INTENSIFICATION OF NEGATIVES (Fr., Renforcement ; Ger., Verstdrkung) A process whereby density or contrast is in creased. Intensification processes in common use vary in the degree of strengthening rather than in their character ; with slight exceptions they do not change the gradation, but strengthen all parts of the scale in a uniform ratio. The exceptions are some of processes that depend on bleaching with a mercuric salt as the first part of the operation. With Monckhoven's intensifier very thin shadow detail becomes reduced, and tones with slightly more strength show no change, while the medium and stronger tones more than double their printing value. In bleaching with mercuric bromide or chloride, exceedingly weak shadow detail is not strength ened, but all other parts of the plate show a uniform increase in intensity.
A negative to be intensified must have been thoroughly fixed, as, otherwise, incurable yellow stains may appear. Thorough washing after fixing is desirable.
The following methods of working will yield negatives of a good black colour, free from any tendency to veiling, staining, or any loss of quality. And, in addition, the results are quite permanent. With all mercurial methods, how ever, the intensified negative should be varnished, so as to protect the metallic deposit in the gela tine film from atmospheric moisture, otherwise an iridescence will appear, but this may be removed by means of methylated spirit (see " Reducing Negatives by Mechanical Means "). A badly stained negative will not become black by intensification ; and, in addition, when a negative depends for its printing quality partly on silver deposit and partly on stain, the result of intensifying will always be uncertain. The intensifier strengthens the silver deposit only, and either decreases the staining or leaves it unchanged.
The chief methods of intensification are : Chromium.—The negative is bleached in an acidified solution of a bichromate salt, and then re-developed. The printing value is multiplied by I. Mercuric Chloride.—The negative is
bleached in a solution of mercuric chloride and then blackened with sodium sulphite, ferrous oxalate or by re-development. With sodium sulphite the printing value is only multiplied by but with ferrous oxalate by 2. Mer curic Bromide.—The working is similar to the preceding, substituting bromide for the chloride, and blackening with sodium sulphite or by re development ; the printing value is multiplied by 2. Mercuric Iodide.—The negative is strength ened in a solution of mercuric iodide, and then re-developed, the printing value being multiplied by 3. Silver.—Various methods of intensifying negatives by means of silver have been intro duced. Lead, Uranium, Copper Bromide, etc.— These are treated under their own headings.
A method that has been extensively employed is bleaching in a solution of mercuric chloride and then blackening in ammonia. This method is generally considered to be unsatisfactory, and the results not permanent.
Of the above, the mercurial processes are the most used and will here be described. For the others. see separate headings.
Mercurial Processes.—Most of the mercurial methods are satisfactory, the intensified negative being a good neutral black or brown-black, according to the process ; there is no staining or veiling ; the increase of density is a definite and known quantity, and the result is permanent.
In all processes of mercurial intensification, the mercuric solution should be followed by an acid bath (first suggested by A. Haddon), its object being to prevent the mercury from com bining with the gelatine to the detriment of permanency. When the plate is removed from the mercuric solution, it should be washed in two or three changes of water, and then placed in a bath consisting of I drm. of hydrochloric acid in 12 oz. of water, remaining in this solution for two or three minutes, being then placed in a second quantity, and finally in a third. Next it is washed for fifteen to twenty minutes, when it is ready for the second part of the treat ment.