Re: Gum "tanning" process

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From: Peter Fredrik (pete@fotem.demon.co.uk)
Date: 05/13/01-06:57:13 PM Z


Katharine Thayer wrote:

> Peter Fredrik wrote:
> >
> The
> > dichromate processes are in fact governed by the effects of oxidation this is the
> > mechanism that produces the tanning effect. The greater the exposure the greater
> > the oxide present. I suspect that the oxide is always present after exposure, even
> > though in small quantities it is invisible

Katharine ,

I am sorry we have got our wires crossed on this one

>
>
> 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).

> I have found in practise that as the expsure increases so does the hardness of the
> colloid but only up to a certain point when the effect tapers off .I have not said
> that it is the dichromate salt that is being tanned that would make no sense at
> all.,however there is a mettalic compound present in the exposed and processed
> colloid.

> The chromium compound, if I
> remember my chemistry right, which of course is a big If, is being
> reduced rather than oxidized, which makes intuitive sense because in its
> original, hexavalent state, it has plenty of oxygen to give up. I need
> to see the chemical equations that govern this process, if someone would
> be so kind, so I can picture what's going on here... Thanks...
>
> Katharine Thayer

You are a much more knowledgeable chemist than I. So I have looked up some information
in Clerc's Photography theory and practice my bible when I was at college but is quite
incomprehensible for me now --:

In the presence of any organic matter the bichromate is decomposed by exposure to light
into neutral chromate which is eliminated in the later washing _and brown oxide or
chromochromate mCr2O3 which by subsequent washing is decomposed into chromic acid CrO3
carried away by the water, and into GREEN OXIDE OF CHROMIUM Cr2O3 which combines with
the gelatine and effects the TANNING action.
In the course of this reaction a very small part of the gelatine is oxidized, resulting
in products which are eliminated during washing.
The insoluble is formed entirely of normal gelatine in combination with chromium
oxide (J,M. EDER, 1878 ).

I hope this puts on the right track

Pete


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