Ferric Sulphate Reducer. A very dilute solu tion of a ferric salt which is quite free from chlorides and bromides (a condition which is generally satisfied by ferric ammonium alum which is a crystalline salt of pale rose violet colour) and which is acidified by a little sulphuric acid forms a reducer which is almost exactly proportional in its action and keeps well (H. Krause, 1919). The following solution may be employed : a 2 per cent solution of the salt in rain water or distilled water containing about o.5 per cent of pure sulphuric acid. After rins ing, the negative is bathed for some time in a very dilute solution of sulphuric acid before the final washing.
Permanganate and Persulphate. A mixture in suitable proportions of the surface acid per manganate reducer with the superproportional persulphate reducer forms a reducing solution of intermediate character, the action of which is almost proportional (N. C. Deck, K. Huse and A. II. Nietz, 1916).
The bath must be prepared when required for use by mixing the following substances in the order given, each one being dissolved separately in a small amount of water— Potassium permanganate . i gr. (o-lo grin.) Sulphuric acid (I% solution) i oz. IA dr. (ho c.c.) Ammonium persulphate . 90 gr. (to grin.) Water, to make . . 20 OZ. (I,000 c.c.) The time required for reduction is from 2 to 5 minutes. The negative is then rinsed and bathed for 5 minutes in a solution containing about To per cent of bisulphite of soda before being washed in several changes of water.
463. Ammonium Persulphate Superpropor tional Reducer. The selective action of alkali persuIphates on the higher densities of photo graphic images was discovered in r9ox by A. and L. Lumiere and A. Seyewetz, who specially recommended the use of ammonium persulphate." Persulphate converts silver into silver sul phate, 2 which, in proportion as it is formed, tends to accelerate the attack on the remaining silver. The presence of chlorides, which is practically inevitable, causes disturbance (E. Stenger and H. Heller, 1910) if the amount of sodium chloride is as much as o-or per cent : this is noticed as an exaggerated superpropor with a discontinuity in the reducer for a certain density ; this value of density is raised by increasing the amount of chloride present (G. Higson ; S. E. Sheppard, 1921). It will thus be understood that reduction by per sulphate may be impossible with tap water which is relatively rich in chloride, such as may be found in some coastal districts.
The rate at which persulphate attacks the silver varies considerably with the acidity of the substance. The action is controlled by the addition of a trace of sulphuric acid or of a very small quantity of a silver salt (even of a small amount of an already-used solution) or of iron alum (Sheppard, 1918-1921) ; all these sub stances slightly diminish the risk of " poisoning " by chlorides.
The reducing bath is prepared when required for use as follows— Ammonium persulphate . 17-5 to 45 gr. (2 to 5 grm.) Sulphuric acid gr. (o.f grm.) Water, to make . 2 oz. (WO c.c..
The reduction, which is comparatively slow at first, gradually gains in speed and may be come too rapid. The action of the bath must be stopped a little before the desired effect has been attained ; this is done by plunging the negative, without intermediate rinsing, in a solu tion of about 10 per cent of anhydrous sulphite of soda, which at the same time dissolves any traces of silver chloride formed in the film. The negative is then washed in several changes of water. 2 464. Indirect Reduction of Heavier Densities. The image is treated in a bath containing about 2 per cent of potassium ferricyanide and 2 per cent of potassium bromide, in which it gradually bleaches. The operation is interrupted by wash ing in plenty of water a little before the higher densities have been completely bleached ; the remaining silver is then dissolved in a solution of potassium permanganate acidified with acetic acid (R. Namias, 1911-1925) as already described in a note to § 439.
After dissolving the residual silver, the nega tive is washed thoroughly and is transferred to a solution containing about io per cent of bind phite of soda, which decolorizes the gelatine and dissolves the acetate and chloride of silver formed ; it is then re-developed in any ordinary developer whilst exposed to weak light.
It is also possible to protect the greater part of the silver against the action of Farmer's reducer by transforming it into silver sulphide, or by toning it with gold or selenium. The silver can also be converted to bromide or chloride and re-developed superficially, the unreduced silver halide being dissolved in a fixing bath.