author did not report the degree of carboxylation that was observed in his experiment so it is not possible to ascertain if carboxylation actually occurred and, if it did, to what degree. This is important since the pyrolysis of cellulose at temperatures above 2000C produces a wide variety of products including formic acid, furfural, levulinic acid, formaldehyde, carboxyl radicals and other beasts in the carbon zoo. There is no way of ascertaining which carbon species were involved in this experiment nor is it possible to easily compare the experimental conditions to the conditions incurred by the Shroud of Turin in the fire of 1532. Nonetheless, the 14C measurements are suggestive. Four sets of measurements were reported:

                                                      Appearance                      pMC

                                                 Virtually no darkening         0.526 +/-0.009

                                                 Light tan color                    0.554 +/-0.009

                                                 Dark toast color                 0.537 +/-0.014

                                                 Nearly black color             0.551 +/-0.001

The author states that statistical tests appear to show no significant difference between the base cotton sample, with a reported  14C level of 0.55 pMC, and the heated samples. However, if it is presumed that the degree of discoloration is directly proportional to the temperature regime experienced by the sample, then the distribution of the observations appears to indicate that some level of change in cotton textile radiocarbon content took place. The chart below illustrates the observed 14C measurements along with the standard error of measurement associated with each.

A linear regression line of these data, shown in red, shows radiocarbon content increasing as the temperature to which the cotton was exposed is raised. However, given the very small number of observations, the degree of correlation is too small (0.57) to be statistically definitive. The 0.55 pMC line on the chart denotes the base level of the radiocarbon measured in the original cotton sample prior to heating. No standard error for this measurement was provided in the original paper.

It may be that the author’s experiment actually produced isotopic fractionation since the experimental vessel was closed while sections of the cotton in it were exposed to different levels of elevated temperature potentially enabling isotopic effects to be captured. Unfortunately, with no measurements of the degree of carboxylation or any other carbon product for that matter, and only indirect measurements of the temperature regime the samples actually experienced, it is impossible to ascertain just what this experiment really uncovered.

The second paper, An Experiment to Refute the Likelihood of Cellulose Carboxylation by R.E.M. Hedges, C.B. Ramsey and G.J. VanKlinken, approached the carboxylation issue somewhat differently. In this experiment, the researchers took 5mg of cellulose extracted from wood known to be older than 45 thousand years BP. Two equal amounts of this cellulose, one of which also contained a “few mg” of Silver powder, were sealed in glass containers of unstated volume in an atmosphere of modern CO2  at 2 bar pressure and H2O at 1 bar pressure. This mix was kept at a temperature of 2000C for 24 hours. Slight charring of the cellulose took place during this exposure.

Prior to this treatment, the extracted cellulose produced a measured 14C content of 0.20 +/- 0.1 pMC. After treatment the 14C content of the treated samples was as follows:

                                      Sample 1                        0.33 +/- 0.2 pMC

                                      Sample 2 (includes Ag)   0.41 +/- 0.1 pMC

The authors concluded that the dates of the treated samples are not younger in a statistically significant sense. Unfortunately, it is not obvious from the information provided in the paper just how this statistical finding was computed. They also stated that their experiment makes “a very strong case that the carboxylation mechanism, if it occurs at all, is very unlikely to make a significant change in the carbon composition of cellulose”.

Unfortunately, the authors do not state the degree of carboxylation, if any, that their experiment produced. Thus, their negation of carboxylation as a potential mechanism of radiocarbon enhancement would appear overstated. The statistical analysis necessary to evaluate these results involves the comparison of 2 treatment measurements – heating and heating in the presence of Ag - of one reported measurement each, compared against the non-treated cellulose sample measurement from which these treatment samples were extracted. The number of runs/trials that were used to compute the mean treatment measurements reported was not stated in the paper. If it is presumed that the values reported were the only values produced, then it is statistically very difficult to draw any conclusions from this experiment.

However, since the standard error of the non-treated cellulose was reported, the 14C content of the non-treated sample can be compared with each of the two treatment modalities to at least examine the differences are observed. Using only one measurement for each type of treatment puts severe limits on any statistical inference that may be drawn; combining the two treatment modalities is an experimental design error.  The effects of the two treatments can be seen on the following chart.

The vertical lines represent the 1 standard error range of the observations reported for each of the treatment modalities as well as the untreated sample. The dotted line is the regression relationship among the three observations. It would appear that the effects of the two types of treatment measured in this experiment point toward radiocarbon enrichment of the cellulose. Unfortunately, because of the poor experimental design, it is not possible to draw any definitive conclusions from these observations.

Conclusion

The definitive set of experiments evaluating carboxylation as a radiocarbon enhancement mechanism has yet to be done. Unfortunately, both experiments described here suffer from poor experimental design, not in the extraction of radiocarbon measurements but in the lack of measurements of any of the basic chemical reactions that may have occurred during the experiment. Further, by using a very small number of samples, it is very difficult to extract useful statistical conclusions about the results of the experiments. Finally, several key experimental parameters are not mentioned in the descriptions of experimental procedures further exacerbating the weaknesses in experimental design.

References

Long, A., 1998, Attempt to Affect the Apparent 14C Age of Cotton by Scorching in a CO2 Environment. In Mook, W.G. and    van der Plicht, J., eds., Radiocarbon, 40(1):57-58.

Hedges, R.EM., Ramsey, C.B., and Van Kinken, G.J., 1998, An Experiment to Refute the Likelihood of Cellulose Carboxylation. In Mook, W.G. and  van der Plicht, J., eds., Radiocarbon, 40(1):59-60.

 

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