From: Katharine Thayer (kthayer@pacifier.com)
Date: 05/13/01-12:04:49 PM Z
Well, my curiosity got the better of me and I did a couple of little
experiments. If the way I think this process works is correct, then the
reduction of the chromium compound proceeds independently of the
hardening of the gum. I demonstrated this independence to my own
satisfaction this afternoon by brushing ammonium dichromate onto unsized
paper and exposing it to sunlight; the dichromate showed the same color
changes as ammonium dichromate mixed with gum. But then I worried
about what if there were something in the paper that could confound the
experiment, so I did the same experiment again, brushing the ammonium
dichromate onto glass, drying it and exposing it to sunlight. Again, the
dichromate quickly changed to green, and then to brown. So I think it's
clear that the presence of the colloid is not necessary for what happens
with the dichromate. Apparently when exposed to light, the dichromate
proceeds with its own reaction, throwing off oxygen and changing to
reduced compounds that are more and more insoluble, and those compounds
will be present independent of what's going on with the gum. I suspect
that Clerc was wrong and that the chromium oxide is not a component of
the tanned gum, but just there as a by-product of the chromium
reduction.
Katharine Thayer
> > 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
>
> I take your point about {if we're operating off such ancient explanations, } but information
> on this subject
> is hard to find and in my case even harder to understand ;--<
> I will take a trip down the library and see what I can find
>
> Pete
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