[pure-silver] Re: Strorage life of diluted fixer?
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Wed, 22 Sep 2004 13:45:25 -0700
----- Original Message -----
From: "DarkroomMagic" <info@xxxxxxxxxxxxxxxx>
To: "PureSilverNew" <pure-silver@xxxxxxxxxxxxx>
Sent: Wednesday, September 22, 2004 9:07 AM
Subject: [pure-silver] Re: Strorage life of diluted fixer?
> Using paper fixer one-shot is wasteful. One-session maybe,
> if it is an
> exhaustive session, but otherwise not. I use film fixer
> one-shot, but paper
> fixer is used until I measure the silver-content limit in
> the first bath.
>
> I don't see, why you wouldn't use the two-bath,
> film-strength method for
> infrequent application. It works well and the fixer shelf
> life is increased
> at higher concentrations. I print every week, but never
> had fixer go bad on
> me. It's exhausted (1 g/l) before it goes bad.
>
>
>
>
>
> Regards
>
>
>
> Ralph W. Lambrecht
>
I have considerable doubts about the effectiveness of
fixer diluted more than normal. The more it is diluted the
slower it fixes and the lower its capacity to fix
completely. Remember that the fixing process consists of
several steps from insoluble halide to soluble complex. The
ability of the fixing bath to complete this process so that
all the halide is water soluble is dependant on its silver
content. There must be an excess of thiosulfate ions
available to carry out the process, dissolved silver binds
up thiosulfate ions so that they are no longer available.
The reason two bath fixers are recommended for archival
fixing is that most of the silver halide is made soluble in
the first bath leaving the second one relatively free of
silver and able to complete the process. At some point when
a fixing bath is diluted enough the capacity to fix an
emulsion completely will be insufficient to fix anything.
Keep in mind that the fixing process goes well beyond simply
"clearing" the emulsion or making it so that it doesn't
darken in light. If any insoluble or incompletely soluble
complexes are left in the emulsion they will eventually
change chemically and attack the image or themselves stain
the emulsion.
Capacity can be estimated using clearing time for known
film or paper or by using a Potassium Iodide test of known
strength. While Ryuji Suzuki has questioned the need for two
bath fixing with Ammonium thiosulfate fixers it seems to me
that even if their capacity is well in excess of sodium
thiosulfate fixers (which it is) there is still a safety
factor in using two baths.
The capacity of a single bath, especially a sodium
thiosulfate bath, to fix archivally is reached well before
its lifetime. Ilford estimates a single sodium thiosulfate
fixer has a capacity of only 10 8x10 prints per _gallon_. A
two bath system has a capacity of four to ten times this.
High dilution of fixing baths is not good economy because
the capacity of the bath to fix archivally may be
insufficient, even when used one-shot.
Aside from the amount of dissolved silver the life of
fixer is probably limited by oxidation of the sulfite. At
some point enough sulfite will be converted to sulfate to
allow the fixer to sulfurize and become useless. In acid
fixers the acid tends to decompose the thiosulfate and will
do so rather rapidly in the absense of sulfite. Thiosulfate
will also slowly decompose even in neutral or alkaline baths
without sulfite.
The shelf life of rapid fixer concentrates probably
depends on the type of container (oxygen permiability) and
the pH. I would estimat the shelf life of sealed Kodak Rapid
Fixer at around 2 years. Fixer mixed or diluted for use has
a storage life of perhaps 6 months. The life will depend on
how much air is gotten into it during use.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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