The Shroud of Turin

The Oxford Radiocarbon Accelerator Unit has been working with a team from Performance Films Ltd making a documentary about the Shroud of Turin for the BBC. The film (transmitted on BBC2 at 8:30pm on 22nd March 2008) examined the evidence for the authenticity of the Shroud. It also marked the 20th anniversary of the original carbon dating completed by the Zurich, Arizona and Oxford radiocarbon laboratories.  All three labs gave a consistent, mediaeval date for the Shroud.

Another contributor to the film, John Jackson (Turin Shroud Center of Colorado), while not doubting the validity of the original radiocarbon measurements, has developed a new hypothesis, which he believes may explain why the mediaeval date for the Shroud is incorrect.

The hypothesis put forward in the film is that the linen of the Shroud might have been contaminated by carbon monoxide. Unlike most contaminants, carbon monoxide is naturally enriched in radiocarbon when found in the environment and would therefore in principle be able to alter the radiocarbon age significantly. A relatively small amount of carbon monoxide (roughly 2% of the carbon in the linen) could alter the age of the sample by a thousand years. This is the only contamination hypothesis which could affect the radiocarbon age of the Shroud enough to allow it to be 2000 years old.

However there are also a number of reasons to think that carbon monoxide contamination is not likely to have had a significant effect:

• Carbon monoxide is only present in very low concentrations in the atmosphere.
• Carbon monoxide is not very reactive and would not be expected in normal circumstances to react with a long chain polymer like cellulose in linen.
• No contamination like this has been detected before, even on very old samples (up to the 50,000 year limit of radiocarbon) which would be much more severely affected.

The only way to see if this sort of contamination is possible is to do experimental work on modern linen. The key question is whether carbon monoxide reacts to any significant extent with linen. The Oxford Radiocarbon Accelerator Unit has been collaborating with John Jackson's team to test the reaction rates.

So far the linen samples have been subjected to normal conditions (but with very high concentrations of carbon monoxide). These initial tests show no significant reaction - even though the sensitivity of the measurements is sufficient to detect contamination that would offset the age by less than a single year. This is to be expected and essentially confirms why this sort of contamination has not been considered a serious issue before.

The research continues because the effect of the specific storage conditions of the Turin Shroud have yet to be reproduced by John Jackson's team. It remains possible, though not at all likely, that in these specific conditions there are reactions which provide significant contamination. There are also other possible types of contaminant, and it it could be that one, or some combination of these, might mean that the Shroud is somewhat older than the radiocarbon date suggests. It is important to realise, however, that only if some enriched contaminant can be identified does it become credible that the date is wrong by 1000 years. As yet there is no direct evidence for this - or indeed any direct evidence to suggest the original radiocarbon dates are not accurate.

There is a lot of other evidence that suggests to many that the Shroud is older than the radiocarbon dates allow and so further research is certainly needed. It is important that we continue to test the accuracy of the original radiocarbon tests as we are already doing. It is equally important that experts assess and reinterpret some of the other evidence. Only by doing this will people be able to arrive at a coherent history of the Shroud which takes into account and explains all of the available scientific and historical information.

Christopher Ramsey (March 2008)