Rapid (ammonium thiosulfate) fixer is mostly recommended for
emulsions that have a lot of silver iodide in them. That applies
to many high speed emulsions. Silver iodide is very difficult to
dissolve but ammonium thiosulfate has an easier time with it than
sodium thiosulfate. It is not so much speed of fixing as being
able to complete the fixing. By completion is meant the
conversion of all of the undeveloped halide into a soluble form
that will wash out. For emulsions, such as most paper emulsions,
which have little or no iodide in them sodium fixer works fine.
Plus, ammonium thiosulfate may need to be diluted for paper to
prevent it from causing bleaching. This is mainly a problem in
acid hardening fixing baths, a non hardening rapid fixer does not
have this problem.
Another consideration is the use of a fixer clearing bath.
This was developed by Kodak Labs many years ago. The research was
inspired by the discovery that sea water was more effective in
washing emulsions than fresh water. At first it was thought that
it was due to the sea water being alkaline but the addition of
alkali to the wash water or a bath in an alkaline solution proved
less effective than sea water, so it was something else in the
water. Kodak researchers found that the presence of sulfite
acted as an ion exchange agent for thiosulfate reaction products.
They came up with a formula of sodium sulfite buffered to neutral
pH with bisulfite was very effective. The commercial product also
contained a sequestering agent to prevent the formation of an
aluminum scale on the surface due to the alum hardener in many
fixing baths. The reason for making the solution neutral is to
prevent loss of the hardening action of alum hardener which
functions only in a narrow window of pH. If made too alkaline the
hardener looses its power. However, there is another factor; the
molecular binding of thiosulfate and its reaction products due to
the electrical charge of the molecules. Photographic gelatin is
what is called amphoretic, that is, it has no specific pH but
takes on the pH of the last bath it was in. However, it does have
a characteristic pH for neutral charge. In the case of
photographic gelatin that is very slightly on the acid side of
neutral. If in an acid condition the gelatin molecules tend to
bind the thiosulfate and its reaction products tightly making
them much harder to wash out. By making the emulsion neutral or
slightly alkaline this binding is eliminated so the reaction
produces wash out easily. The washing aid was made to be in the
range of pH where the electrical binding was broken but where the
hardening action still was effective. So the wash aid has two
functions: one to act as an ion exchange agent; 2 to adjust the
pH of the emulsion. Sulfite wash aid is very effective on the
emulsion, reducing wash time by a factor of nearly ten, but it is
much less effective on the fiber support of conventional printing
paper. It still is effective but needs a longer treatment time
and a longer wash than is the case for film.
The instructions for Kodak Hypo Clearing Agent specify the
treatment times and following wash times. Ilford makes a very
similar product. The formula is in the Kodak patents and is
reasonable stuff to make at home. Wash aid has the further
advantage that it works on otherwise insoluble reaction products
so will compensate to some degree for incomplete fixing.
Note that this gives less advantage to rapid fixer over
conventional fixer although rapid fixer still is faster and
better for difficult to fix emulsions.
I would recommend using a non-hardening fixing bath unless
you find you have specific problems with soft emulsions during
fixing and washing. Also, the use of a two bath fixing system
will much extend the life of either kind of fixer and promote
complete fixing. For both archival and economic reasons a
two-bath fixing system and the use of a sulfite wash aid are
recommended.
A rather obscure note: It IS possible to over fix. Many years
ago it was discovered (I think at Fuji in Japan) that a tiny
residue of thiosulfate in the emulsion actually acted to
_preserve_ the image by causing a very slight amount of
sulfiding. This was so amazing that the results were not
published until similar results were reported by Kodak Labs. This
resulted in a change in the recommended wash times and handling
of all photographic materials. Up to this time some heroic
washing processes were prescribed, many hours of washing, etc.
Turns out that could result in the oxidation of the silver image!
The old commercial standard of washing turned out to be much
better. Of course, the use of a toning bath will also help
prevent oxidation of the image.
The is a ton of stuff in the literature about this.
Somewhere I have the patent for Kodak Hypo Clearing Agent,
which I think has been discontinued and will see if I can find it
and post it here. Again, I think the current Ilford wash aid is
either identical or at any rate produces the same result.
This has gotten pretty far afield from your original question.
There are two ways of making a rapid fixer: one is the use
of an ammonium compound added to conventional fixer, as has been
posted here already, the other is to use ammonium thiosulfate
directly. The problem is that ammonium thiosulfate does not store
well. It is typically supplied as a liquid concentrate but the
formulas are available. In fact, little difference from
conventional fixer other than the use of ammonium rather than
sodium thiosulfate and adjustment of the quantity.
Again, a plain fixing bath, without hardener or its necessary
acid, works fine. Most modern emulsions do not need the
hardening. About 15 grams/liter of sodium sulfite is enough to
prevent breakdown of the fixer from oxygen absorbed from the air
or carried over acid from the stop bath (actually 5 grams/liter
is probably enough but the extra does no harm).
Enough already.
On 11/6/2019 3:27 AM, Miha Golobic wrote:
Greetings from Slovenia. I have been reading pure-silver archives for quite some years now and I never cease to be amazed by the wealth of knowledge available over there.
As for me, I've developing film since I was 15, which is over 25 years now, setting up my first darkroom in the bathroom when I was 17, now having a permanent one in the basement of my house. For the last 5 years or so I've been practicing LF (4x5) photography as well which meant I had to source out an appropriate enlarger. I was lucky enough to find a mint Durst locally from a private seller. It sits happily next to my Kaiser which can only go up to 6x6.
Recently I have been playing with the idea of using powdered chemicals only. Developer is of no concern to me, however fixer is another matter (so they say!). Fot too many times a message came across from many experts and "experts" advocating that with today's materials ammonium based fixer is a must and that sodium fixer can produce less than permanent images. Is that really the case? Kodak, Foma, Tetenal (as well as Formulary, etc) offer such without reservations. I would really like to know if anyone has tested modern papers fixed with sodium fixer for residual silver?
Cheers! Miha
