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silver, copper, hypo, plate, metallic, cent and washing

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REPEATING BACK.—A form of single plate-holder so arranged that two separate nega tives may be made upon one plate. There are many forms of studio cameras with repeating backs, enabling the operator to take two cabinet or c.-de-v. pictures upon one plate. Repeating backs are also used for single stereoscopic cameras, with one lens only, the two negatives being made upon the one plate, the lens having been moved about two inches previous to making the second exposure.

RESIDUES.—The recovery of the unused gold, silver, and platinum used in photography is now Practised by all who use large quantities.

.Silver Residues.—It is well known that only about three per cent. of the silver used in sensitizing albumen paper remains in the finished prints; although it is not possible to recover the whole of the remaining 97 per cent., yet by careful practice about 8o per cent. should be saved. The following table, showing where the different losses take place, has been carefully compiled by MM. Davanne and Girard: Silver per cent.

Draining papers. 1-o28 First and second washing waters Hypo bath Washing water of fixed proofs Cuttings and clippings 4'570 Remaining on proofs 3'100 Loss 2,232 100'000 It will be observable that the greater portion of the silver is to be found in the first wash ing waters and in the hypo bath—nearly 85 per cent., so that it will well repay one to save these, if only a fair portion of the silver can be extracted.

Residues may be divided into three classes, i. e.: 1. Clippings, cuttings, spoilt paper and prints.

2. Washing water used previous to fixing.

3. Old hypo baths.

Unless large quantities of paper are used, the first-class can be entirely abandoned if the unreduced silver is fixed out from the spoilt prints, etc. If they are saved, however, they should be stored away until a good quantity is obtained. They are then made into a pile, and burnt by setting fire to the top of the pile and allowing it to descend. The ashes are then collected and sent to the refiners, or, instead of burning, the paper may be beaten up to a pulp, with dilute sulphuric acid and strips of metallic zinc or copper placed in the mixture. Metallic silver is pre cipitated, and the zinc or copper is dissolved.

With regard to the second class of residues, the first and second washing waters only should be saved in a large tank or tub, and hydrochloric acid or common salt added, and the liquid al lowed to settle. When the top is quite clear, the silver chloride will have settled to the bottom, and the liquid decanted or syphoned off. VVhen a fairly good portion of the silver chloride is obtained, it should be collected, dried, and sent to the refiners.

The third class, i e., the old hypo bath, cannot, of course, be subjected to the same treat ment, as the silver chloride is soluble in them, and would not precipitate. To reduce the silver from the fixing baths, the old solutions are kept separately in a tub or earthenware vessel, and some sulphuretted potash (liver of sulphur) added, which throws down the silver as a black deposit of sulphide In this state it should be sent to the refiners. It can be reduced to metallic silver by to free it from the the sulphur. and fusing. There are serious draw backs to this process, however. The stench is abominable and injurious, and if free sulphur be left in, explosions are likely to occur when the mass is fused.

Davanne's method of recovering the silver from old hypo baths consists simply in immers ing a plate of copper in the hypo bath, and letting it stand for about four days. The silver becomes gradually deposited upon the copper plate, whilst the copper is dissolved in the liquid. The copper plates are generally stood up on end against the sides of the tub, and are occasionally brushed with a stiff brush. The metallic silver will then settle gradually to the bottom, and when a sufficient quantity has accumulated it is filtered and washed. The filtrate, consisting of metallic silver and a little copper, can easily be converted into silver nitrate, slightly blue owing to the copper. If only a small portion of copper be present it will have no injurious effect ; if a large quantity is detected, it can be thrown down as an oxide in the form of a black powder. The silver oxide should be added until the blue color has disappeared.

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