Tanning theory of dichromated colloids (was gelatin

Ryuji Suzuki wrote:
>
>
> Take that idea for a moment and comment on how similar gelatin and
> leather are, and why discussion on one is useful for the other, and to
> what extent. (in 250 words)
>

Not very much. At all. Not very useful. At all.

But unfortunately it's been received wisdom in photography for 125 years
or more, under the "tanning" theory of dichromated colloids, that the
chemistries are essentially the same. Kosar: "All tanning theories
suggest that the hardening of dichromated colloids is in principle the
same as the chromium tanning of leather." This model has never felt
like a good fit to me, but lately I decided that before I say so, I
needed to know more about the chemistry of leather tanning.

My goal is to show that the "tanning" model is not at all useful to
explain the chemistry of dichromated gum. The leather tanning model, as
it turns out, doesn't even explain leather tanning, let alone carbon
printing, and even less gum printing, and not at all PVA printing. If I
could get people to stop using the word "tanned" when they refer to
hardened gum, I would feel as if I'd done something useful, but I expect
I'll be no more successful with this than with the crusade to get people
to refer to specific pigments rather than to paint names when talking
about color.

Hope that clears it up. I don't have time to count, but I think that's
still under 250 words.
Katharine Thayer
Received on Fri May 7 09:11:03 2004