Gum analogies that steer us wrong

Recently I've had occasion, searching through the archives for something
I know I wrote but can't find anywhere (searching the archives on the
chance that maybe I wrote it as a post to the list rather than as a page
on my website as I thought) to reflect on the many mistaken analogies
that have been used to explain how gum printing works.

The first and oldest one is the idea that gum printing (or any
dichromated colloid photography process for that matter) is accomplished
through the same mechanism as the chrome-tanning of leather. How deeply
that wrong analogy is engrained into the collective psyche of alt-photo
workers is demonstrated by the fact that many people continue to use the
term "tanned" to refer to crosslinked gum or gelatin in alt photo
processes, even though it seems like it should be abundantly clear by
now that the crosslinking of colloids in our processes has very little
similarity with how tanning processes work.

Then there's the idea that gum is like dye transfer, or like carbon
transfer. Coming across suggestions that one could harden a very thick
layer of gum and then peel it off a hard surface as a block, or that the
best way to adhere a gum image to glass would be to make a pigmented gum
"tissue," expose it and then adhere it to the glass with cool water
before developing it, one doesn't know exactly whether to laugh or to
cry. For anyone who really believes that these things are possible with
gum, I'd just say well, tell you what--- why don't you actually try it
and get back to us. My experience tells me that the nature of gum
(particularly the hypersolubility of dried unhardened gum even in very
cold water), the way hardened gum attaches to surfaces, and the fact
that dichromated colloid processes in solution don't behave like
dichromated colloid processes in film, argue against the likelihood of
any of these suggestions actually working.

Another analogy that has been suggested is that gum printing is like
commercial printing; it's just a matter of laying down pigments on
paper. Well, again, it's actually not very much like it at all. Gum
printing is about hardening gum, not about laying down color; the
pigment is encased within the hardened gum and doesn't behave anything
like the pigments in wet inks printed on paper with a commercial
printing press.

And then there's the one about the adhesion of hardened gum to a surface
being like the adhesion of paint or glue to a surface. No, actually a
more apt analogy for how hardened gum attaches to a surface would be
velcro, not paint. The gum is like the passive side; the "tooth" on the
surface is like the hooked side of velcro that catches into the passive
material and holds it tight. Having had some experience scraping
hardened gum off mylar, where I created tooth by sanding the mylar with
rough sandpaper; the tooth consisted of bits of mylar sticking up from
the surface, I can tell you that the "hooks" hold very tight. When I
tried to get all the gum off, I found that I was cutting off the "tooth"
along with the gum; to get the gum without bits of mylar, I had to use a
dull blade for scraping and be resigned to leaving some of the gum
behind, because the "tooth" was never going to let go of that last
little bit.

It's a lot like the story about the blind men and the elephant, I
suppose. One guy says the elephant is like a rope, another that the
elephant is like a tree trunk, another that it's like a wall, another
that it's like a hose. Perhaps it's useful to think of the different
parts of the elephant being like those things, and if individual gum
printers have actually been able to print gum better by considering that
gum printing in part may be somewhat like something else, then I say
more power to them; I have no criticism of analogies that are useful in
improving practice. But even in that case, it's important to remember
that there's a difference between saying "the elephant is *like* a hose"
and saying "the elephant *is* a hose."

My 25cents worth.
Katharine Thayer
Received on Tue Apr 19 13:37:57 2005