However you don't need formula. A ferricyanide bleach as used for sepia
toning is followed by a brief wash, then immersion in a warm mixture
containing copper ions and hydrogen peroxide. Concentrations are not too
important, though they - and temperature - will alter the speed of
working. Copper sulphate is often cheaper to buy and works fine. I suspect
the solution I used to take from the shelf was 125g/litre. The peroxide I
used was '20 vol'.
Having etch bleached you can redevelop if you want to get a positive
result but with the dark grey and black missing - just paper base. You can
also use dyes that will absorb into the gelatine and give a flat colour
image. If you etch bleach before fixing the print I think you can then
develop it to give a black everywhere except where the emulsion has been
removed - like a lith effect.
There is (or used to be) a Tetenal kit that gave the same results as etch
bleach - they have a different name for it again though. The chemistry was
different and I think more convenient.
Peter Marshall
On Fixing Shadows and elsewhere:
http://www.people.virginia.edu/~ds8s
Family Pictures, German Indications, London demonstrations &
The Buildings of London etc: http://www.spelthorne.ac.uk/pm/