The capacity of a fixing bath to fix depends on the
amount of free thiosulfate ions available to form complexes
with the silver halide. It takes three thiosulfate ions to
complex one halide ion. The conversion of the halide from
the insoluble form it has in the emulsion to a fully soluble
form is complex, going through about four steps. If there
are not enough thiosulfate ions available the process is not
completed and some silver-thiosulfate complex remains in the
film. After a period of weeks this complex changes to a form
which can no longer be made soluble by additional fixing.
  The speed of fixing depends on the concentration of
thiosulfate in the fixer, the higher the concentration the
faster the fixing. Early research at Kodak Labs showing a
definite optimum value for thiosulfate concentration was
later found to be in error. The research was done using dry
film. When wet film is tested there is no peak value, the
fixing speed continues to rise as the thiosulfate
concentration is increased.
  While a highly diluted solution of either sodium or
ammonium thiosulfate might contain enough thiosulfate ions
to complete the fixing of a small amount of film the time
required might be very long. If the idea is to insure
complete fixing by using an unused fixing bath each time it
is misleading because the diluted fixer may not be able to
complete the fixing in any reasonable time.
  The dissolved silver in a fixing bath is a measure of the
ability of the fixer to fix completely, however, it is only
one test and not a replacement for measuring the actual
residual halide and thiosulfate complexes in fixed and
washed emulsion. That is the only real way of determining
the degree of fixing and whether fixing is "archival".
Testing for halide or complexes is simple, using using
either a 2% solution of Sodium sulifide (not sulfite) and
the other is a 1:9 dilution of Kodak Rapid Selenium Toner.
The sulfide test can be used at any time after fixing, the
KRST test fails if there is much residual hypo in the
emulsion so should be used only on well washed material.
Both tests leave a yellow to brown stain if there is any
residual silver halide or complexes in the emulsion.
Properly fixed film or paper should show no stain after
using the test.
  The tollerance for dissolved silver of Ammonium
thiosulfate fixers (rapid fixer) is about double that of
Sodium thiosulfate fixer. Also the ammonium salt is more
tollerant of dissolved Iodide from high speed films which
tends to act as a restrainer of the fixing bath. However,
the capacity of either salt for complete fixing in a single
bath is rather limited. By using a two bath system the
capacity is extended by a factor of at least four and as
much as ten. This can be further extended by using a sulfite
wash aid, such as Kodak Hypo Clearing Agent, following
fixing. The Sulfite will release some thionates which are
otherwise tightly bound to image silver allowing them to
wash out.
  Ammonium fixing baths are often used at two different
dilutions, one for film, a weaker one for paper. The only
reason for the weaker dilution is as a precaution to prevent
bleaching. Bleaching is not a problem for Sodium fixing
baths but can happen when an acid Ammonium bath is used.
Ammonium thiosulfate is a fairly good solvent for metallic
silver when acid. The bleaching does not take place unless
fixing time is extended well past the point of complete
fixing and is noticable mostly as a cooling of the color of
warm tone paper. Nearly any paper will fix out completely in
2 minutes in a film strength Ammonium thiosulfate bath, many
papers will fix out in 30 seconds. Fixing time for paper
should not be extended beyond about 2 minutes unless testing
has shown definitely that more time is needed for a
particular paper. Film emulsions for various reasons take
longer to fix. The images on film are also composed of
substantially larger grains of silver than those on paper
making them much less susceptable to the bleaching effect.
It is not an issue for film. Neutral or alkaline Ammonium
thiosufate fixing baths do not have the bleaching property.
If one is reasonably careful there is no reason a film
strength rapid fixing bath should not be used for paper.
 Testing for fixer exhaustion:
 There are two means of testing for fixer exhaustion, one
is to test for clearing time of a standard emulsion. This is
compared to the clearing time of that emulsion in fresh,
unused, fixer. The film should be wetted before testing
because dry film fixes at a different rate than wet film.
The rule of thumb is to replace the fixing bath when the
clearing time doubles but this is probably to much silver so
some shorter time should be allowed.
 The second test is the Potassium Iodide test. A
standardized solution of Potassium Iodide is added to a
diluted sample of the fixing bath. If more than a certain
amount of silver is present it complexes to form insoluble
Silver Iodide which precipitates as a cream colored or
yellow cloud in the solution which does not redissolve when
agitated. The sesitivity of the test depends on the relative
dilution of the fixing bath. This test is useful for
determining the end of life for the first bath of two bath
fixers or for single fixing baths where commercial rather
than archival fixing is required. Formulas for the Iodide
test, the Sulfide test and others are found in the
literature, for instance the _Kodak Black-and-White Darkroom
Dataguide_.
  A last word. Complete fixing is necessary for long life
of film or paper because any insoluble material left in the
emulsion will eventually either decompose and attack the
image or be attacked itself by atmospheric polutants. Where
prints or film are to be toned for image protection the
fixing _must_ be complete because the toners will tone the
residual materials as effectively as the image silver
resulting in permanent stains.
  Some general recommendations:
  While either Sodium or Ammonium fixers will work for film
Sodium is much slower and is more vulnerable to accumulated
Iodide and dissolved silver.
  Either Sodium or Ammonium fixer can be used for paper. If
well controlled darkroom procdeures are followed film
strength Ammonium fixer has the advantage of greater
capacity and faster fixing times.
 So, For film or paper use film strength rapid fixer.
   Use a two bath fixing setup.
   Use sulfite wash aid for both film and paper.
   Test fixing baths periodically by using  both the
clearing time method and the accumulated silver test using
Potassium Iodide.
   Test the results of fixing by using the Sulfide or KRST
tests. A scrap of paper can be processed along with the
prints for tests on paper.
   Unless there is a good reason for using hardenign fixing
use non hardening fixer. Rapid fixers without hardeners or
with the hardener in a separate container which can be left
out are available from all chemistry manufacturers.
   If there is a question about the control of fixing time
use neutral ph Rapid fixer. This is available from several
sources and is used in color processing. An example is Agfa
Universal Fixer. Such fixer must not be used with an acid
stop bath. Use a running water wash of a couple of minutes
for film or paper. This procedure is to prevent bleaching.
  For paper Sodium thiosulfate fixer is quite suitable.
Make tests using the sulfide test to find out what the
minimum fixing time actually is since clearing is hard to
determine on paper. Many papers will fix out in 2 minutes in
conventional fixer. For these there is no particular reason
to use rapid fixer.
  Film or paper given a reasonably good fixing, and a
reasonably good wash should have an expected life of about a
century if stored well. By treating film and paper with a
protective toner, such as Kodak Brown Toner, the expected
life will be more than a century and the materials will be
reasonably free of vulnerability to airborne peroxides and
sulfides.

Wow - Patterson tanks all leak because we've been putting on the caps wrong?
Maybe Triumph motorcycles all leaked oil for the same reason! ROFL

Greetings;

I also use the Paterson System 4 tanks for 120 roll-film; inversion
agitation is not the only successful method. The rotation handle which
comes with that system allows the alternative of rotation agitation
which I use a lot, and gives perfectly successful results, and also
guarantees even development.

Just my 2c worth - F.C. Trevor Gale