Kevin O'Brien (kob@paradise.net.nz)
Sun, 14 Feb 1999 17:05:54 +1300
Having been printing for nearly 50 years, I have made up, and used, a wide
variety of bleaches. The basic formulae all go back to last century and the
use of chromium seems to have been initially as an a metallic intensifier.
Looking at the many formulae, acidulated chromium or pot permanganate are
readily interchangeable for silver bleaching, but not dissolving silver
halides. I used whatever I had at hand. The chrome bleaches don't stain the
hands, or other organic matter, the same way and that's probably why they
became more popular. Now with eco-considerations permanganate seems the
obvious choice. It's easy to get too, as many pharmacys stock it, under the
name of 'Condy's crystals'.
The purpose of the acid is to create soluble silver salts that can be washed
out. Sulphuric seems to be preferred but hydrocloric is also popular. Some
formulae show nitric, acetic and others. If using acids they can be kept
easily and safely well diluted. Sulphuric can be had from many sevice
stations in a dlilute form. Hydrocloric can be found in plumbing suppliers
as "spirits of salts" or muriatic acid. Nitric is still used by printmakers
and by industrial engravers. (For those into also printmaking, ferric
cloride can be had from printed circuit board manufacturers).
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A safe substitute for sulphuric is sodium bisulpate and one formulation of a
reversal bleach shows:
Water 1l
Pot bichromate 6g
Sod bisulphate 20g
Clear by rinsing and treat with 5% sod bisulphite to remove staining.
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Another reversal film bleach:
Sol A
Water 500ml
Pot permanganate 2g
Sol B
Water 500ml
Sulphuric acid 10ml
Use equal parts freshly mixed. Both keep well separately.
Clear by rinsing and treat with 2.5% soln of sod or pot metabisulphite.
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>From these two formulae it seems that a friendly general purpose silver
bleach could be made up:
Sol A
Water 500ml
Pot permanganate 2g
Soln B
Water 500ml
Sod bisulphate 10g
Mix equal parts freshly prior to use.
Clear by rinsing and after treatment with 2.5% of sod or pot metabisulphite.
Precise compounding is not essential.
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It is a worthwhile project to re-create modern, safe, simple, inexpensive,
easy to mix and keep formulations of the older working chemistry. Its a
wonder that past users lived to report their success.
Kevin O'Brien
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