From: Peter Marshall (petermarshall@cix.co.uk)
Date: 10/05/01-03:19:26 AM Z
> Well,the REAL mordancage uses a bleach formula on a print that is
> DEVELOPED but NOT FIXED. Therefore there is metallic silver (the black
> image) as well as remaining silver halide (unexposed original emulsion
> not
> yet washed away).
>
> In this formula, the copper sulfate with hydrogen peroxide (and for all
> I
> know you could do it with copper chloride instead) and an acid will
> attack and dissolve BLACK SILVER only, leaving the non-metallic
> undeveloped silver halide intact. My formulas all called for a bath in
> hot water after the bleaching -- you felt with your fingers until the
> silver was raised and starting to come off, then swabbed it away with
> cotton, which took more or less time depending on your personal virtue,
> heat of the bath, strength of the bleach, position of the planets, etc.
>
> Then you had a print with raw uncoated paper where there had been image,
> and silver halide where there hadn't. At this point you exposed the
> undeveloped part and developed again, which gave you a REVERSAL of the
> original, no need to fix. Since I solarized my original, which often
> reversed, I could end up with a positive, or near positive.
Yes, what you have described is exactly one of the techniques we used to
use. The copper bleach dissolves the silver and loosens the emulsion as
you say. It doesn't have this effect in lighter gray areas of the print,
so on re exposure and development you get the original white areas going
to black and the mid tones staying more or less the same.
We also did it with fixed prints, where the bleach has exactly the same
result, leaving a print with some lighter grays, particularly around the
edges of what were previously light areas. There is some bleaching so you
need to redevelop to get the areas as dark as possible. Doing it this way
gives a positive print except the original shadow areas which are now
white.
You can also simply fix after bleaching at either point. This gives you an
image that is simply in gelatin - with some areas simply the paper base.
At the time I did this there were a range of materials sold with intensely
coloured emulsions on a white base especially for use in this process -
giving a 'lith' effect in colour, which was the primary use of the
material. However we mainly worked with ordinary RC photo paper, so the
image was not very visible at this stage.
We called all variations of this process etch bleaching, and the copper
bleach was sold as an etch bleach, though I used to just take a couple of
bottles from the chem lab shelves and make up my own.
Some students then went on using any of the variations of etch bleaching
to use dyes or litho inks, water-soluble dyes soaking into the remaining
gelatin areas, whereas oily inks were repelled by them. (You could also
use knives, needles etc to work by hand to remove emulsion as well or
instead of as using the chemical method.)
Not my kind of photography, and I no longer teach this kind of thing, so I
have no personal need for the information. It may still be in one of my
filing cabinets at work, but I just haven't got room for everything at
home.
Peter Marshall
Photography Guide at About http://photography.about.com/
email: photography.guide@about.com
_________________________________________________________________
London's Industrial Heritage: http://petermarshallphotos.co.uk/
The Buildings of London etc: http://londonphotographs.co.uk/
Also on Fixing Shadows: http://www.people.virginia.edu/~ds8s
and elsewhere......
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