It's spring cleaning time, and while most of us are thinking of packing a way our winter linens and airing out our summer clothes, a historian at the Vatican has decided to drag out another mouldering old bit of cloth to dangle before us. 

In a well-timed news story that could easily be confused with the plot of a Dan Brown novel, the Vatican newspaper has reported that a historian working in the Secret Archives has uncovered proof that the Shroud of Turin was guarded and venerated by the Knights Templar. The earliest confirmed mention of the shroud, which is believed by some to be the burial cloth of Jesus, was in 1353, when it was put on display in France. It later passed into the hands of the Savoy family, and has been in the Royal Chapel of the Turin Cathedral since 1694.

The cloth was largely thought to be unremarkable, until photographs in 1898 showed what appeared to be the full body of a crucified man. Efforts to trace the history of the shroud back in time have proven difficult. Before the cloth's appearance in Turin in 1353 there are only sketchy references to other objects that may or may not be the same thing. A cloth with an image of a man on in is mentioned in Constantinople in 1204, and there are vague earlier references to something similar. 

Luckily for us, samples of the cloth were permitted for radiocarbon analysis, and the results of three independent labs showed that the cloth was produced in the first part of the 14th century, which fits in quite well with the historical evidence. So we all went home in our acid wash jeans to tease our hair and listen to Duran Duran and worry about what the Soviets were up to. 'Cause all this happened in the late '80s you see. When it comes to the Shroud of Turin, we've been there, done that.

But high tops and big hair are back in style, and so too is the Shroud of Turin. The story of the latest archival research in the Vatican has been picked up by dozens of news agencies around the world, many of them reporting the document from 1287 which describes “a long linen cloth on which was impressed the figure of a man”. Many of the reports take the angle that this document might provide an explanation for where the shroud supposedly had been hiding, since it was last seen in 1204, but very few of them mention the radiocarbon dates that make it very unlikely that the shroud is an older than AD 1300. 

But radiocarbon dating doesn't really work, does it?

Yes it does. 

No, it really does work, and I would put far greater faith in the workings of radiocarbon dating than I would in the likelihood of relics of biblical or historical characters happening to turn up on our doorsteps. 

There is a lot of carbon in the world. And it comes in three different flavours. Most of it is plain old boring carbon-12. About 1% of it is carbon-13, which is so boring that we don't even need to mention it. An even tinier amount (0.0000000001%) is carbon-14, which is radioactive, and decays at a known rate. Carbon-14 is constantly being produced in the upper atmosphere by those crazy cosmic rays. As long as you're alive, you're constantly exchanging carbon with the atmosphere (aka "breathing") which means that you have the normal amount of carbon-14 in your body. 

When something dies, it stops exchanging carbon with the atmosphere, so the amount of carbon-14 in its tissues decreases at a known rate. The older something is, the less C14 it has, and by measuring the proportion of C14 left, we can tell how long ago it died.  

The dating in 1988 was completed using the then relatively new technique of AMS (Accelerator Mass Spectrometry) dating, which was able to count the number of carbon-14 atoms directly. AMS dating made it possible to take extremely small samples sizes, only a few square centimetres of cloth, compared to the then standard technique which indirectly measured the radiation produced by decaying carbon-14 atoms, and which would have required upwards of 500 cubic centimetres of cloth. The results were calibrated by comparing them with the standard graph of tree-ring dates, which takes into account variations in the atmosphere over time, and produced an age that was 95% likely to fall between 1290 and 1360. The farther a proposed date is away from that timeline, the less likely it is to be correct. 

For each sample that is prepared for dating, they also prepare a specimen of known age, such as animal bones that have come from a historically dated shipwreck. If there was contamination or something wrong with our understanding of radiocarbon dating, the date for the known date sample would come back wrong. Archaeologists did not just test radiocarbon dating once and walk away from it. Every single date is also performed alongside known age samples, so that the method is continually being checked and re-evaluated.

Every single published dated is double-checked this way, and if the known date sample was off, the entire run would be discarded, regardless of what the dates might have proved.

There are some people who continue to insist that the radiocarbon dating of the shroud must be wrong, by invoking explanations ranging from the fact that the wrong area was sampled (despite being selected by textile specialists), and that something in the way the shroud was handled or treated could have resulted in contamination. In the original 1989 report, the three different labs that had subsamples of the shroud tried a variety of pretreatment and cleaning methods. No matter what pretreatment method was used, the dates all turned out to be the same. The 1988 analyses were also performed alongside similar pieces of cloth and linen which had previously been dated using other methods. All the dates came out correct. 

The people who are asking us to accept that the radiocarbon dates are wrong are either asking us to reject radiocarbon dating altogether, or to believe that there is something special about the cloth of the Shroud of Turin that makes it unlike any other material ever dated. The implied assumption is that there is just something inherently funky about Shroud Carbon. After turning water into wine, and curing lepers, doesn't sticking a few extra carbon-14 atoms on a piece of cloth seem pretty easy?

What's the harm in all this?

From one point of view, nothing, I suppose. This is how science works, by constantly testing and retesting hypotheses. It is extremely unlikely that that the Shroud of Turin does not date to the middle ages, because it is extremely unlikely that our understanding of how radioactive carbon-14 atoms become incorporated into living tissues is wrong. If it was wrong, it would mean we would have to seriously re-evaluate almost every single radiocarbon date ever performed. The lab that I am familiar with has performed over 10,000 analyses in the last twenty years. The amount of proof that would be required to overturn such an accepted record would have to be extraordinary. The fact that radiocarbon dating can withstand such scrutiny today is proof of its validity, and of the success of the scientific method. So we can say as scientists, go ahead, hit radiocarbon dating with your best shot. And it will withstand the blow, almost certainly. Because if it doesn't, 50,000 years of human history would have to be rewritten, or at least re-examined.

Every now and then, there are problems with how radiocarbon dates have been performed. Filtering samples ensures that only well-preserved organic molecules or analyzed, but the filters ended up contaminating the samples with modern carbon. These problems were detected by comparison with known-age samples, and the method was fixed. Early on, archaeologists and scientists discovered that the production of C14 hasn't been constant over the earth's history, introducing the added complication of having
to calibrate all reported dates against a sequence of known-age tree ring dates. 

So in many ways, I agree with the head of the Oxford lab, who has allowed further tests to be conducted, to determine if sources of contamination were possible. As far as I know, the tests to date have only supported the original date. If people have several hundred dollars to throw around on some spare radiocarbon dates, it's not up to me to suggest that they might have a
better use for their money. 

What I do object to is that every time the Shroud of Turin is dragged out of its dusty box and trotted before us, the media fail to take a critical, scientific view. It is not up to us to decide whether we "believe in" the Shroud. It should not be the conversation starter between believers and non-believers that it has been. As Charles Greenia said: "if someone has based their faith in Jesus on the presumed legitimacy of the shroud, then there is something profoundly flawed about their understanding of their faith."

The first thought that I had upon reading the Vatican report of the knight's account of seeing an object like the Shroud was that it might attest to the fact that such relics were relatively common during the later middle ages. It is not possible that the idea of of a shroud with a picture of a person on it had already been an "urban legend" of sorts by the time that the forgers created the Shroud of Turin in the early 1300s? This possibility has not been considered by any other source. 

It's time for the Shroud of Turin to go back in the box. There are plenty of other scientific and historical mysteries that deserve our attention, and plenty of other dirty laundry waiting to be aired. 

References:

Damon, P.E., Donahue, D.J., Gore, B.H., Hatheway, A.L., Jull, A.J.T., Linick, T.W., Sercel, P.J., Toolin, L.J., Bronk, C.R., Hall, E.T., Hedges, R.E.M., Housley, R., Law, I.A., Perry, C., Bonani, G., Trumbore, S., Woelfli, W., Ambers, J.C., Bowman, S.G.E., Leese, M.N.&Tite, M.S. 1989, "Radiocarbon dating of the Shroud of Turin", Nature, vol.337, no. 6208, pp. 611-615.