Re: Gum "tanning" process

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From: Katharine Thayer (kthayer@pacifier.com)
Date: 05/13/01-03:23:23 AM Z


Hi Pete,

Thanks for this explanation below. It's quaint but not terribly helpful,
as it was written before scientists understood much of what they
undertand now about chemistry. The chemistry of chromium is more
complicated than this; there are actually two dichromates, one
hexavalent and one trivalent; the hexavalent dichromate is probably
reduced to the trivalent dichromate before being further reduced to one
of the four possible chromium oxides.

I do appreciate seeing this, in fact it shows how some of the confusion
arises, if we're operating off such ancient explanations, but I need a
modern explanation. I spent all day yesterday, when I had more important
things to do, searching for this information on the internet and came up
empty-handed. The only thing I did find was a reference recommended by
someone on this list: Jaromir Kosar, Light-sensitive systems: chemistry
and application of nonsilver halide photographic processes, 1965, part
of the Wiley series on photographic science. This sounds just jim-dandy,
but since I'm out in the sticks with no library access, it's not
terribly helpful to me.
Katharine

Peter Fredrik wrote:
>
> 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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