RESTRAINER (Fr., Moderateur ; Ger., Zartich halter) Any compound which will check the too ener getic action of a developer. The most popular restrainer for alkaline developers is a soluble bromide, which works well with the caustic alkalis, and in some cases with the carbonates. When the alkali is ammonia, ammonium bromide may be used ; otherwise, potassium bromide may be employed. Bromide has more effect in restraining the least exposed portions of a nega tive—that is, the shadows—than on the better lighted portions, and as a result it gives greater contrast and brilliancy. In cases of over exposure, where normally a flat and lifeless negative would be the result, this property of bromide is turned to account, the addition of a very few drops of a to per cent. solution to the developer often sufficing to save an under exposed plate. The generally accepted theory is that the bromide forms a double compound with the silver salt in the film, and that this double compound is less readily acted upon by the developer. With stale plates, a restrainer is absolutely necessary, as the silver salts in the film seem to undergo a certain amount of decom position, and reduction (development) takes place where the light has not acted. Bromide is also necessary with bromide and gaslight papers, as it serves to keep the white parts of the print dear while the image is being developed to its proper density.
Some authorities recommend the omission of a restrainer from the developer entirely, while others advise its use with extreme caution. Watkins, for example, says that the use of a restrainer, such as potassium bromide, is a legacy left from the early days of pyro development, that its use introduces complication and varia tion from the usual simple course of develop ment, without sufficient compensation, and that with a good modern dry plate and sodium car bonate as the alkali, the restrainer is best omitted from the normal developer and used only exceptionally to " hold back " the lowest tones or fog. Watkins illustrates this " holding back " power with two diagrams, A and B, the former representing four steps of gradation, A, B, C, and D, produced by, say, five minutes' development without bromide. B gives an idea
of the result of the same exposure if the same developer, with a little bromide added, is used for the same length of time. The steepness of gradation (and therefore the contrast in the print) is identical in the two cases ; but the bromide has retarded all the tones and prevented the lowest one, A, from appearing at all. It has, in fact, at this stage reduced the speed of the plate. If development with bromide were continued, the tone A would develop out, and in due time take exactly the same place as if no bromide were used. To utilise the restraining power of a bromide, therefore, development must be terminated while the holding-back power is still in force. Watkins also points out that with high factor developers the lower tones come out so early in development, they rush up, so to speak, that the holding-back power of bromide is exhausted before a useful degree of contrast is attained, and bromide is of very little use with this class. But if a low factor developer is used, the tones naturally come out very late in development, and the bromide decidedly alters results. It is with a low factor developer and plenty of bromide that gross over-exposure can be made to give good negatives. But when once the tones have appeared it is quite useless to add a bromide with the idea of holding back fog or the lower tones. With high factor num bers, bromide has practically little result, except to make development slower.
The only restrainers that are of any service when development has once started are the citrates (which see); they stop the shadows and allow the high lights to go on developing.
The usual dose of potassium bromide is 5 drops per ounce of mixed developer ; and, of course, the ammoniu_m and sodium bromides may be used in a like manner. Potassium bromide is bulk for bulk the weakest restrainer ; ammo nium bromide the strongest ; and sodium bromide the medium, the most suitable propor tions being potassium 120, sodium 103, and ammonium 98.