Kevin O'Brien (kob@paradise.net.nz)
Wed, 10 Mar 1999 11:34:37 +1300
In reversal processing the bleach removes the silver produced. I have
only seen two agents used for that: the chomates and pot permanganate.
Sulphuric acid is normally used. Hydrochloric can't be used because of the
chloride; we want to remove the silver not reuse it, as is done in
intensification. Except for the hazardous materials this part is not
critical. The strengths of active solutions is quite weak. Ten mils of
sulphuric acid in a litre of water is no big drama. One formulation has
only ½%! A formulation using acetic acid from Ilford, given for stain
removal, but appears generally useful as well as safe:
Pot permanganate 6g
Common salt 13g
Acetic acid, glacial 50ml
Water to 1l
Leave the salt out for this purpose. The Acetic acid in the pure glacial
form is
unpleasant and can cause burns. However it can be bought as white vinegar
which is about 3%-6%; so one would just add the permanganate to the vinegar!
The commercial (photo) strength is 28% and that's unpleasant enough.
I suspected citric acid may work and I have just found in my old notebook
this one:
SOL A
Citric acid 25g
Copper sulphate 20g
Nitric acid 8ml (Optional - speeds up working)
Hot water (60%C) 240ml
SOL B
Hydrogen peroxide 3% sol 300ml (normal medical strength)
Working soln 1pt A + 1pt B just before use.
I have not tried it but it was given as an etch (bleach) for reversal
processing of lith film. It may be that the citric acid/copper sulphate
combo works on its own. Another formulation with copper sulphate doesn't
use peroxide. Another possibility is is adding a little pot permanganate to
a citric acid/copper sulphate soln. Another version uses copper sulpate &
sulphuric acid so the citric/copper sulpate looks like its well worth a
trial- any piece of developed film should show the image being removed if it
works.
Kevin O'Brien
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