What are the biggest ‘plus factors’ when comparing the previous (LUWU) and the new (LOTTO) imprinting procedure? The old LUWU method (Linen Underneath, with Underlay) required very brief contact time, at most a few seconds, with a template initially neither too hot nor too cold and with the ever-present risk of over-scorching, especially as the geometry makes it difficult to monitor degree of scorching.
The new LOTTO method (Linen On Top. Then Overlay) needs the template very hot to start with, and a surprisingly long contact time (minutes rather than seconds) to get a decent scorch. During those minutes there is time to mould the fabric manually around the contours, but importantly there is very little risk of overscorching, especially as one can lift a corner to check on progress. Manual moulding offers a far greater degree of flexibility than the passive moulding to relief that occurs when a solid template is prodded down into linen and underlay with a wooden stake or similar – with the ever present risk of tenting of stretched linen between prominences, and thus failure to imprint the parts in between.
The LOTTO image, perhaps not surprisingly, looks softer and fuzzier than the instant LUWU imprint. I’m not sure how one compares fuzziness on a scientific basis*, but it’s always a factor that seems to be implicit or explicit in aesthetic judgements when folk look at any of our feeble attempts to reproduce the Shroud image. The result, before being mercilessly subject to 3D imaging and microscopy, MUST have that certain indefinable wraith-like quality you understand if it is to stand any chance of being considered at least a “decent effort, a jolly good try old chap, but still lacking that certain je ne sais pas quoi, know what I mean like?”
*Update: 25 Jan have just this minute comes across a reference to and ‘edge-finding’ on Mario Latendresse’s site:
I’ll need to study it carefully before commenting in detail. For now, it’s worth pointing out that the Shroud image lacks edges – which is another way of saying it is fuzzy, an observation that should surprise no one, but backed up by a measurement process. Quite how it works at the pixel level I shall need to find out. It’s presumably something to do with gradation of density across given linear distances. Is there an arbitrary criterion built into that one wonders?
Theoretical footnote: why should it make so much difference as to which of the two procedures one uses to imprint, with one producing a scorch in seconds, the other in minutes? Both have a hot template, a sheet of linen and a thick piece of damp cloth (folded I might add to get two or more layers).
I don’t pretend to understand all the subtleties, but would suggest the following for starters. It’s to do mainly with applied pressure and the degree of contact between metal and linen. In the earlier LUWU method, there was a strong force applied to the template from above, pushing it into the linen. That resulted in a high degree of atom-to-atom contact between metal and linen, allowing heat transfer by conduction. Steam and pyrolysis gases had to escape as best they could in an unnatural direction (downwards and sideways) instead of upwards, the preferred direction for convection currents.
When linen is draped over a hot template, followed by the damp overlay, the only pressure is from the weight of the fabric itself, and the manual patting. That results in much less atom-to-atom contact, so initial heat transfer by conduction will be smaller. However, another possibility for heat transfer arises – convection. Even with relatively little contact initially the air space between metal and linen will heat up, the expanded air will tend to escape, and, IMPORTANTLY, its place will gradually be occupied by steam (linen is reckoned to have about 8% moisture typically which needs to be driven off before pyrolysis can take place). Maybe that steam become a means of transferring heat from metal to linen via a gaseous but still material carrier. In other words, the pyrolysis is less about metal-linen contact (producing sharp image edges) and more about pyrolysis caused by laminar flow of superheated steam (and other pyrolysis gases). The image is still primarily a contact scorch but is softened , i.e. made fuzzier, through being a convection scorch too. Whilst pyrolysis is in essence temperature-dependent, it is probably immaterial whether the temperature rise is due to direct contact with hot solid metal at, say 225 degrees C, or contact with superheated steam and other gases at the same temperature. What matters is the temperature, not the means by which heat was supplied to produce the temperature increase.
Summary: how to produce a ‘Shroud of Turin’: slow- roast linen over a shaped metal template, with an overlay of damp cloth, and occasional moulding of the fabric layers to the template to capture relief.
“Slow roast”? Ring any bells. Think Knights Templar. Think Jacques de Molay. Think slow-roasting over charcoal, Paris, 1314.
Update, Monday 2 December: another posting on The Other Site (mine, this time):