Sometimes negatives are obtained which are so dense that they are difficult to print. Other negatives are so contrasty that they give harsh prints. In order to improve these negatives recourse may be had to the process called "reduction," that is, to the removal of some of the silver by treatment with a chemical which dissolves the metallic silver of the image.
It is unfortunate that the word "reduction" is used in English for this purpose. In other languages the word "weakening" is used and it is undoubtedly a better word because the chemical action involved in the removal of silver from a negative is oxidation, and the use of the word reduction leads to confusion with true chemical reduction such as occurs in development.
In order to produce the best results it is necessary that the reduction should be suitable for the negative which is to be treated. Thus, in the case of a negative which is too dense all over it is necessary to remove the density uniformly, while in the case of one which is too contrasty what is required is not the removal of the silver from highlights and shadows alike, but the lessening of the deposit on the highlights without affecting the shadows.
In Fig. 89 we see a diagram which represents a negative originally dense from which by the removal of an equal amount of silver from shadows, halftones and highlights, there can be obtained a negative of proper gradation. A reducer which effects this uniform removal of density is generally called a "cutting reducer". The typical "cutting reducer" is that known as Farmer's reducer, which is made by preparing a strong solution of potassium ferricyanide, otherwise known as Red Prussiate of Potash, and adding a few drops of this to a solution of plain hypo until the latter is yellow. This reducer will not keep when mixed so that the ferricyanide must be added to the hypo only when required for use. It is especially useful for clearing negatives or lantern slides and is often used for local reduction, the solution being applied with a wad of absorbent cotton to the part which is to be lightened. Another cutting reducer is permanganate, which is supplied under the name of the 'Eastman Reducer." Permanganate, however, tends to act more proportionally on the highlights and shadows than is the case with ferricyanide.
Fig. 89. Diagram Showing How Cutting Reducer Acts.
Proportional reducers are those which act on all parts of the negative in proportion to the amount of silver present there. They thus exactly undo the action of development since during development the density of all parts of a negative increase proportionately. A correctly exposed, but over-developed negative should, therefore, be reduced with a proportional reducer. This effect is shown in Fig. 90 where it is seen that the contrast of the negative is far too great owing to over-development, and that by removing the same proportion of the silver from the shadows, halftones and highlights, a negative of correct contrast can be obtained.
Fig. 90. Diagram Showing How Proportional Reducer Acts.
Unfortunately there are no single reducers which are exactly proportional in their action but by mixing permanganate, which is a slightly cutting reducer, with persulphate, which is a flattening reducer, a proportional reducer may be obtained.
Flattening reducers are required for negatives which have been under-exposed and then over-developed. In these cases the negative is much too contrasty but it is important not to remove any of the deposit from the shadows, since owing to the under-exposure, there is already insufficient deposit in the shadows.
What is required in this case is shown in Fig. 91, where a large amount of deposit is removed from the highlights, a smaller amount from the halftones, and very little or none from the shadows. This can be accomplished by the use of ammonium persulphate. Ammonium persulphate attacks silver deposit with the formation of silver sulphate and this attack is increased by the silver salt which is produced, the rate of attack increasing as the attack goes on. Such chemical actions are called "auto-catalytic," a "catalyst" being a substance which increases the rate of a chemical action without actually taking part in it, and an auto-catalytic action being one in which the rate of action increases of its own accord. Since the action of ammonium persulphate is auto-catalytic it acts most rapidly where the greatest amount of silver is present, and consequently it attacks the highlights far more energetically than it attacks the shadows of the negative and is, therefore, suitable for the reduction of under-exposed, over-developed negatives. (Whether any silver will be removed from the shadows will depend on how long the reducer is allowed to act.) Because it is auto-catalytic in its action, however, it is very likely to go too far and get out of control so that it is not by any means an easy reducer to handle, and it is not recommended that it be used upon a valuable negative unless the user has had considerable experience of its action.
Fig. 91. Diagram Showing How Flattening Reducer Acts.
a. Negative too dense all over.
b. Result of using Farmer's Reducer.
For some time after ammonium persulphate was introduced as a reducer for negatives its action was very uncertain; some samples would reduce silver while others would not. When this peculiarity in its behavior was investigated by the Research Laboratory of the Eastman Kodak Company the reason was found to be a chemical difference in some of the samples tested.
a. Correctly exposed but over-developed negative.
b. Result of reducing with a Proportional reducer.
a. Too dense in highlights, deep shadows not clear.
b. Effect of the Eastman Reducer on such a negative.