The scorch theory should be discountable by examining the range in grey-scale values. The Shroud should have a greater range in grey-scale values compared to a scorch, which would presumably have more uniform values.
The first question that needs to be asked is what mechanism is being proposed for the Shroud image formation if not a scorch? Why suppose it has a wider range of grey-scale values than other mechanisms? Do I not detect a hint here that the Shroud image is being thought of as a photograph? But there is no evidence that it is a photograph, whether a pseudo-negative or not. A scorch from a hot template however immediately accounts for the pseudo-negative character, so there are no grounds for making the scorch mechanism a poor relation so to speak, especially as I have yet to hear any mechanism for how rival hypotheses, credible or otherwise, can produce a negative image.
Now let’s consider the mechanics of producing a scorch image (“thermal imprint”) in detail. They can vary significantly. One could have the figure/subject/template lying horizontally (“recumbent”) on a raised support, heating it and then merely draping linen over the top. Thermal imprinting will be greatest where there is closest contact, e.g. the tip of a nose, and least where there is no contact, say the eye hollows. But as I said many moons ago, I do not believe that was the mechanism – the image produced would register too little detail. A more realistic mechanism is to lay the linen over a bed of yielding material, say sand, or snow, or wool, or many layers of fabric, and to press the heated template DOWN into the linen to get a better imprint.
The next step is to consider the nature of the template. If it were a very shallow bas-relief, like a coin say, the end product would be little different from that of a rubber stamp image, with at best a crude differentiation between raised/not raised relief.
But the Shroud image is too good, especially when seen in the Secondo Pia dark/light reversed form, to have been obtained from a shallow bas relief. I believe it was formed from something that was probably intermediate between a shallow bas relief and a fully-rounded statue, and have suggested a plaster cast replica of a real person as a possibility, essentially a death mask (see my recent post), with separate casts perhaps for the torso, limbs etc
When you analyse the likely points of contact, and, equally if not more important, the pressure at those points of contact with the linen in a “sand bed” model, it quickly become clear that the gradation of image densities in the final imprint can approach the complexity and subtlety of a photograph. Without labouring the detail, one could have two quite different reasons for a particular feature of the anatomy making a good imprint. The first is that it is prominent (“sticks out”) like the nose, and given that it probably digs deep into the underlay, compressing lots of sand as it goes, a good imprint is assured. But there are other parts that do not “stick out” that could still imprint well, namely the planes that are most “square on” to the linen, like the forehead, say, which quickly encounter resistance when pushed into the linen/sand. Conversely, there are planes that are “side on” rather than “square on” like the two sides of the nose, the peripheries of the face near the ears etc that will not press firmly against the linen and underlay material, that will leave a weak imprint. The end result is a complex image: small wonder then that it may respond in a spectacular fashion to light/dark inversion, followed by transformation in a 3D-enhancement program that interprets image density as height. See my banner graphics for what was achieved simply by making a thermal imprint of a metal trinket in a sand bed system.
So, in summary, I would say to Chris that we already know from modelling studies that there is nothing inferior about a thermal imprint (“scorch”) if taken from a good template with ample 3D contours, and arranging the presentation onto linen so as to best capture and record those contours.