Yes, hemicellulose is, I now believe, the answer. Each fibre in the yarn, or which there are reckoned to be some 200 per thread, comprises a core of physically and chemically resistant cellulose microfibrils, and the entire assembly is then held together by a coating of hemicelluloses and (in the native plant) some pectins too – the latter being lost we are told in retting.
Here is a diagram culled from the internet (Annenberg Learner Site)
Note the rod-like cellulose cores and the relatively open matrix of enveloping filamentous hemicellulose. See my preceding post for information on the chemical make-up of hemicelluloses (which is entirely different from cellulose, being better described as predominantly xyloglucan in nature , rather than simple glucan)
Here’s a second imported picture (with original caption) to show the structure of fibres at a lower magnification, in which I have coloured the highly crystalline cellulose blue, the more loosely packed hemicellulose yellow.
Yup, I now believe that hemicellulose is the key to understanding the nature of the image on the Turin Shroud – and yes, the latter has most if not all the characteristics a heat-induced scorch, but a highly selective one that was largely confined to the hemicellulose coatings of the fibres, leaving the cellulose cores largely unaffected.
Why do I say that? One of the major challenges has been to explain the highly superficial nature of the Shroud image, reckoned to be a mere 200nm (that’s approximately the thickness of gold leaf) , so thin and weakly attached that it can be stripped off with adhesive tape, as described by the late great Raymond Rogers.
Let’s do a thought experiment to see how that Turin Shroud could have been produced using simple technology that was available in medieval times, supported by a theoretical framework that is in tune with physics, chemistry and botany.
Here goes: a bas-relief representation or bronze statue of the crucified Christ (the problem re arms, blood etc will be discussed some other time) was heated until hot enough to scorch linen, and then pressed down into the linen with an underlying bed of sand or similar, as discussed earlier in my miniature metal trinket system (see banner).
The first part of the linen with which the hot metal impacts would be the primary cell walls of the flax fibres, and it is the latter that are predominantly hemicelluloses.
Hemicellulose degrades easily when heated, far more so than cellulose, and at a lower temperature. The respective temperatures for the onset of pyrolysis (“scorching”) measured as weight loss are 220-315C for hemicellulose and 315-400C for cellulose respectively (H.Yang et al, 2007 ).
The scorching will initially be confined to those parts of the fabric that are in IMMEDIATE contact with the hot metal; no air gap is permissible, since radiated heat will not scorch white linen. What is more, the scorch will be confined to the outermost fibres of the thread, because the scorch will tend remain trapped within the first-encountered fibres, rather than being able to “jump across” to adjacent fibres. Why is that? It is because the resistant cellulose cores that are unaffected are able to conduct away heat rapidly, bringing the temperature of the hot template down to below that which will induce scorching Is it realistic to suppose that cellulose fibres could conduct away heat without themselves becoming degraded? Yes. I believe it is – being the basis of a well-known “party trick” at scout camp, which involves boiling water in a paper bag. More about that later.
But there is another curious detail about the Shroud image which is capable of explanation in the hemicellulose model, and once we have addressed that we shall be in a position to explain the so-called “half tone “effect.
Whereas the pyrolysis (“heat-induced degradation”) of cellulose is endothermic, i.e. requiring constant application of heat to be sustained, the pyrolysis of hemicelluloses is reported to be exothermic ( see previous post) In other words, once started it can, in principle (thermodynamic principle that is) be continued after the initial source of priming heat has been removed or gradually conducted away). A burning match is a model: provided it is held with the lit end slightly down, it will continue to burn, because heat from the flame provides the energy for pyrolysis of fresh wood, carbonising it, releasing more flammable gases, allowing more combustion, more flame, more pyrolysis, and the potential for a runaway reactions . I believe there is a low-level (flameless) conflagration, a micro-runaway reaction if you like, when hot metal encounters hemicelluloses-coated fibres: the hemicellulose pyrolyses (“chars”) where it is in immediate contact, but a spreading zone of pyrolysis then runs around the complete encircling cylinder of hemicelluloses, leaving a bare, largely untouched core of cellulose. That would explain the thinness of the image (representing the primary cell wall/hemicellulose thickness) but would also explain the fact that the image is in the ENTIRE coating of each of the (relatively few) affected fibres.
We are now in a position to understand the half-tone effect. The latter description refers to a curious feature of the Shroud image. It might have been expected that where the image is most dense, each fibre would show a greater degree of coloration. That is not in fact the case. Image areas are a mixture of coloured and uncoloured fibres. A denser image represents a greater number of coloured fibres per unit area, but coloured fibres are all said to be much the same hue. In other words, there is an all-or-nothing effect about image-forming at the level of individual fibres. The hemicellulose model accounts for that neatly. As soon as a scorch forms on a hemicelluloses coating, it will tend to propagate around and along the fibre, using the thermodynamic enthalpy of reaction (nett heat release) to sustain a “runaway” reaction until a point is reached, due to unfavourable geometry etc where there is insufficient heat to produce the Arrhenius activation energy, and the pyrolysis promptly ceases (rather like a lit match going out when one holds it at the wrong angle). This model accounts for other strange observations too, like the coloured region extending far along individual fibres without affecting adjacent ones. That is fully consistent with the self-propagation effect of an exothermic pyrolysis that prefers to stay within the one thread, there being insufficient energy to make the jump from thread to thread or even fibre to fibre.
Earlier I mentioned a strange and unexpected phenomenon that has a bearing on this model in which the hemicellulose is selectively degraded by contact with heat, while the cellulose survives relatively intact. That was the “boil water in a paper bag” effect.
Yes, a naked flame can play on the underside of a paper bag without the latter catching fire, or even scorching, while the water inside the bag boils. That suggests that cellulose fibres can conduct sizeable amounts of heat without degrading. It may be that water in the interstices between fibres helps, either to conduct heat or protect the cellulose, but it has to be remembered that the same effect can be achieved by heating water in a semi-inflated rubber balloon. In other words, interstitial water in the envelope is not obligatory, given that rubber is a water-repellent hydrocarbon.
There is probably enough to be getting on with for now. Suffice it to say that this “selectively discriminating” hemicellulose model that leaves the cellulose intact can explain some other curious features of the Shroud, including the fuzzy image of a face that has been seen and photographed on the reverse side of the fabric, with no sign of degradation of cloth between the two external surfaces of the fabric. That too is probably down to the “paper bag/rubber balloon” effect, with the difference that when the conducted heat reaches the opposite side, it quickly dries out any moisture that is present (including sand in my model) and can then initiate selective pyrolysis of hemicelluloses on that opposite side too (leaving the observer then asking “Who -or what – managed to pull off that trick?”).
Finally, for those with a chemical bent, here’s a handy little graphic I’ve just spotted that summarises the kind of fate that awaits a hemicellulose molecule if provided with enough thermal energy to push it over that Arrhenius activation energy hump. Thereafter it’s downhill all the way:
Postscript: arising from discussion on another site, I’ve thought of a name for my safe, self-limiting pyrotechic model – one in which the open plan design of the primary cell wall rules out a major conflagration – “the fuse wire strung between scaffolding model” (see first graphic).
Colin Berry, aka sciencebod
18th February 2012