This section is from the "Studio Light Incorporating The Aristo Eagle - The Artura Bulletin 1917" book, by Aristo Motto. Also see Amazon: Studio Light Incorporating The Aristo Eagle - The Artura Bulletin 1917.
There are two methods of removing the stain - (a) chemically, (b) photographically.
Oxidation stain may be removed by bleaching the silver image to silver chloride, and redeveloping, this process, incidentally, removing the stain. In the case of both negatives and prints, it is a wise precaution first to harden them in a 5 per cent, solution of formalin, and wash, otherwise the gelatine is apt to soften and frill during the treatment. The permanganate bleach is made up of the following two stock-solutions:
Avoirdupois Potass, permanganate . 64 grains Water......32 ounces.
Sodium chloride(table salt) .... 5½ ounces Sulphuric acid (strong) . . 1 ounce 160 grains Water......32 ounces
For use, take two parts of water and one part of B, and to this add one part of A.
A point that must be observed in the mixing of the solutions is that the stock-solution A must be added to the diluted B solution, that is, one part of A must be added to the combined two parts of water and one part of B. If the stock-solutions are mixed before B is diluted, chlorine gas will be given off. When the solutions are mixed as directed, chlorine gas is not liberated, but remains in solution and converts the silver image into silver chloride, which is wanted.
The solutions A and B keep well if kept separately, but not when mixed, and for this reason the bleaching-bath should be prepared as required.
When preparing the solution A, be sure that no particles of undissolved potassium permanganate remain, or they will give trouble in the way of spots and blemishes in the negative.
The bleaching should be complete in about three or four minutes, after which the negative should be rinsed and put into a weak solution of sodium bisulphite, rinsed and developed in a strong light (daylight, if possible) with an ordinary developer, say Nepera solution one part, water four parts.
In the case of a pyro-develop-ed. negative, the image of which consists partly of a silver and partly of a pyro stain image, the above process removes the stain image entirely, leaving a pure silver image, the process therefore being equivalent to reduction. By using a weak pyro re-developer, much of the original stain image may be re-formed, though, incidentally, considerable general yellow stain is produced at the same time.