One of the common misconceptions about the chemistry of the gum process is the idea that the photohardening of the gum is accomplished by the same process as the tanning of leather. As Kosar (4) says, "All tanning theories suggest that the hardening of dichromated colloids is in principle the same as the chrome tanning of leather," and in fact to this day people refer to gum or gelatin as "tanned" when it has been crosslinked by a photoredox reaction.
The logic connecting gum printing to leather tanning seems to go something like this: leather tanning uses chromium. Dichromated photoprocesses use chromium. Leather is made from collagen. Gelatin is made from collagen. So dichromated gelatin must work like leather tanning. And dichromated gum must work like dichromated gelatin, of course, so therefore......dichromated gum must work like leather tanning!
I don't know about you, but I have some problems leaping the gaps between the steppingstones of this logic without getting my feet wet. Let's start by looking at leather tanning, and thinking about what relevance this process might have for our process.
Leather tanning in the older days started with Cr(VI) just as our process does, but Cr(VI) was then converted to Cr(III) in a separate set of reactions (not involving light and not involving tanning) and the Cr(III) was then used to tan the leather. For 100 years or so, I'm told, most leather tanning has started with Cr(III) and skipped the first step altogether, to avoid the environmental and health hazards of Cr(VI). So right away there's one important difference: leather tanning uses Cr(III); dichromated gum uses Cr(VI). If we started with Cr(III) for our process, let's say a Cr(III) that is known to react with gum to harden it, such as chrome alum, the gum would simply harden, all of it, without reference to which part was exposed to light and which part didn't have exposure to light. Leather tanning is not a photoprocess, and dichromated gum is. And when the leather is tanned, it turns blue. When gum is tanned, it is clear, unless there is dichromate stain present, in which case it is brown or tan. So those are the first obvious differences.
But there's more. The leather tanning process is incredibly complex, involving pickling liquors, tanning liquors, masking liquors, and other liquors whose names I don't remember. The Cr(III) compounds form complicated complexes with elements of themselves and with the other materials in the liquors; one researcher identified fifteen different chromium complexes in the tanning liquor. As one reviewer said, "The knowledge about chromium complexes, although far from completeness, is quite extensive. However, relating it to tanning chemistry is very difficult. [Physical Chemistry of Leather Making]
As one example of the difficulties and uncertainties in the chemistry of leather tanning, it's never been completely understood what happens to the chromium. It attaches to the pelt and turns the pelt blue, but for a long time it seemed to be most likely that the chromium compounds were simply deposited in the collagen, rather than being complexed with it. Some of the evidence supporting that idea was electronic microscopy that showed clearly that chromium was deposited in the collagen fibers in widely- and randomly- spaced bunches, in areas of disorder in the collagen, rather than being distributed throughout the leather as would be expected if the chromium coordinates with the collagen.
But current theories of tannage seem to accept a proposed mechanism involving a ligand exchange in which carboxylic groups of collagen are complexed with the chromium complexes described above. This theory is supported by the fact that when carboxylic groups are blocked by methylation, then collagen binds less chromium and the temperature at which shrinking of the leather occurs (a measure of the extent that leather is tanned) is lower. It also rests on acceptance of the chromium-collagen carboxylic group as the most stable of the complexes known. However, it does not explain the electron microscopy that shows chromium deposited in randomly-spaced clumps rather than evenly distributed.
At any rate, current wisdom in the chemistry of the leather tanning process goes like this, from [Physical Chemistry of Leather Making]:
"Chrome tanning may be defined as the control of four competitive reactions:
(1) entering hydroxylic groups into the chromium complex to give it the required charge;
(2) binding of the anion in the first sphere of the complex in order to make it susceptible to the masking;
(3) the masking reaction: entering of the organic acid residue into the complex
(4) reaction of the protein carboxyl group with the chromium complex.
Crosslinking could be believed to be the fifth reaction, i.e. reaction 4 proceeding when the unipoint binding has already been formed. But this reaction has not been tested separately. It is presumably slow; probably it still proceeds during the piling of the leather after tanning."
Whether or not this is an adequate explanation of the chemistry of the leather tanning process must be determined by the chemists of leather making, but one can be quite sure it isn't a good explanation of the chemistry of the gum process.