[pure-silver] Fixing Bath Formulas
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Tue, 25 Jan 2005 17:20:41 -0800
A couple of notes applying to this discussion:
Citric acid is not suitable for a hardening fixing bath
using white alum (Potassium Aluminum Sulfate) because it
binds aluminum. This interfers with hardening and can
generate an aluminum sludge.
There is no advantage of an alkaline fixing bath over a
neutral fixing bath. If a non-hardening and non-acid bath is
desired it can be had using just Thiosulfate and Sulfite.
About 5 grams/liter of sulfite is sufficient to protectect
the thiosulfate in the absence of acid.
Emulsion swelling is about minimum at neutral pH. The
isoelectric point of most photographic gelatin is very
slightly acid. Minimum swelling exists at this pH. By
adjusting the gelatin to neuteral pH the swelling is close
to the minimum but the binding of thiosulfate and fixer
reaction products due to charges in the emulsion are
eliminated. Also, the diffusion path is minimised, a
desirable condition for washing. Alkaline or acid fixers
will increase the swelling. The further they are from the
isoelectric point of the gelatin the greater will be the
swelling.
Wash rates of material fixed in various baths is the same
once the material is treated in a sulfite wash aid.
Buffering the wash aid to neutral insures minimum swelling
of the emulsion consistent with the conditions of charge
required to maximise washing rate. Sulfite has the
additional function of being an ion exchanger so it
accelerates washing to a much greater extent than would be
accomplished by simply adjusting pH.
The function of the hardener in the fixing bath is to
reduce the emulsion swelling from all processing steps.
The amount of swelling of gelatin depends on many factors
including, among others, the inclusion of hardeners in the
gelatin at the time of manufacture, the exact nature of the
gelatin itself, the addition of non-gelatin materials to the
emulsion (synthetic polymers for instance), which may
substantially reduce swelling, temperature of the processing
solutions, others.
Modern films have, for the most part, very hard emulsions
which will withstand quite high or low pH without damage and
quite high processing temperatures.
Most modern developers are relatively low in pH.
A non-hardening fixing bath can be made to be close to
the isoelecric point of gelatin without losing its ability
to stop development.
The pH required by a developer to be active depends on
the developing agents used. Some developing agents can
operate in slightly acid environment. Metol and Amidol are
examples. D-25, for instance is pH 7.0, and some Amidol
paper developers are made slightly acid.
There have been a number of scientific papers written
about the swelling effect of processing solutions on
emulsions. I can probably find and cite them but some of the
older books like _Theory of the Photographic Process_ C.E.K.
Mees, have extensive bibliographies and citations to papers
which are still for the most part valid. I will now be like
Ryuji and suggest excercizing the local library facilities.
BTW, if the citizens of your locality have seen fit to
shut down library services to avoid taxes please SCREAM
about it. Libraries are among the greatest bargains in the
world. Don't get me started on this.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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