Sandy King (sanking@hubcap.clemson.edu)
Sat, 13 Feb 1999 23:26:56 -0400
Hi Kevin,
Thanks very much for posting your ideas and formulas for silver bleaches.
What is the primary use of these formulas, 1) contrast control via
bleaching and redevlopment, or 2) bleaching for reversal processing?
Sandy King
>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).
>-------------
>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.
>-------------
>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.
>------------------
>>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.
>---------------------
>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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