Bromide. Cyanide

HgBr.AgBr + AgK(CN)2 = Hg(CN)2

Potassium Bromide and

Bromide. Metallic Silver.

+ KBr + Ag.AgBr

From the above, it will be seen that the intensified image is composed of metallic and silver bromide, so that should the deposit, after the treatment, be too intense, it can easily be reduced by a bath of dilute hypo, which dissolves the bromide.

The formula which Spiller recommends for intensifying by this process, although possessing little that is novel, may prove of value to many of the readers who have no experience with the method. The following solutions are required: -

(a) Mercury bichloride .. 1 dr.

Potassium bromide .. 1 dr.

Water........ 16 oz.

(6) Silver nitrate .. .. 1 dr.

Potassium cyanide . 1 1/2 dr.

Water........ 8 oz.

The commercial potassium cyanide is quite pure enough for this purpose; but if the pure salt be used, only 1 dr. should be taken. The solution of cyanide should be made at least 24hours before required, and the liquid shaken briskly from time to time, to ensure the saturation by the silver. Even after standing for the above period, a large precipitate will remain undissolved.

A negative to be treated by this method is first soaked in (a) till the image is more or less bleached, according to the amount of intensification required; it is then washed in 2 or 3 changes of water, placed in another dish containing (6), and there allowed to remain until the white deposit is blackened throughout the whole film. The latter is finally very thoroughly washed, preferably in running water, for about 1/2 hour, in order to remove every trace of the silver. Negatives treated by this means ought to be permanent, as the cyanide acts like hydrochloric acid in dissolving out every trace of mercury.

In conclusion, Spiller advises the addition of a small proportion of hydrochloric acid to the mercury bath for all processes in which the chloride per se is used; but when expense is no object, the mercuric bromide and silver cyanide is the most satisfactory method for gelatine films. {Photo. News.) (3) The chemical now mostly used in intensifying gelatine plates is mercury bichloride in combination with ammonia, or potassium iodide or cyanide. The main difficulty of such intensification has been that it was not stable; in a short time the image on the plate, if exposed much to the Tight, fades out, and spoils the negative. The intensifier given below has been found to work well, and at the same time possesses the quality of being absolutely stable.

(a) A stock solution of iron sulphate is made as follows: -

Iron sulphate .. .. 15 gr. Citric acid .. .. 15 gr. Water ...... 1 oz.

(6) A second solution is made as follows: -

Water ...... 1 oz.

Silver nitrate .. .. 10 gr. Acetic acid .. .. 10 m.

To intensify, take enough of (a) to cover the plate, and add thereto 6-10 drops of (6); flood the plate, and the intensification will proceed in a clear, gradual, and satisfactory manner. To produce a great degree of intensity, more of the silver solution should be added, a few drops at a time.

(4) Dr. Eder has frequently used the mercuric chloride and sulphite method for intensifying gelatine negatives, and recommends the process.

The washed negative is placed in a solution of mercuric chloride, and allowed to remain a longer or shorter time according to the degree of intensification required. After this, the plate is rinsed with water - a thorough washing being superfluous. To blacken the plate, it is now immersed in a strong solution of sodium sulphite (say 1 part of sulphite in 8-10 of water), and when the darkening has reached its maximum, the plate is well washed.

The special advantage of this method is the fact that there is no necessity to wash away all traces of the mercuric chloride before placing the plate in the sulphite bath, as mercuric chloride and sodium sulphite do not react upon one another. The white and insoluble mercurous chloride which is deposited upon the plate is, however, rapidly reduced to the metallic state by the sulphite. As metallic mercury forms a stable image, negatives intensified by the method described may be regarded as permanent; moreover, the colour is a very good one for printing, and there is but little fear of losing the more delicate shades of the subject from over density.

If the intensification has been carried too far, it is easy to reduce the negative by treatment with a very weak potassium cyanide solution.

(5) W. T. Wilkinson has been experimenting with a new intensifier, which, as its principal ingredient is platinum, induces the hope of greater permanency than the usual mercurial intensifier. The formula stands thus: -

(a) Ammonium chloride .. 5 gr.

Mercury bichloride .. 10 gr.

30-gr. solution of platinum bichloride .. 1 oz.

Water........ 20 oz.

(6) Liquor ammonia .. .. 1/2 oz.

Water........ 20 oz.

Two solutions are given, but it is rarely that more than the first solution is needed.

Immerse the negative to be intensified in (a), and watch carefully the action. Directly the requisite density (a dark-brown colour being the result) is reached, remove and wash thoroughly. If, however, through extreme weakness or not stopping exactly at the right time, the image begins to bleach, let it continue until nearly white, and then wash and immerse in (6).

For negatives requiring only a small amount of strengthening, this process is splendid; and even when carried out so far as to render the use of 2 solutions necessary, there is no clogging of the shadows or intense yellow films, as is frequently the case with mercury alone. After washing thoroughly and immersing in (b), the change takes place very slowly, the high lights gradually assuming a bluish black, and the shadows clearing if the negative be an overexposed one. This clearing of the shadows is very valuable, and, instead of having a thick negative taking hours to print,the result is a negative harmonious from high light to clear shadow. All the changes are slow, and under perfect command.