(3) Potassium Permanganate.—The use of an acidified solution of potassium permanganate followed by a weak solution of oxalic acid for reducing negatives is due to Prof. Namias. The oxalic acid bath, however, is injurious to some plates, particularly those composed of soft gelatine. It softens the film and causes the more transparent parts to become opalescent and useless for printing. With some plates it works satisfactorily. The following modified method of working is quite satisfactory and free from the objections that apply to oxalic acid. Two stock solutions are necessary : A. Potassium perman ganate . 20 grs. 4'5 g.
Water . . . io oz. i,000 ccs.
B. Sulphuric acid (pure) . . too mins. 2i ccs.
Water . . . io oz. i,000 „Take t dram or io ccs. each of A and B, and add sufficient water to make t oz. or 8o ccs. For considerable reduction use a larger proportion of A and B. The negative should be soaked in water for an hour, placed in the solution, the dish being rocked until the reduction is sufficient, rinsed rapidly in two or three changes of water, immersed in an acid fixing bath for a few minutes, and then washed and dried. A solution of potassium metabisulphite may be substituted for the acid " hypo," but the latter is prefer able. The reduction effected by acidified potassium permanganate is almost uniform throughout the scale, but there is slightly greater reduction in the strong tones than in the weak shadow details.
(4) Cerium Peroxide.—This is obtainable in the form of a concentrated solution which keeps moderately well. To i drm. of this solution add sufficient water to make i oz. ; immerse the negative in this until the desired reduction has been attained; then wash and dry. It acts in a greater degree on the shadow details than on the stronger parts, but the difference is not quite so great as in the case of the ferri cyanide reducer. The plate, after reduction, is very clean and the image is of a good colour.
(5) Ferric Chloride.—This reducer is different in character from the preceding ones. The following stock solution may be prepared, but it does not keep well for more than a few weeks : Ferric perchloride . 6o grs. 14 g.
Citric acid . . i2o „ 28 „Water . . . io oz. i,000 ccs.
For use, add to oz. of this solution enough water to make I oz. Allow the negative to remain in the bath for about two minutes, rinse, transfer to an acid " hypo " bath for a few minutes, and wash and dry. Longer immersion in the reduc ing solution will produce no greater effect, as the reduction is a fixed quantity. The degree of reduction is slight, the negative losing about one-eighth of its density uniformly throughout the scale. The operation may be repeated, one eighth of the strength being removed each time.
Reduction by re-halogenisation is treated in the article on re-halogenisation.