This section is from the "Studio Light Incorporating The Aristo Eagle - The Artura Bulletin 1919" book, by Aristo Motto. Also see Amazon: Studio Light Incorporating The Aristo Eagle - The Artura Bulletin 1919.
By reduction in photography is meant the removal of some silver from the image so as to produce a less intense image. Thus, in the case of an over-developed plate there will be too much density and contrast, and the negative may be reduced to lessen this. In the case of an overexposed negative there may not be an excess of contrast but the negative will be too dense all over, and in this case what is required is the removal of the excess density.
It is unfortunate that the word "reduction" is used in English for this process. In other languages the word "weakening" is used, and this 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 the true chemical reduction, such as occurs in development.
All the photographic reducers are oxidizing agents, and almost any strong oxidizing agent will act as a photographic reducer and will remove silver, but various oxidizing agents behave differently in respect to the highlights and shadows of the image. Reducing solutions can be classified in three classes:
A. Cutting reducers
B. True scale reducers
C. Flattening reducers
A. The cutting reducers remove an equal amount of silver from all parts of the image and consequently remove a larger proportion of the image from the shadows than from the highlights of the negative. The typical cutting reducer is that known as Farmer's Reducer, consisting of a mixture of potassium ferricyanide and hypo, the potassium ferricyanide oxidizing the silver to silver ferrocyanide and the hypo dissolving the latter compound. Farmer's Reducer will not keep when mixed, decomposing rapidly, so that it is usually made by making a strong solution of the ferricyanide and then adding a few drops of this to a hypo solution when the reducer is required. It is especially useful for clearing negatives or lantern slides which show slight fog, and is also used for local reduction, the solution being applied with a brush or a wad of absorbent cotton.
Another cutting reducer is permanganate. The permanganates are very strong oxidizing agents, and if a solution of permanganate containing sulphuric acid is applied to a negative, it will oxidize the silver to silver sulphate, which is sufficiently soluble in water to be dissolved.
Permanganate has only a very weak action on a negative if acid is not present and this may be made use of for the removal of "dichroic" fog, the yellow or pink stain sometimes produced in development. Dichroic fog consists of very finely divided silver and this is attacked by a solution of plain permanganate which will have no appreciable action on the silver of the image. An important difference between the behavior of ferricyan-ide and permanganate when used for reducing pyro-developed negatives should be noted. In a negative developed with pyro the image consists partly of the oxidation product of the pyro associated with the silver. When such a negative is reduced with ferricyanide the silver is removed but the stain is unattacked so that the negative appears to become yellower during reduction, though the ferricyanide does not really produce the color, only making it evident by removal of the silver. Permanganate, on the other hand, attacks the stain image in preference to the silver and consequently maks the negative less yellow. Permanganate can also be used as an alternative to ferricyanide for bleaching negatives, since if bromide is added to the solution silver bromide will be formed and the same bleaching action obtained as with ferricyanide.
Portrait Film Negative, Artura Print By Geo. C. Bell Madison, Wis.
Potassium Permanganate occurs in dark purple crystals which dissolve to form a purple solution. It is easily obtained pure but there is a good deal of impure permanganate on the market; Eastman Tested Permanganate is a very pure product.
In addition to its use for reduction and bleaching, permanganate is employed as a test for hypo, since it is at once reduced by hypo, and the colored solution of the permanganate therefore loses its color in the presence of any hypo. It may consequently be used to test the elimination of hypo from negatives or prints in washing. When permanganate is reduced in the absence of an excess of free acid, a brownish precipitate of manganese dioxide is obtained and sometimes negatives or prints which have been treated with permanganate are stained brown by this material. Fortunately, manganese dioxide is removed by bisulphite, which reduces it still further, forming a soluble manganese salt. The brown stain can therefore be removed by immersion of the stained material in a solution of bisulphite.
A very powerful cutting reducer is made from a solution of iodine in potassium iodide, to which potassium cyanide has been added to dissolve the silver iodide formed during reduction. Iodine is not soluble in water but is soluble in a solution of potassium iodide, and to make up the reducer a few iodine crystals are dissolved in a 10% solution of potassium iodide, and five parts of this are added to one part of a 10% solution of potassium cyanide, making up to 100 parts with water for use.
Portrait Film Negative, Artura Print By Geo. C. Bell Madison, Wis.
B. 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 the negative increases proportionally. A correctly exposed but over-developed negative should be reduced with a proportional reducer. Unfortunately, there are no single substances which form exactly proportional reducers, but by mixing permanganate, which is a slightly cutting reducer, with persulphate, which is a flat-ening reducer, a proportional reducer may be obtained.
C. In order to have a flattening reducer, we require one which acts very much more on the heavy deposits than on the light deposits of the negative, and which will consequently reduce the high-lights without affecting the detail in the shadows. Only one such reducer is known, and this in ammonium persulphate. Ammonium persulphate is a powerful oxidizing agent and attacks the silver of the negative, transforming it into silver sulphate, which dissolves in the solution. It must be used in an acid solution and is somewhat uncertain in its behavior, occasionally refusing to act, and always acting more rapidly as the reduction progresses.
Ammonium Persulphate is a white crystalline salt, stable when dry. It has recently been found by the research laboratory of the Eastman Kodak Company that the action of persulphate depends largely upon its containing a very small amount of iron salt as an impurity, and that its capricious behavior is due to variations in the amount of iron present. The persulphate supplied among the Eastman Tested Chemicals may be relied upon to give a uniform action in reduction.