In his controversial book about the radiocarbon dating of the Shroud of Turin (The Shroud Unmasked), David Sox, who was visiting the Zurich laboratory with the BBC Timewatch team, tells an anecdote about Professor Willy Wölfli, the head of the department.
“He was also worried about the results he had with the linen tablecloth of his wife’s mother. It was 50 years old, but carbon dating said it was 350 years old. There might have been a problem created by detergents which had been used on the cloth.”
This little story has been taken up, and embellished, by the Shroud fraternity and often used to demonstrate the inadequacy of radiocarbon dating in general. However, I believe that Wölfli was in fact not at all worried about the apparent discrepancy, and that Sox mistook levity for concern. Detergent has nothing to do with it.
A cloth which is 500 years old contains 94.1% of the radiocarbon it had when it was a growing plant. By convention, the standard atmosphere is that of 1950, before a series of nuclear bomb tests greatly enhanced the Carbon-14 content of the atmosphere, so the ‘Before Present’ (BP) date is actually ‘Before 1950’. This is not a problem with archaeological artefacts, but leads to clear anomalies in dating modern materials. A cloth made in 1950 is now (2020) actually 70 years old, and contains 99.2% of its original radiocarbon, but is considered to be from 1880 in terms of its “years B.P.” Willy Wölfli knew very well that a 50 year old cloth contains 99.4% of its original radiocarbon, but that it would calibrate to 1900 (50 years before 1950), regardless of its actual date (probably about 1940).
But there’s more. The radiocarbon proportion of the atmosphere fluctuates over time, and overall, between about 1650 and 1950, gradually decreased, so that objects from later years (which have lost less radiocarbon than those from earlier) appear to have about the same amount of radiocarbon as those from earlier in the period, and thus appear to come from the same date. If Wölfli’s mother-in-law’s table cloth really had been radiocarbon tested – something I think extraordinarily unlikely – and found to have 99.4% of its original radiocarbon, corresponding to 50 years, say with an error of plus-or-minus 10, then it would genuinely have been impossible, from the results alone, to say if it came from about 1715, 1830, or 1900.
Diagram from the OUAU online calibrator OxCal, showing how a BP date of 50 ± 10 (the red curve on the left) can calibrate to c.1715, c.1830, and c.1900.
Wölfli knew all this perfectly well, and was most unlikely to have been worried that any such consideration reflected badly on the performance of his laboratory.
He may, however, have been alluding to a similar fluctuation in the radiocarbon proportion of the atmosphere, which occurred in the 14th century, exactly the time when the Shroud was probably manufactured. Although he seems to have made his remark before his laboratory had tested its sample, he would have known that a radiocarbon proportion of, say, 92.8%, corresponding to 620 BP, would calibrate to about 1315, 1360 or 1390, with no way, from the results alone, of determining which was correct.
Diagram from the OUAU online calibrator OxCal, showing how a BP date of 620 ± 20 (the red curve on the left) can calibrate to c.1315, c.1360, and c.1390.