FERRICYANIDE REDUCER.
The ferricyanide reducer was introduced ay Mr. Howard Farmer, and is perhaps the one most used for general work. As already stated, it tends to reduce the shadows of the negative in greater propor tion than the lights. It is therefore a valuable agent in all cases of fog, flatness due to over-exposure, and other kinds of deficient contrast. It may be employed for either general or partial reduction, and is made up as follows: No. 1.—Potassium ferricyanide ... 20 grs.
Water I oz.
No. 2.—Sodium thiosulphate (hypo.)... 1 oz.
Wa ter ... 20 oz.
The hypo. solution must be clean and fresh ; it will not do to take it from an old fixing bath. If dry, the negative is im mersed in the hypo. until well soaked ; otherwise, the reduction may be proceeded with at once. A few drops of the feria cyanide solution are poured into a glass measure ; then the hypo. is poured off the negative into the measure, allowing the two solutions to mix. The mixture is afterwards poured back over the plate. Reduction immediately commences, and proceeds gradually. Take care not to use
too much ferricyanide, or the solution may be too energetic. Progress should be carefully watched, and the moment the negative is sufficiently reduced it should be removed from the solution and washed thoroughly. The negative should be taken out while still a trifle too dense, as reduction will continue slightly during washing. What is left in the dish may be thrown away, for this reducer will not keep, and requires to be mixed fresh each time. If the solution does not work quickly enough, add more ferricyanide. Avoid using too much, however, or it will cause a yellow stain. When a strong solu tion is to be employed, or when the nega tive has to be kept in it for a long time, it is advisable to render the bath alkaline with ammonia, which will prevent yellow ing of the negative. The ferricyanide is poisonous, and must be used with care. Both the salt and its solution keep better in the dark.