Liam Lawless (lawless@vignette.freeserve.co.uk)
Fri, 12 Mar 1999 02:50:58 +0000
Hi all,
One of Kevin's bleaches that I forgot to mention yesterday does work: pot.
permanganate plus acetic acid. It leaves a very faint, brownish residual
image that is strongest in the highlights; if left long enough, it does
redevelop, but to such an extremely low density that I'm sure it can be
ignored. I have only used teaspoons to measure the chemicals in my
experiments, so I cannot give the best strength or proportions, but I would
suggest initial trials with around 50g pot. permanganate + about 75ml
glacial acetic per litre, and dilution 1+9 for use.
The thing I don't like about permanganate bleaches is that they're so dark
that it can be difficult to locate a piece of film with forceps, so I've
also been playing with dichromate formulations. Sulphuric acid, which we
are trying to avoid, is H2SO4, and silver sulphate, the soluble form to
which silver is changed by reversal bleaching, is Ag2SO4 (the H2 of the acid
being replaced by Ag2), but there are other soluble silver salts, such as
silver sulphamate, NH2SO3Ag (as in MW's Argyrotype process). Instead of
sulphuric acid, therefore, why not try sulphamic acid (NH2SO3H)?
Sulphamic acid (which comes as crystals) with either pot. dichromate or pot.
permanganate will form a bleach that works. Dichromate + sulphamic again
leaves a very faint residual image, but this disappears in the wash. With a
little zinc sulphate added (I didn't have any sod. sulphate, which is what I
wanted to use), bleaching is faster and there is no residual image, but
dichromate + sulphamic alone is quite satisfactory.
Permanganate + sulphamic, with or without sulphate, also work fine, though
both leave a residual image that does not wash off. Again, this is so
slight that I'm sure it can be ignored.
For what it's worth (and to stop others wasting their chemicals), I also
tried copper sulphate + sulphamic, and also with zinc sulphate as well.
Neither worked.
So, either pot. permanganate or pot. dichromate with sulphamic acid (and,
optionally, zinc or sod. chloride) gives a safer alternative to the bleach
we have been using until now. For a starting point, try 50g of each
chemical in 1 litre (I hope all will dissolve!), and dilute 1+9. One
advantage of including the sulphate seems to be that it gives a cleaner
result with less dichromate staining, but I doubt this is worth worrying
about.
Hope this is useful - probably there are zillions more suitable bleaches if
anyone else cares to look for them. I now have to do some work for
Post-Factory, so please don't bother me for a few days unless it's urgent.
Thanks,
Liam
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