RESIDUES (Pr., Residus ; Ger., Racksteinde) The collection of residues, from the amateur's point of view, is hardly worth the trouble unless he uses a very large quantity of material, though probably about 6o to 7o per cent. of the total sensitive salts are not used in the formation of the actual images. Residues may be divided into four kinds : (I) emulsion residues, (2) fixing bath residues, (3) gold residues, and (4) platinum residues.
(r ) Emulsion residues .—The general method of collecting spoilt emulsions, etc., is to mix them with plenty of water acidulated with impure nitric or hydrochloric acid, and boil till the set ting property of the gelatine is destroyed. The sensitive salts settle to the bottom of the vessel in time, allowing of the decanting or syphoning off of the supernatant fluid ; then the silver salts are collected and dried.
(2) Fixing bath residues.—The fixing baths both from negative and positive work should be saved and mixed together, and when a sufficient quantity is obtained they may be mixed with a saturated solution of liver of sulphur, which precipitates the silver as sulphide. This process should be done outdoors, as sulphuretted hydrogen is evolved, which is not only evil smelling but is detrimental to any sensitive sur faces. A better method is to add zinc filings or
magnesium powder, which decompose the silver hyposulphites with the precipitation of metallic silver.
(3) Gold residues.—Spent toning and com bined toning and fixing baths should be acidu lated with hydrochloric acid, and then an acidu lated saturated solution of ferrous sulphate added till a precipitate is no longer caused.
(4) Platinum residues.—The developer and 'first washing water of platinotype should be col lected and preferably boiled down to a smaller bulk, a saturated solution of ferrous oxalate added, afterwards boiling.
The various precipitates may be collected and all the silver ones mixed, but the gold and platinum should be kept separate, and they can then be sent to the refiners, who will allow the market value of the metals less a small percentage for smelting.
Paper clippings of all kinds should be burnt and the ashes added to the general residues.
Silver is obtained from the residues by fusing with borax, charcoal, etc.