RESTORING PRINTS.--Prints when kept some time previous to toning will often turn yellow or brown, and become completely discolored. This may, however, be restored by placing immediately before toning in a two per cent. solution of strong ammonia. After toning they are washed in a bath of similar strength, and, further, about one part of ammonia is added to the fixing solution. With this treatment the whites of the prints should be completely restored.
Finished prints which fade by the action of light or by other influences can never be satis factorily restored.
RESTRAINER.—Any substance employed in order to lessen the energy of the reducing or developing action of the chemical solutions upon the exposed film. If a plate has been ex posed and it is immersed in an ordinary developer the effect would be that the image would flash up rapidly, allowing the operator no control over its density, and when finished it would have a very unsatisfactory appearance, owing to the absence of detail in the high lights and other defects.
The simplest restrainer is water added to the developing solution which naturally weakens its action upon the film. Another method is to tan or harden the film, with tannin or chrome alum, for instance, causing the gelatine to be less permeable to the developer.
The soluble bromides and the citrates of potash soda and ammonia are all used as restrain ing or retarding agents The following formulae for restrainers are from Wall's " Dictionary of Photography."
The restraining power of the bromides of ammonium, potassium and sodium bear the following proportions to each other : Bromide of ammonium 98 parts are equal to I 19 parts of potassium bromide and equal to 103 parts of sodium bromide. Ammonium bromide is consequently the strongest, and potassium bromide the weakest, while the sodium salt ranks between the two.
The citrates of potassium, sodium and ammonium, have apparently quite a different action, for while the bromides prevent to a certain extent the excessive deposit on the high-lights, and permit the detail to make its appearance, the citrates, on the other hand, appear to prevent the detail from making its appearance, and permit the density to be obtained.
To make these restrainers the bromides are simply dissolved in water as follows : Ammonium bromide 98 grains. Distilled water to make 960 minims.
of solution, each drachm of which will then contain six grains of bromide of ammonium.
Potassium bromide .119 grains.
Distilled water to make .98o minims.
of solution, each drachm of which will contain 73 grains of bromide of potassium, equal in re straining power to six grains of ammonium bromide.
Sodium bromide .103 grains.
Distilled water to make .980 minims.
of solution, each drachm of which will contain grains of sodium bromide, equal in restrain ing power to six grains of ammonium bromide, or to 73 grains of potassium bromide.