[pure-silver] Re: Pyro in a Jobo Processor
- From: Ryuji Suzuki <rs@xxxxxx>
- To: pure-silver@xxxxxxxxxxxxx
- Date: Mon, 21 Mar 2005 14:42:30 -0500 (EST)
From: "John Black" <jblack@xxxxxxxxx>
Subject: [pure-silver] Re: Pyro in a Jobo Processor
Date: Mon, 21 Mar 2005 13:55:27 -0500
> That reminds me. I have some trouble with 35mm film "cupping" or "bowing"
> rather badly after drying and this must be due to excessive shrinkage of the
> emulsion relative to the backing or any other layers on the reverse. Does
> anyone have a remedy for this and would it benifit to cross-link or "harden"
> the emulsion to prevent excessive shrinkage? I know hardening was used to
> toughen up older, soft, thick emulsion films and mostly isn't needed now,
> but could it help prevent shrinkage?
I think that has more to do with the film base, whether cellulose
triacetate or polyester, which is poly(ethylene telephthalate). It
also depends on the plasticizer used in the emulsion. Modern emulsions
incorporate nongelatin polymer macromolecules in part to achieve
plasticizing effect, maybe in addition to large sugar alcohol
molecules. In old emulsions small molecules like glycerol and other
small polyols were used but these agents would only help plasticizing
unprocessed materials, since they will be lost during processing due
to their diffusibility in gelatin.
> Agreed. My own tests show pH to be the most important issue affecting
> graininess followed by temperature. Assuming proper developing time and
> exposure. A&T and even Haist continue to speak of "grain clumping" though
> in their literature.
I wouldn't take A&T to the accuracy of every word they printed. The
value of their work was to make some parts of the material more
accessible to the average darkroom workers.
> > Also, I think electrophoresis is done on crosslinked polyacrylamide or
> > maybe some other polymer, which is quite different from gelatin.
>
> Maybe not so different as you might think. Until it was halted, I
> understand resources at Kodak were being spent in pursuit of "artificial
> gelatin" or a sythetic gel with the AgX binding properties of natural
> gelatin and progress had been made. Remember, polyacylamide gel came about
> as a substitute for highly purified agar.
Polyacrylamide is/was used as a gelatin blend in commercial
materials. I know Fuji has been and is still active in this area. I
don't know if Kodak halted this research either.
As far as I know no polymer blend is capable of displacing gelatin in
photographic emulsion even today. The fraction of synthetic polymers
as well as modified gelatin polymers has been increased, but still
it's not large.
There are some recent patents that use non-gelatin molecules like
starch as the peptizing agent but clearly such an emulsion has to
incorporate another binder (such as gelatin) later in the process,
making the process very complicated. I doubt that kind of technique is
used in current production.
--
Ryuji Suzuki
"Keep a good head and always carry a light camera."
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