Re: plain hypo for fixing film?

[Richard Knoppow <dickburk@ix.netcom.com>]

At 02:57 PM 05/30/2002 -0700, you wrote:
>I have some plain hypo crystals that I want to use to fix films. I have 
>always used Ilford liquid rapid fixer up to this point.  What kind of
>solution should I make with plain hypo to fix film?  Is the fixing time the
>same (5 minutes has been my time)?
>
>--shannon
>
  For crystaline hypo use 250 grams/liter. You must add some sulfite to
prevent staining from carried over developer. Sodium Sulfite, dessicated 15
grams / liter is about right. 
  Note that crystaline hypo is very endothermic. That is, it absorbs heat
as it dissolves. So, start with pretty hot water and have some means for
keeping it hot as you dissolve the hypo. About 125F is right. Much hotter
will tend to decompose the hypo. For anhydrous hypo do not dissolve in
water hotter than about 90F. 
  The sulfite should protect the hypo from decomposing from any reasonable
amount of acid carried over from an acid stop bath.
  The acid in the usual fixer is for the benefit of the hardener, which
requires the pH of the solution to be within narrow limits to work. Without
the hardener the pH is unimportant. 
  All "rapid" fixers use Ammonium thiosulfate rather than Sodium
Thiosulfate. The fixing time is shorter, particularly for films with any
amount of Silver Iodide in them. That is practically all films. Fixing time
should be determined by fixing out a scrap of film. Note the time it takes
to make the film clear. Fix for twice that time. Most films will take about
three minutes to clear. T-Max, Delta, Tri-X, and some others will take
considerably longer.  
  The capacity of a single fixing bath to produce archival fixing is very
limited. Use two successive baths. The capacity is four to ten times that
of a single bath. This is particularly important with film since most of
the iodide will leach out in the first bath leaving the second one working
at full speed. 
  Despite Ilford's claim that no wash aid is necessary if a non-hardening
fixing bath is used, I think a wash aid will still accelerate washing. Alum
hardener acts as a mordant for thiosulfate and fixing reaction products
over a narrow range of pH. By changing the pH of the emulsion to neutral
before washing the mordanting effect is eliminated without losing the
hardening action. However, sulfite wash aid, such as Kodak Hypo Clearing
Agent, also has an ion exchange function which greatly accelerates washing
regardless of the pH. It also tends to displace some othewise insoluble
fixer reaction products, which extends the life of the fixing bath. 
----
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com