Re: direct positive

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From: Liam Lawless (lawless@ic24.net)
Date: 11/23/00-03:45:08 AM Z


Hi Gerard,

I believe that what happens when dichromated gelatin is exposed to light is
that a chromium oxide is formed within the gelatin. This is what hardens
it, and gives the pale image you describe. Clerc says:

"When gelatine is impregnated with a bichromate, dried in a current of air,
and exposed to light, a brown image is formed on a yellow ground. It can
then be ascertained that the parts which have darkened to a brown tint have
lost their solubility in warm water; or, at least, can only be dissolved in
water much hotter than that which would dissolve those parts which have been
protected from the action of light. The property of swelling in cold water
is also very much reduced in the exposed parts... "

And in a footnote, he says:

"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, nCrO3, 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 oxidised, resulting in products which are
eliminated during the washing. The insoluble image is formed entirely of
normal gelatine in combination with chromium oxide (J.M. Eder, 1878). When
ammonium bichromate is used, the neutral chromate formed is decomposed in
giving up bichromate which can participate in the reaction."

My Clerc is the 1937 edition and our understanding of the chemistry might be
a bit different today (or not?), but I'm sure that this tanning/staining is
not resposnible for the stains I have been getting - they're much darker. I
don't know for sure that they would occur on every FB paper, or that they
would not on any PE, but I think the difference between the two types (apart
from the plastic!) is that FBs have a baryta layer. That's my current
theory, which, if correct, means I need to find a non-staining bleach. I
haven't tested permanganate (or if I have it's so long ago that I've
forgotten), but all I remember about permanganate is that I didn't much like
it when I used it in my very first reversal experiments (circa 1990).
Permanganate does not tan gelatin, as far as I'm aware. I think I've found
an alternative, non-staining, bleach (with help from Mike Ware and Don
Gaines), and am testing it at the moment.

Liam


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