[pure-silver] Re: Mixing My Own Fixer
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Mon, 18 Oct 2010 01:11:32 -0700
An adendum to my previous post.
Anhydrous sodium thiosulfate = 65% by weight of
crystalline thiosulfate, that is, where the formula calls
for 240 grams of the crystalline form use about 156 grams of
the anhydrous form.
Ammonium thiocyanate is usually sold as a 60% solution
since the crystalline form is not very stable. You can make
a rapid fixer out of any standard fixing bath by using about
160 ml of the liquid concentrate where 240 grams of
crystalline sodium thiosulfate is called for. This is for
_working_ strength fixer at about "film" strength. Because
ammonium thiosulfate is much more soluble than the sodium
form very strongly concentrated stock solutions can be made
up.
Any form of thiosulfate can be made into an acid,
neutral, or alkaline fixing bath, the pH is not important to
the fixing properties.
Again, while both cations can cause some silver
bleaching when in an acid environment ammonium is more
likely to do so than sodium. Neither will cause bleaching
when neutral.
A good mild silver reducer can be made by adding about
15 grams per liter of citric acid to an acid rapid fixing
bath at film strength. Kodak recommends this for removing
dichroic silver stains from negatives.
Another thought, dichroic silver deposits are another
reason to be careful of carried over developer in neutral or
alkaline fixing baths since it can convert dissolved silver
complexes in the fixing bath to metallic silver which is
then deposited on the emulsion. This can also happen when
acid fixer is used well beyond exhaustion. Since this silver
is likely to be extremely fine grained (colloidal silver) it
is especially vulnerable to oxidation causing black or shiny
silver stains. Just another reason not to push fixer beyond
its limits. Remember that fixer will clear film or make
paper light proof long after it has stopped being able to
fully convert the halide to a soluble form.
--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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