The Kodak FT-1 and Subclub, Fix-A-Sure tests referred to in
another post this thread, represent the extrems.
   The Kodak test and the other one-drop-in an-ounce tests are at
opposite ends of the spectrum. One drop in one ounce is no margine
at all.
   I see I've 5% KI. The 10% you use should show no precipitate
with a few drops in 10ml. That is my opinion. I think I would feel
safe with that. Do the test slowly, one drop at a time, and
stir or shake.
   I've yet to read anything at all which delves into the subject
of fixer testing.  Considering the importance of fixation, I think
the lack of information a real oversight.
   Some films and most papers have no silver iodides. Fixer has a
great capacity for the chloride and bromide. Is the iodide test
relevant? If so how is it related?
   The test itself, is it for film, RC or FB paper? In all cases
capacity is not the measure. In fact the capacity of all usuall
fixers surpass their usefullness; the more true with FB
archivaly processed.                                           Dan

   The test for a single bath is 5 drops of test solution _and_ five
drops of water to 5 drops of fixer. That's not 19%. The concentration
of the mixture is about 6.3%.
    The tollerable concentration for the second bath is much less
than for the first since it must have low enough silver concentration
to function as the first bath.
     I do agree that one would expect a more sensitive test for a
single bath than for the first bath of a two bath system. It may be
that there is an error is the instructions. They came from the _Kodak
Black and White Darkroom Dataguide_ I've found examples of
instructions in Kodak publications that have lost something in
translation at some point. I don't know what the original source of
the test formulas is. Probably one of the Kodak Labs research papers.
  The capacity of a single bath for "archival" fixing of paper is
astonishingly limited. According to Ilford figures its only about 10
8x10 sheets per _gallon_. The capacity of a two bath system is at
least four times this and probably much more than that. Kodak used to
rate both fixing and washing as either "commercial" or "archival".
Commercial processing had a life expectancy of 25 years. Since the
capacity of a two bath system for archival fixing is enough to make it
economical there is no good reason to use the less permanent standard.
Since neither Kodak or other makers of Iodide test solutions seem to
publish the approximate silver concentration that results in an
indication they are of use mainly as a guide. Further, the use of a
sulfite wash aid will make some otherwise insoluble reaction products
soluble, or at least removable, thus indirectly increasing the
capacity of the fixing bath.
  Capacity depends on the nature of the emulsion. Fixing time and
capacity is actually about the same for fiber paper as it is for RC.
The emulsions are about the same. Film has a different kind of
emulsion, generally thicker, with a greater concentration of halides,
and with more silver iodide. All of these result in longer fixing
times. Typical paper emulsion will fix out completely in two minutes
in a Sodium thiosulfate fixer, perhaps even faster. Typical film
emulsion takes perhaps 6 minutes in fresh sodium fixer but can take
longer. Emulsions with a lot of Silver Iodide, like Tri-X or any of
the tabular grain films, take considerably longer, probably ten
minutes in sodium fixer. Ammonium thiosulfate has a distinct advantage
in fixing these emulsions.
  Fixing time varies with the silver concentration in the fixing
bath. A very large excess of free thiosulfate ions must be present to
complete the fixing. Note that clearing of the emulsion, that is,
making it visually transparent, does not signify complete fixing.
While a fresh fixer may fix in the clearing time used fixer will not.
The old rule of fixing for twice the clearing time is a good one but
sufficiently used fixer may not fix in any amount of time although it
still clears the emulsion. Since any resudual silver halide will
eventually decompose and attack the image good fixing is important.
  Probably a better test of fixing is a residual silver test of the
emulsion rather than a silver concentration test of the fixing bath.
Residual halide may be tested by using a Sodium Sulfide solution or a
1:9 dilution of Kodak Rapid Selenium Toner. Both stain the halide. The
toner is a more stable test solution but works only on well washed
emulsion, it fails where there is a large excess of hypo present. Well
fixed emulsion should show no stain. One problem with either test is
that its hard to see the stain on tinted stock. Very little of that is
currently in use so it is a very useful test.

Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com