REDUCING NEGATIVES BY CHEMICAL MEANS Most of the methods of reducing negatives modify the gradation by acting to a greater degree at one end of the scale than at the other. The special character of each being known, advantage may be taken of its properties to improve or modify an unsatisfactory negative. The following are the chief methods of reducing negatives by chemical means :— (i) Potassium Ferricyanide in a solution of " hypo " (Howard Farmer's reducer). Prepare a io per cent. solution of potassium ferricyanide (red prussiate of potash) ; this solution will keep indefinitely. When required for use from io to 6o mins. of the solution are added to an acid solution of 2 oz. or 2f oz. of " hypo " in 20 oz. of water. The " hypo " must not have been pre viously used for any purpose. A solution strong in ferricyanide should be employed if consider able reduction is required ; for less reduction a weak solution is more under control. The ferri cyanide should be added to the " hypo " at the moment of commencing reduction, as the mixed solution deteriorates rapidly. Soak the plate in water for half an hour before treatment, and rock the dish during reduction to ensure even action. When desired, the negative is with drawn and well washed. The general quality will be improved by immersing it, after a slight rinsing from the reducing solution, for a few minutes in an acid " hypo " bath of the same strength as that used for mixing the reducer.
This reducer acts more on the weak tones, or shadow portions, of a negative than on the strong tones, or high lights. A moderate reduction of the high lights may be accompanied by com plete obliteration of the feeble details in the shadows. It is a very valuable method for dealing with over-exposed, foggy or veiled negatives, since it increases contrasts as it reduces density. The character of the result is not affected by the strength of the solution or proportion of ferricyanide present. A weak solution acting for a long time gives the same result as a strong solution for a shorter time.
(2) Ammonium Persulphate.—H. W. Bennett's method is to prepare a stock solution containing ammonium persulphate, sodium sulphite, sul phuric acid (pure), and water ; the exact pro portions will be found under the heading " Ben nett's Reducer." The working solution is a
mixture of i part of the stock solution and 4 to 8 parts of water.
The negative must be soaked in water for an hour, immersed in the solution, and the dish rocked until sufficient reduction has taken place. The solution will become slightly opalescent or milky in appearance, but this is an indication of its working satisfactorily. When desired, the plate is removed from the solution, rinsed rapidly, and placed for not longer than six minutes in an acid " hypo " bath of the strength already stated, afterwards well washing. A shorter immersion in the " hypo " solution would be sufficient to check the progress of the reduction, but by remaining for six minutes the plate is left in such a condition that subsequent in tensification or any other treatment can be successfully applied if desired. Without the " hypo " bath after treatment is practically possible.
The character of the result given by reducing with ammonium persulphate varies according to the degree of reduction. In the early stages the dense parts of the plate only are attacked, and they may be appreciably reduced without any perceptible action on the weak shadow details. With a moderate degree of reduction, the high lights will decrease considerably in strength with only a slight loss in the shadows. After this, however, the action gains in force in the shadows, and prolonged reduction results in a loss of strength in the shadow details equal in proportion to that of the strong tones. The most useful character of ammonium persulphate is that of correcting or harmonising harsh con trasts, which it accomplishes by reducing the strong tones to a much greater degree than the weak details. But in order that full advantage may be derived from this quality, it is essential that the action should not be prolonged. The plate must be carefully watched, and withdrawn from the solution as soon as any action can be detected in the weaker parts.