The principles underlying the process are almost ridiculously simple, and it seems to me that if gum printers grasped the simplicity of the process, they would be less likely to fall for magic formulas and incantations.Gum and pigment in watercolor painting vs gum printing
A simple way to ease very gently into thinking about the mechanism of the gum bichromate process is to consider the difference between a watercolor painting and a gum print. In both cases, the color medium is pigment held in gum arabic. But in the case of the watercolor painting, the gum arabic remains water-soluble in the final product, whereas in a finished gum print, the gum arabic is insoluble in water. What permanence there is to a watercolor painting is a function of pigment staining: where the pigment stains the paper, that permanent stain constitutes the permanent image. Whereas the permanence of a gum print is due to the insolubility of the hardened gum. (In both cases, it's important to use lightfast pigments in order to assure permanence of the color, but that's a different issue from permanence of the image itself).
Some watercolor painters add gum arabic to their colors to increase the ability to lift off color and decrease the staining power of their paints. I wonder if they realize that by doing so, they are making a painting that is perilously water soluble. I haven't ever seen such a warning in any of the many watercolor books that I have read, but it is certainly and demonstrably true. I fell into this mistake myself by accident once: a small container of gum-pigment mixed for gum printing got knocked over and a puddle of it dried on the glass surface of my gum-coating table before I discovered it. I was so enchanted by the quality of the dried paint-- a meltingly smooth passage of gorgeous deep color with a lovely organic sheen that simply couldn't have been replicated in acrylic-- that I made two paintings using thick applications of this gum-heavy paint, before I came to my senses and remembered the first principle that governs the gum bichromate process: Dried unhardened gum dissolves in water.
Just to prove to myself what I already knew was true, I took a dampened brush and ran it across one of these paintings, and sure enough, there went the picture, dissolved instantly. That was the end of my short "gum painting" period. (I have done a few experiments since then using hardening agents such as chrome alum or glyoxal to harden the gum to make it insoluble, but it seems that the hardening process destroys the limpid melting sheen of the dried soluble gum; without that lovely sheen one might as well paint in acrylic.)
So while watercolor painting and gum printing both use pigment held in gum arabic, in watercolor painting the gum is there only to bind the pigment; the image itself is made of pigment staining the paper. In gum printing, the gum is there to be hardened and form the image. For the two purposes, a different ratio of gum in relation to pigment is required: for watercolor painting, only enough gum to hold the paint in suspension is necessary, but for gum printing, enough gum must be present to form the image.
Beginning gum printers, including myself as a beginner, almost always want to use too much pigment and even imagine that if they could figure out a way to use pigment directly from the tube without adding any gum, that would be ideal. But such a wish simply shows a lack of understanding of what the process is. Remember, it's GUM printing; the object is to harden GUM. So there needs to be enough gum to become hardened and to create the image, which though rendered visible by the entrapped pigment, is actually made of gum arabic, hardened differentially according to the amount of exposure. The pigment is just there, trapped in the hardened pigment like a beetle encased in amber.
Another reason to add gum to the paint is that paint without gum added will stain paper, which makes sense given that in watercolor painting, the goal is pigment stain. But in gum printing, pigment stain is usually anathema, as the stain interferes with the visibility of the image. So the added gum serves two purposes: it enables the formation of the image by providing sufficient gum for hardening, and it prevents pigment stain which would obscure the image.
Gum arabic, dichromate, and UV radiation.
Pigment is added to render the image visible, but isn't part of the process; the gum process is about hardening gum. The pigment is just along for the ride.
(1) dried unhardened gum arabic is very soluble in water, which allows the gum and its suspended pigment to clear completely from areas which are protected from ultraviolet radiation.
(2) gum arabic which has been hardened by exposure to UV in the presence of dichromate is very insoluble in water.
Consider the two color swatches below.
The left swatch is gum arabic and pigment, mixed for gum printing, brushed on a piece of paper, dried for several weeks, then one drop of cold water was dropped onto the dried pigment/gum mix and immediately blotted. As can be seen, the dried gum arabic dissolved instantly on contact with water. And in fact, anyone who has accidentally splashed a drop of water onto a gum layer that's freshly coated and dried and ready for printing, will have seen this principle in action.
The right swatch is the same pigment-gum mix with dichromate added, exposed and developed; in other words it has gone through the normal gum printing process. To show how invulnerable to dissolution the hardened gum is, this swatch has not only had water dropped on it without effect, it has of course been soaked in water during the gum development process. It has also been soaked in boiling water, in household ammonia, and in 7% acetic acid without effect, and has had boiling water poured on it with force, again with no effect. The one thing that fazed it at all was household bleach; the visible spot on the swatch was made by dropping full strength household bleach on the hardened gum; you can see from the spot where bleach was dropped on it that bleach does dissolve hardened gum, (although not as fast as plain water dissolves unhardened gum; as you can see it didn't dissolve all the way through to white paper before I blotted it) but not much else does.
[The mottled appearance of the jpeg on the right must be an artifact of the scan, as it's not in the swatch itself. It seems to be a function of the slight texture on the scrap of museum board I used to print the swatch on; the scanner picked up this texture and accentuated it in the jpeg]
That's really all there is to the process, in brief: unhardened gum is very soluble; hardened gum is very insoluble. Once exposed, the print is placed in water to separate the soluble gum from the insoluble gum, and voila: a completed gum print. This may seem almost a foolishly simple way to explain the process, but I think it helps sometimes just to peel away all the jargon and esoterica and see the process in its bare bones. The bones are really quite elegant if you think about it.
When gum is mixed with dichromate and exposed to ultraviolet radiation, the ultraviolet radiation triggers an electron transfer between the gum and the chromium. The electron transfer opens the way for elements of one gum molecule to bond with elements of another gum molecule in a process called crosslinking. The result is a matrix of crosslinked gum; the crosslinking renders the gum insoluble.
One physical chemist explained it to me this way: "it's like making a net out of ropes." Now that I know more about what gum molecules are like, I'm not sure the analogy fits entirely, since gum molecules seem to be more like a series of spheres than like ropes, but never mind about that; for now I think the analogy is useful to help you visualize how the creation of a matrix makes the gum insoluble.
That's the simple explanation; see the page on chemistry of the process for a more technical account.