From: Richard Knoppow (dickburk@ix.netcom.com)
Date: 06/24/01-04:10:46 PM Z
At 10:55 PM 06/24/2001 +0100, you wrote:
>Hi,
>
>Have a vague memory of having read somewhere that it is an oxidation product
>of pyro that causes staining of gelatin. Since more oxidation takes place
>in the heavily exposed areas, staining should be proportional to developed
>silver densities.
>
>
>
>Liam
>
>
This is exactly correct. The oxidation product is related to humic acid,
it is probably longer lasting than the silver.
Pyro can be made into a non-staining developer by using lots of sulfite.
In fact, before M-H developers became the defacto standard most of the
research into Pyro was directed to making it non-staining. These days the
stain image is considered desirable. For printing materials which are blue
sensitive the stain is opaque and adds linearly to the printing density.
Pyro negatives are likely to look too light visually but print well.
The reaction which causes tanning of gelatin is related to the stain
reaction, both take place together.
Presumably the stain reaction should take place in a collodion layer but
I've never seen any experimental data so its only a guess.
---- Richard Knoppow Los Angeles,Ca. dickburk@ix.netcom.com
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