From: Peter Fredrik (pete@fotem.demon.co.uk)
Date: 05/14/01-12:29:33 AM Z
Katharine Thayer wrote:
> 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.
You have lost me girl
>
>
> 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
Do we need to know the precise chemistry ? from practical testing I know that there is a
chemical compound present in the exposed colloid, and almost certain that it related to the
tanning effect. I have done tests that show as exposure increases, the tone of chemical
reaction deepens and makes more saturated the chemical present, which again I presume
demonstrates the production of a tanning chemical itself . If we can see it
its there. However if our exposures are minimal it doesn't show. The problem lies with the
highlights of the image which often need's protracted exposure and it then can
significantly degrade the hue, and give a blue green/grey bias which is difficult to correct
without recourse to my 1% sulphuric clearing bath. I am also doing tests with caustic soda
that show promise.
>
>
> 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
This archive was generated by hypermail 2b30 : 07/12/01-11:29:39 AM Z CST