It has been shown that the mixture of bi chromate and hydrochloric acid may be re placed by a pure solution of a chlorochrornate (Lumiere and 1919), or by mixtures of chromic acid with a chloride (C. H. Bothamley 1918), or of a bromide (L. J. Bunel, 1923).
For the practical application of this method of intensification two stock solutions are pre pared. These by themselves will keep indefi nitely : (A) 10 per cent solution of potassium bichromate.
(B) Ordinary hydrochloric acid diluted to ten times its volume with boiled water.
If it is required to obtain varying degrees of intensification, heavy, medium or slight, one of the following mixtures must be made up at the time required— 2 The various operations in intensification must be carried out in weak daylight or in artificial light in order to avoid solarization of the silver chloride. The negative is allowed to remain in the bleaching bath until all traces of black image have disappeared when viewed from the back. Negatives should not be allowed to remain in this bath for a longer time than necessary, otherwise irregular markings may be produced.
Negatives must be washed in several changes of water until the coloration of the gelatine has almost completely disappeared. The washing process may be considerably shortened by immersing the negatives, after rinsing, in a solution containing about 5 per cent of soda carbonate.
The image is then re-developed, preferably in a metol and hydroquinonel developer. In some developers the image comes up very rapidly at first and then appears not to change, though in reality it increases slowly for at least a quarter of an hour.
If the intensification is thought to be insuffi cient it may be repeated ; there is, however, very little to be gained by intensifying more than Images intensified in this way are quite permanent ; their warm black tone renders the method very suitable for the intensification of lantern slides or paper prints, either for the purpose of intensification as such or, by slightly increasing the amount of acid, to improve the colour. It may be noted that this method of intensification, which does not make use of any poisonous substance, is extremely The sensitometric measurements of Nietz and Huse (1918) have shown that, contrary to con clusions drawn from photometric measurements, the action of this intensifier is not proportional, but is greater in the least dense regions, a property which we have already seen to be very valuable.
455. Great Intensification with Copper and Silver. A method which has been used for a very long time, especially for collodion nega tives, consists in treating the negative in a solution of cupric bromide in which the silver is converted into bromide and at the same time fixes an equivalent amount of cuprous bromide. The negative, after rinsing, is transferred to a solution of silver nitrate, some of which is reduced to metallic silver by the cuprpus bromide ; this silver is thus precipitated with an equivalent amount of silver bromide in the image along with the existing silver bromide. After washing, and reducing the silver bromide, the amount of silver is exactly three times that originally present (Abney, 1877). This method is not easily applicable to negatives made with gelatino-bromide emulsions ; during washing, which is much slower than with collodion plates, the cuprous bromide is partially re-dissolved or re-oxidized and so escapes reaction.
The method has been improved by Luther and Schreiber (1923), and by G. Zelger (1924), by using in the first operation a solution which deposits in the image not cuprous bromide but a cuprous salt which is absolutely insoluble and non-oxidizable, such as cuprous sulphocyanidc or cuprous iodide.
As before, the density is exactly trebled by intensification ; it may be again trebled by repeating the process, and thus sufficient con trast may be obtained to print an image which exists as a mere ghost, as is sometimes obtained with films in which regression of the latent image has occurred.
The " bleaching " bath (which, as a matter of fact, gives a yellow image) is prepared by pouring solution (A) into solution (B)— Heat is generated when the solutions are mixed, and the bath must not be used until cool. The mixed solution, which is clear blue in colour, should be slightly acid ; if it is not, a little acetic acid must be added until blue litmus paper is faintly reddened by it ; it is quite stable and may be used until exhausted.