From: Richard Knoppow (dickburk@ix.netcom.com)
Date: 05/12/01-06:27:52 PM Z
At 01:02 AM 05/13/2001 -0400, you wrote:
>Katharine Thayer wrote:
>
>>Sorry, I thought I was done with this, but this above isn't making sense
>>to me. Yes, there is an oxidation process going on, but the way I've
>>always understood it, it's the colloid that's being
>>oxidized, hardened, "tanned", not the dichromate (and for that matter,
>>it was always my understanding that "tanning" in this case means
>>hardening, rather than changing color).
>
>It does appear that tanning of some colloids involves changing color. For
>example, in developing films with tanning developers like pyrogallol and
>pyrocatechin the gelatin changes color as it is tanned or hardened.
>Depending on the combination of film and developer the color can be brown,
>yellow or even green.
>
>This may have nothing to do with gum printing, or then again it might.
>
>Sandy King
>
I think the color change with Pyro is not due to the hardening of the
gelatin but rather a pigment generated as a reaction product of the
development. I think it is another reaction product which does the tanning
but the same conditions (low sulfite) allow both reaction products to form
so the stain image is incidental to the tanning.
---- Richard Knoppow Los Angeles,Ca. dickburk@ix.netcom.com
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