[pure-silver] Re: Mixing My Own Fixer
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Sun, 17 Oct 2010 16:18:14 -0700
----- Original Message -----
From: "Robert Hall" <robert.g.hall@xxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Sunday, October 17, 2010 10:47 AM
Subject: [pure-silver] Mixing My Own Fixer
I can buy ammonium thio from thatcher chemical here in salt
lake city
for about $6 a gallon. About a third the cost of commercial
fix.
For a while I was seeing ilford fix come in from Spain. Now
from
germany. I can't imagine what I pay for shipping.
Some have suggested adding boric acid, others suggest
sulfite. Could
someone comment on benefits of either and how much should be
used?
The two serve different purposes. Sodium sulfite is a
preservative which absorbs oxygen preferentially so that the
thiosulfate does not become oxidized. It should be used in
any fixer. For a plain fixer about 5 grams per liter is
sufficient but a greater amount is insurance if developer is
carried over. For an acid fixing bath more sulfite is
necessary because without it the acid will decompose the
thiosulfate. Most acid fixers have about 15 grams per liter
in them.
Boric acid is used as part of a buffer solution to
acidify acid fixing baths. Fixers using boric acid and
acetic acid in a buffer combination were originated by Kodak
and the familiar F-5, which is the bases for Kodak packaged
powder fixer, is very similar. Boric acid is never used on
its own and should not be arbitrarily added to a fixing
bath. The reason for the buffer is to maintain the pH of the
fixing bath at a value that allows the alum hardener to
work. It is supposed to compensate for carried over alkaline
developer. When an acid stop bath is used it is probably not
necessary. Sodium bisulfite is used in some fixer formulas
where a lesser degree of acidity is desired. For instance
Kodak F-24 is a non-hardening fixing bath that is nontheless
made acid to inactivate carried over developer. Kodak
offered a formula, given below, which is claimed to be low
odor using Kodalk as a buffer.
Any standard fixer formula specifying sodium
thiosulfate can be made up as a rapid fixer by using the
correct amount of ammonium thiosulfate. Ammonium thiosulfate
is not very stable in dry form so is usually sold as a
concentrated liquid. Sodium thiosulfate is available as both
crystals and in anhydrous form. Kodak's packaged powder
fixer contains anhydrous sodium thiosulfate so it you weigh
the package it will be considerably less than if you are
expecting the crystaline form. Note that crystaline sodium
thiosulfate is extremely endothermic so very hot water is
needed to dissolve it or additional heat should be applied
when mixing. Anhydrous thiosulfate has little heat of
solution but can be decomposed by high temperatures so the
water should not exceed 90F when mixing.
Here are some typical fixer formulas, thiosulfate amounts
are given for the crystaline form:
Kodak F-5
Water, at 125F or 50C 600.0 ml
Sodium thiosulfate, crystaline 240.0 grams
Sodium sulfite, desiccated 15.0 grams
Acetic acid, 28% 48.0 ml
Boric acid, crystaline 7.5 grams
Potassium aluminum sulfate (Alum) 15.0 grams
Water to make 1.0 liter
Beside acting as a buffer the boric acid bath is
supposed to produce less scumming of negatives than fixing
baths without it.
Kodak F-6 (Low Odor)
Water, at 125F or 50C 600.0 ml
Sodium sulfite, desiccated 15.0 grams
Acetic acid, 28% 48.0 ml
Sodium metaborate 15.0 grams
Potassium aluminum sulfate (alum) 15.0 grams
Water to make 1.0 liter
Kodak F-24 Non-hardening Fixing Bath
Water, at 125F or 50C 500.0 ml
Sodium thiosulfate 240.0 ml
Sodium sulfite, desiccated 10.0 grams
Sodium bisulfite 25.0 grams
Water to make 1.0 liter
Note that the sodium bisulfite of commerce is usually sodium
metabisulfite and it is the latter chemical that is actually
specified in most Kodak formulae calling for bisulfite.
For comparison the standard AGFA/Ansco fixing bath is AGFA
201. If you adjust the amounts to standardize for 1 liter of
solution (the formula results in 1.5 liter) it is virtually
identical to Kodak F-5 minus the boric acid. AGFA also has a
formula for a non-hardening bath which specifies potassium
metabisulfite in place of the sodium bisulfite in the Kodak
F-24 formula. The results are identical.
In addition to these there are also formulas for baths
employing chrome alum (potassium chromium sulfate). Chrome
alum is a much more effective hardener than white alum but
must also operate at much lower pH. It also has a tendency
to leave a green stain so was mostly used for negatives. I
don't think there are many applications for it these days.
There are other variations, such as a thiocyanate fixer
given by Defender, but the above are pretty much the
practical ones.
--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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