How did the Turin Shroud get its image?

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Source: BBC

On Sunday, Pope Francis will “venerate” the famous Shroud of Turin, which is thought by some to be the burial wrapping of Jesus Christ – and by others to be a medieval fake. Whatever it is, it’s a mystery how the cloth came to bear the image of a man. Science writer Philip Ball discusses the theories.

In a carefully worded announcement, the Archbishop of Turin says that the Pope “confirms the devotion to the shroud that millions of pilgrims recognise as a sign of the mystery of the passion and death of the Lord”.

You’ll notice that this says nothing about its authenticity. The Catholic Church takes no official position on that, stating only that it is a matter for scientific investigation. Ever since radiocarbon dating in 1989 proclaimed the 14ft by 4ft piece of linen to be roughly 700 years old, the Church has avoided claiming that it is anything more than an “icon” of Christian devotion.

But regardless of the continuing arguments about its age (summarised in the box at the bottom of this page) the Shroud of Turin is a deeply puzzling object. Studies in 1978 by an international team of experts – the Shroud of Turin Research Project (Sturp) – delivered no clear explanation of how the cloth came to bear the faint imprint of a bearded man apparently bearing the wounds of crucifixion.

There’s no shortage of hypotheses. Some suggest that the image came about through natural processes; some impute considerable ingenuity to medieval forgers of relics; others invoke wondrous physical processes associated with the Resurrection. But do any have any merit?

1. It’s a painting

If this were true, it should be possible to identify the pigments used by chemical analysis, just as conservators can do for the paintings of Old Masters. But the Sturp team found no evidence of any pigments or dyes on the cloth in sufficient amounts to explain the image. Nor are there any signs of it being rendered in brush strokes. In fact the image on the linen is barely visible to the naked eye, and wasn’t identified at all until 1898, when it became apparent in the negative image of a photograph taken by Secondo Pia, an amateur Italian photographer. The faint coloration of the flax fibres isn’t caused by any darker substance being laid on top or infused into them – it’s the very material of the fibres themselves that has darkened. And in contrast to most dyeing or painting methods, the colouring cannot be dissolved, bleached or altered by most standard chemical agents. The Sturp group asserted that the image is the real form of a “scourged, crucified man… not the product of an artist”. There are genuine bloodstains on the cloth, and we even know the blood group (AB, if you’re interested). There are traces of human DNA too, although it is badly degraded.

That didn’t prevent the American independent chemical and microscopy consultant, Walter McCrone, who collaborated with the Sturp team, from asserting that the red stains attributed to blood were in fact very tiny particles of the red pigment iron oxide, or red ochre. Like just about every other aspect of the shroud, McCrone’s evidence is disputed; few now credit it. Another idea is that the image is a kind of rubbing made from a bas-relief statue, or perhaps imprinted by singeing the fabric while it lay on top of such a bas-relief – but the physical and chemical features of the image don’t support this.

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