(ed: things have moved on a bit since writing this post. For a more up-to-date account of my theory, see this later post, link and the one preceding it link)
ed: a lot of the step-by-step photographs have mysteriously disappeared from this post. I will endeavour to restore them ASAP.
The technique is essentially one of 'branding' (yup, as onto the hide of cattle) but the 'branding iron' was more probably a bronze statue - which did not need to be red hot to create a superficial scorch on linen.
This is intended as the briefest of summaries. I propose using my own comments section to flesh out the details, but only in response to queries. In the absence of comments my time is probably better spent in further experimentation (continuing the work I have described in previous posts on thermo-imprinting or thermo-stencilling).
Essence of the new model:
1. It uses a statue or bas relief of a crucified Christ - the kind of icon that would have adorned (if that is the correct term) many a medieval church or cathedral.
2. A shallow sandpit (US: sandbox) is made with fine dry (ed. or maybe moist?) sand and levelled off with a rake.
3. The sheet of linen is stretched over the top of the sand and smoothed out. The linen may have been impregnated to make it more receptive to acquiring a heat-imprinted image (think "invisible writing" that uses dried-on lemon juice or similar).
4. The statue is evenly heated in a kiln or oven until a test shows it to be hot enough to make a yellow or brown impression, i.e. 'scorch mark', on a side-sample of the linen.
(Whether one calls it a scorch or not is a matter of semantics that can be discussed later. Certainly it does not have to be hot enough to degrade or scorch bulk cellulose per se, except perhaps for a highly superficial imprint).
5. The heated statue is then placed face-down horizontally onto the linen, and pressed down lightly.
Except for awkward bits like the feet (see later) the statue is probably pressed no more than a cm or two into the sand, just sufficient to imprint as a light scorch the most prominent frontal features of the statue onto the linen, with little of the side features that might otherwise later give distortion when the cloth is removed and flattened.
The image would of course be a "negative", but that has nothing to do with photography, primitive of otherwise. The technology here is better described as thermal-imprinting by direct contact, relying mainly on heat conduction rather than radiant energy..
6. When a satisfactory image has formed of the ventral (frontal) side, the second unused half of the sheet is positioned over the newly raked sandpit, and the process repeated for the dorsal (rear) side of the statue.
7. What about the blood stains - getting them correctly positioned etc? That can be arranged by a slight modification of the procedure.
It requires a "dry run", or more correctly a "cold run". Firstly, the cold, unheated statue is pressed onto the cloth so as to penetrate the sand a little, leaving an indentation in both the sand and the linen. The statue is then carefully removed, and blood is then applied to the appropriate parts of the anatomy, as judged from the indentation. It may be left to clot and dry first. The statue is then taken away and heated, and then deposited carefully back into its original indentation to ensure consistent alignment, then pressed down a little more into the sand in order to get a good impression.
8. Created in this way, I believe the image would meet some subtle criteria that so far have not been fully achieved, e.g. in Jackson's work with his bas-relief models (see earlier post).
I believe a balance can be struck that can achieve a good compromise between a shallow bas-relief and a fully 3D statue.The compromise gives enough relief to account for 20th century "3D-encoded information" - which if the truth be told is really just an (over-hyped) analogue- to-digitized impression of 3D, ie differential scorching- to-computer-aided graduated relief. The dynamics of the pushing process, ie. - the gentle pressure on the statue, impressing it into the fabric and into the receptive cushioning sand to achieve progressively greater contact between cloth and hot metal is what helps to achieve a softer-focus more natural effect.
9. Because the statue is pressed downwards into the sand, i.e. at right angles, that would account for the so-called "directionality" of the image-forming process.
Attempting to explain the latter with radiation and projected images has been problematical, in the absence of lenses, concave mirrors, collimating systems etc., none of which are credibly medieval or indeed achievable today
10. This procedure might also explain some of the curious - or at any rate unexpected - features of the Shroud, e.g. the somewhat elongated fingers.
To get a better idea of how soles come to be imprinted, and how they look on the flattened sheet, imagine yourself lying down on a sheet with muddy boots, first with heel-only contact, then imagine you ask someone to raise and then press the end of the sheet against the muddy soles to leave an imprint. Then imagine how the imprint will look when you get up and view the flattened-out sheet from above.
Comments - premoderated - are invited, but ad hom attacks will not be tolerated (or published) If posting as "anonymous" please append an initial or two.