Postscript (correction: ‘prescript‘) added July 2019:
You have arrived at a 2014 posting. That was the year in which this investigator finally abandoned the notion of the body image being made by direct scorch off a heated metal template (despite many attractions, like negative image, 3D response etc. But hear later: orchestral DA DA! Yup, still there with the revised technology! DA DA! ).
In its place came two stage image production.
Stage 1: sprinkle white wheaten flour or suchlike vertically onto human subject from head to foot, front and rear (ideally with initial smear of oil to act as weak adhesive). Shake off excess flour, then cover the lightly coated subject with wet linen. Press down VERTICALLY and firmly (thus avoiding sides of subject). Then (and here’s the key step):
Stage 2: suspend the linen horizontally over glowing charcoal embers and roast gently until the desired degree of coloration, thus ‘developing’ the flour imprint, so as to simulate a sweat-generated body image that has become yellowed with centuries of ageing.
The novel two-stage “flour-imprinting’ technology was unveiled initially on my generalist “sciencebuzz” site. (Warning: one has to search assiduously to find it, and it still uses a metal template, albeit unheated, as distinct from human anatomy):
So it’s still thermal development of sorts, but with a key difference. One can take imprints off human anatomy (dead or alive!).
A final wash of the roasted flour imprint with soap and water yields a straw-coloured nebulous image, i.e. with fuzzy, poorly defined edges. It’s still a negative (tone-reversed) image that responds to 3D-rendering software, notably the splendid freely-downloadable ImageJ. (Ring any bells? Better still, orchestral accompaniment – see , correction HEAR earlier – DA DA!))
This 2014 “prescript” replaces the one used for my earlier 2012/2013 postings, deploying abandoned ‘direct scorch’ technology.
Thank you for your patience and forbearance. Here’s where the original posting started:
Original posting starts here:
January 27, 2014 at 11:42 am | #19
“Question for Colin Berry: How in the world the same exact source of heat (with pretty much the same intensity) could only color the first fiber at the top surface of a linen cloth while the source of heat is located at 3 or 4 cm away from the cloth and, AT THE SAME TIME, could also color ONLY the first fiber at the top surface of the same linen cloth while the very same source of heat is located in direct-contact with the cloth?
Theoretically speaking, I think it’s fair to say that such a scenario seem close to absolute zero in term of probability…”
My immediate reply (posted to same site, i.e. shroudstory.com)
January 27, 2014 at 1:00 pm | #20
If you want my opinion, then all you have to do is ask. But kindly wait for an answer before attempting to preempt my reply with your probability assessments. I do not take questions under any kind of duress.
In fact, I will make a posting of your question, and provide a short answer on my own site.
And here’s that reply (a stiff whisky first may help)
In a simple conduction model there would be no scorching if there were even the tiniest air gap. In practice, there is scorching at a distance, due to convection processes (superheated steam, pyrolysis gases etc). However there are limited amounts of target hemicelluloses in the most superficial primary cell wall (PCW), the latter probably accounting for less than 5% of the total fibre diameter. So most or all of that thin PCW may become maximally discoloured in affected threads even with a small air gap (but not excessive – millimetres rather than centimetres) . When there is no air gap, then of course heat transfer becomes a lot more localised and efficient (direct atom-to-atom contact/conduction), but there is no greater maximal image intensity due to the presence of the same LIMITING amount of hemicelluloses. In other words, the system is one that shows what might be described as a low saturation effect, and it’s a result of the co-axial geometry, with susceptible PCW hemicelluloses most concentrated on the outside (though some in the core too – see diagram above) with a more scorch-resistant core comprising the secondary cell wall (SCW), the latter containing large amounts of heat-resistant crystalline cellulose). This explains scorching both by contact/conduction, but also at a distance (albeit not very much) primarily by convection. Easy saturability also provides an explanation for the peculiar half-tone effect (though I have previously expressed doubts as to whether it’s really as digitally, non-analogue, all-or-nothing as we have been led to believe).
PS: What I’ve described above is an illustrative example where a convection scorch from a distance could (in principle) be as intense as a contact scorch. But I’m not suggesting that is the norm. In practice, the norm would be a fainter image from a distance giving a range of contrast values that give depth, probably best appreciated after Secondo Pia-style light/dark inversion, and no doubt contributing to that iconic 3D-enhancibility.
Update: 29 Jan 2014