[pure-silver] dichroic fog (was re-using D-76 w/Ilford Films)
- From: Ryuji Suzuki <rs@xxxxxxxxxxxxxxx>
- To: pure-silver@xxxxxxxxxxxxx
- Date: Sat, 01 Apr 2006 03:13:34 -0500 (EST)
Changed the subject.
From: Eric Nelson <emanmb@xxxxxxxxx>
Subject: [pure-silver] Re-using D-76 w/Ilford Films
Date: Fri, 31 Mar 2006 22:40:18 -0800 (PST)
> With Ilford 120 films I have been getting, what was
> once diagnosed over the phone, dichroic fog, using
> this replenished D-76.
> No Kodak films are affected and I've made a new batch
> with the same result.
> I've also tried UN-replenished D-76 with no problems
> straight or 1:1. The unreplenished is used only once.
What you described sounds like dichroic fog. It often appears when the
developer contains silver complex ions that can be reduced by the
developing agent at or near the interface between the coated gelatin layer
and the bulk solution. Its appearance is rather unpredictable but what
you described makes perfect sense to me.
Replenished (reused) developer contains silver complexed by sulfite,
while fresh developer does not, at least until the developer is poured
into the loaded tank. Some films are more susceptible to dichroic fog
than others. There are MANY factors that affect this.
> It has no effect in printing and to
> my knowledge, cannot be removed.
You can remove it in acidic ammonium thiosulfate fixer with very low
pH. It's easy to make one by mixing ammonium thiosulfate, sodium
metabisulfite and citric acid. It may take many minutes to lighten and
disappear. Very weak bleach may also be used. Either case, test the
bleach with an unimportant specimen from the same film/developer
combination.
> From what I was able to read about dichroic fog, it is
> the result of low acidity of the stop bath but mine is
> made according to directions.
Unless one is doing something stupid, those are just harmless (and
useless) suggestions. The rationale behind that suggestion is that, if
a film developed in powerful developer (like D-19) is transferred to a
nearly exhausted fixing bath without a complete stop, dichroic fog
will occur. The reason for this is because developer is still active
in the fixer, where lots of silver ion is available to form dichroic
fog. As long as the developer is well removed from the emulsion, plain
water rinse is completely capable of preventing this
mechanism. However, based on what you described, your dichroic fog is
formed in the developer stage and the chocie of stop bath is irrelevant.
> Anyone care to hazard a guess as to what is going on
> and if Ilford films "don't like" replenished D-76?
I won't list but there are many factors affecting whether dichroic fog
will appear or not.
My guess in this particular case is the difference in the
gelatin-polymer blend used for the emulsion and overcoat.
Perhaps the easiest way to reduce the risk of dichroic fog like yours
is to dilute the developer. However, diluted developer suffers from
poorer keeping property and is not suitable for replenished system.
The next easiest way is to change the developer formulation. You can
reduce the amount of sulfite, but for the above reason, this is not
a very good idea to go very far on this road. There are dozens of
compounds that are known to prevent dichroic fog. Henn and Haist are
two experts in this area. Many organic antifoggants tend to reduce or
eliminate this type of fog, but the amount sufficient for this role is
probably undesirable in film developers.
One compound known to be effective are resorcinol-like 1,3-dihydroxy
carbocyclic compounds (Kitze and Rosecrants, US Patent 3380828).
Another well known compound is 2,4-dihydroxybenzophenone (Henn, USP
3161513). Bill Troop argues that this is the antistaining agent used
in Microdol-X, but he has no hard evidence for this.
Another interesting example is poly(N-vinyl) compounds such as
poly(vinylpyrrolidone). (Henn, King and Surash, USP 3552969) This is
the HC-110 patent. PVP is a highly water-soluble solid, but it makes a
wide range of useful copolymers and I think some of them are
incorporated in the binder system of modern emulsions. It has
excellent adhesion and coating properties, although the polymer is
water sensitive. But this last problem is something polymer engineers
can solve by playing with copolymerinzation, blending, etc. PVP is
also used to clarify beer and white wines (to remove excessive tannin,
etc.)
Silver stain similar to dichroic fog is a common problem in roller
transport processors. Obviously, Ilford had a problem with it, and
they got US Patent 5770351 (Long and Parker). Examples of this patent
used two common antifoggants 1-phenyl-5-mercaptotetrazole and
benzotriazole, which were largely effective in inhibiting stain (they
call physical development in this patent) but PMT in particular didn't
allow the print to be fixed completely. I have no experience with
compounds in Formulae II and III of this patent.
Similar problems occur in monobath developers, because monobath
contains a lot of strong silver halide solvents, such as thiocyanate
and thiosulfate. Haist published a number of compounds for this
application, and if I remember correctly, compounds where thiol group
and carboxylic group are attached to the same carbon are most
effective. Examples are mercaptoisobutylic acid and mercaptoacetic
acid. Both are nasty compounds.
Of these compounds, certainly the easiest to obtain and safest to
human health and environment is PVP. Resorcinol is easy to obtain but
it is said to be not as effective as its alkylated or halogenated
derivatives. It may be that dissolving some PVP in the developer may
solve the problem...
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