It’s a simple matter to produce a series of scorches of decreasing image intensity, right down to ones that are scarcely visible – or merely a pressure imprint with no apparent pyrolysis – at least to the naked eye. But measuring their thickness without specialist microscopes is another matter entirely. I tried a different tack a short while ago, which was to strip off the epidermal sheets of cells that line the inner scale leaves of onions – which are just a single cell thick – to dry them overnight, and then to press a hot metal template onto the sheets with an underlying layer of linen. It’s easy to get an intense scorch on the epidermis with little or no scorching of the underlying linen. It’s an approximate model of course – the onion epidermis is not a perfect model for the outermost cell layer of linen. It probably scorches primarily by Maillard reactions (sugar and protein) rather then by caramelisation of carbohydrate, which begins according to one literature reference at 154 degrees C. However, the templates I used that scorched the epidermis were sufficiently hot to scorch unprotected linen too, so I doubt if I had chosen too low a temperature.
What I don’t know, of course, is whether the image on the epidermis, with suitable choice of experimental conditions, might be be confined to the 200nm(?) thick PCW, or whether the entire cell layer is scorched (the latter being much, much thicker, say 200 or 300 microns).
What the experiment demonstrates is that scorching by thermal energy – in this instance conduction by direct contact – should not be assumed to be some catastrophic all-or-nothing process that penetrates the entire thickness of fabric – as Paolo Di Lazzaro seemed to be proposing with his hot coin experiment. It can be fine tuned, making it progressively more superficial – even if the thickness or precise location of the final limiting image cannot be defined at present. I believe he and his ENEA team were much too hasty in assuming that the Shroud discoloration could not be modelled using thermal energy, including simple conduction. As for the immediate recourse to uv energy – from an excimer uv laser no less – words fail me – especially when one reads the press reports in the UK that accompanied ENEA’s work last December ("miraculous flashes of light").
Dawkins? Forgive my saying, but I don’t see why he has to be involved in a discussion of the authenticity of the Shroud as the burial shroud of Christ, as distinct from a medieval fake, distinguished scholar though he is. Yes, I know he has expressed views on the C-14 dating, but I don’t see that being regarded as a chief spokesman (bogeyman to some ;-) on behalf of an atheistic world view should give him any privileged insights re dating technology, and in any case risks creating an unnecessary distraction, conflating scientific scepticism with atheism. It should be possible to put the Shroud under a metaphorical microscope without having one’s own religious beliefs (or lack thereof) similarly scrutinised and commented upon That’s just my opinion, which you are free to ignore…
I have been giving the Dawkins Challenge a lot of thought. I tend to agree with the last paragraph that Colin wrote above. I expect we’ll see a lot more of him in David’s neighborhood.