Re: Tanning theory of dichromated colloids

Thanks for your explanations!

"But the real problem for silver gelatin processors is that monoaldehydes as
well as
aldoses (sugars containing aldehyde) can be oxidized by silver-ammonium
complex and in this reaction silver gets reduced to metallic silver. This is
one (if not the) reason why photographic gelatin is highly purified to
remove sugars and aldehydes (among many
other things), because, otherwise, silver gelatin emulsions get fogged
(useless!)."

Yes, that's what I noticed with some home-made silver halide emulsions. I
gathered a few aldehydes like glycol aldehyde did not have such unwanted
side effects though. However, all (?) aldehyde hardeners were said to
continue their hardening action over a period of several weeks (unless
agents like resorcinol were added).

"What are you trying to do?"

I am particularly interested in a process called "SHSG" (= Silver Halide
Sensitized Gelatin). Basically, it is a method of getting a DCG-like effect
out of a silver halide emulsion. At the end of the process all silver being
bleached/fixed, the image is modulated through the refractive index
differences produced by local hardening of the gelatin. Since this
technology is based on silver halides, speed is far better than for DCG
systems.

Martin

----- Original Message -----
From: "Ryuji Suzuki" <rs@silvergrain.org>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Monday, May 10, 2004 8:01 AM
Subject: Re: Tanning theory of dichromated colloids

> From: MARTINM <martinm@SoftHome.net>
> Subject: Re: Tanning theory of dichromated colloids
> Date: Sun, 09 May 2004 12:16:43 +0200
>
> > "...Incidentally, primary alcohols, alkenes and alkynes can also be
> > oxidized to carboxylic acid with strong oxidizing agents like
> > permanganate or chromium (VI)."
>
> > That has been interesting me for quite a while. Do you know what
> > kind of condition (pH/temperature/time) is required to initiate that
> > reaction?
>
> I don't really have a direct experience with this type of reactions
> except for a few special cases. But it's a typical organic chemistry
> textbook matters (nothing advanced) so you should be able to find a
> good deal of info there. Generally high temp and high pH are preferred.
>
> Alcohols can be oxidized directly to carboxylic acids with strong
> oxidizing agents, but even with mild oxidizing agents, the reaction
> can happen with intermediate steps, typically in aldehyde form. This
> latter type of reaction is common in many places... ethanol can be
> oxidized by aerial oxygen to acetaldehyde while aging wine and
> whisk(e)y.
>
> Aldehydes can be readily oxidized by many things as well, one of the
> reasons why formaldehyde solution doesn't keep well. But the real
> problem for silver gelatin processors is that monoaldehydes as well as
> aldoses (sugars containing aldehyde) can be oxidized by
> silver-ammonium complex and in this reaction silver gets reduced to
> metallic silver. This is one (if not the) reason why photographic
> gelatin is highly purified to remove sugars and aldehydes (among many
> other things), because, otherwise, silver gelatin emulsions get fogged
> (useless!).
>
> > "So one possible test might be to oxidize gum with suitable
> > oxidizing agents and see if chrome alum can harden it better. But
> > the amount of oxidizing agent must be suitably chosen, because you
> > don't want gum molecules to break down at once before they get
> > hardened."
>
> > What would be an appropriate oxidizing agent? Thinking of some
> > special bleaches occasionally used for holographic silver-halide
> > materials and involving Cr bleaching, copper sulfate or potassium
> > persulfate come to my mind...
>
> What are you trying to do?
>
>
> From: MARTINM <martinm@SoftHome.net>
> Subject: Re: Tanning theory of dichromated colloids (was gelatin
> Date: Sun, 09 May 2004 11:45:17 +0200
>
> > I assume that's the Canadian group (Lessard, Couture, Changkakoti,
Bolte,
> > Solano, Capollo etc.)...
>
> That's them. The paper we are actually talking about is:
>
> G. Mannivannan, R. Changkakoti, R. Lessard, G. Mailhot, and M. Bolte,
> J. Phys. Chem. 97, 7228 (1993).
>
> There's also similar study on primary photochemical process of
> dichromated gelatin by Lanford et al (2000), Opt. Eng. vol 39, no 3,
> 610-615. There are a couple others I read but they're in my office
> and no reading note in my computer. I think the recent one used sol
> preparation instead of typical dried films.
>
> --
> Ryuji Suzuki
> "You have to realize that junk is not the problem in and of itself.
> Junk is the symptom, not the problem."
> (Bob Dylan 1971; source: No Direction Home by Robert Shelton)
Received on Mon May 10 07:46:33 2004