It is a fact that only about 5 per cent. of the gold and silver used in producing a photograph remains on the finished picture; the balance is lost, and below are given a few short and simple methods of saving and reducing photographic wastes and residues.
(a) Old baths and the washings of the prints should be precipitated with ordinary salt, thereby forming silver chloride. Add the salt gradually, stirring up the solution, until it forms no longer a precipitate, which you may easily determine by taking a sample of it in a tumbler or white bottle, holding it up to the light when adding a little salt. Do not add too much, as an excess will redissolve the chloride. When the silver is all down, pour in a little acid, either nitric, sulphuric, or muriatic, which will clear the solution; allow it to stand for about 24 hours, then draw off your clear water and you have the chloride on the bottom of the vessel.
(6) The hypo or fixing solution is very rich. It should be precipitated with potassium sulphide previously dissolved in water, also adding it as long as it will form a precipitate. The latter, when down, may be thrown on a plain muslin filter to allow the water to drain off. Such a filter may be readily constructed by taking a piece of common unbleached muslin, say a yard square, tying loops to the four corners, and hanging it up on sticks.
(c) A good many photographers are in the habit of precipitating their washing solutions with metallic zinc expanded in sheets therein. The action of zinc, however, is slow, and must be accelerated by acidifying the solution. Now it frequently happens that the fixing solution is allowed to run into the same vessel, and, the hyposulphite being an alkali, suspends the action of the zinc. In the course of time a deposit out of the water is formed; but the proprietors of the "mud" are sadly disappointed in its value, as it is sometimes even so poor as not to pay for the trouble of refining.
(d) All prints should be trimmed before toning, as it saves gold, and, besides, toned paper is of hardly any value. Keep the untoned clippings and filters clean by themselves; do not throw sweepings, pieces of glass, and spoiled ferrotype plates among them, as their bulk only decreases the real value. If you wish to burn the paper, have your stove cleared of cinders and ashes, and proceed slowly, for a good draught will carry many particles of silver through the flue.
(e) Your toning solution throw down with iron sulphate, but bo sure and have the solution "acid," as otherwise the iron will be precipitated, and your gold goes up the chimney. Save your developer and collodion skins; they will also amount to something in the course of time.
(/) Sometimes the wood of barrels which contained waste solutions for a number of years is quite impregnated with silver, some barrels yielding as much as thirty ounces of metal; so, when yours are unfit for further use, you know what to do with them.
(g) Last, but not least, do not send small lots of waste to be refined, but wait until you have a reasonable quantity for expenses, and charges are then comparatively less.
(h) Common salt is without action upon the liquids which hold silver in the state of a double salt, and will rather aid the solution than the precipitation; such are the double tartrate of silver and potash, whitening bath, the double sulphite of soda and silver, and the bath for dipping. Before employing common salt, add sulphuric acid, which, displacing the other acids, restores the silver to the state of a simple salt, easily precipitated by common salt. Hydrochloric acid alone precipitates silver well from these solutions. Liquors containing silver as cyanide are also exceptions; to extract all the metal, use the process employed for similar combinations of gold, evaporate to dryness, and reduce the mass in a crucible, with an addition of soda carbonate and powdered charcoal. The metallic silver remains at the bottom of the crucible.
(i) Baths of silver cyanide, the residues from galvano-plastic establishments, are precipitated with sulphuric acid. The precipitate contains all the silver, along with copper, zinc, and iron. It is ignited, and the residue is treated with nitric acid, which dissolves out the silver, zinc, and copper. From this solution the silver is thrown down as chloride. The portion insoluble in nitric acid contains carbon, ferric oxide, and traces of silver, which may be extracted with ammonia.
(k) Filter the solution of silver proposed to be operated upon until it is clear, and place the filtrate in a clean white bottle of suitable capacity. To each pint of the liquid add 4 oz. or more of mercury, and allow the mixture to remain at perfect rest for a few days.
In a very few hours a beautiful sparkling corusoation will be found forming upon the surface of the mercury by what is known as double elective affinity, and for each atom of the silver so deposited, a corresponding amount of mercury is acted upon by the nitric acid of the silver, and passes into solution as mercury nitrate. The deposition continues until all the silver has been thrown down, when we find over it a strong solution of mercury nitrate, which may be obtained in the solid crystalline form by evaporation. In a few days the deposition will be completed, which can be readily seen if the tree ceases to grow. Shake the bottle thoroughly, so that the branches of the tree are detached and broken, and brought in thorough contact with the mercury, where the spangles of silver are quickly dissolved. The watery part of the mixture can now be drawn or decanted off from the mercury, and the latter placed in a bag, or, better, in a large piece of fine tough buckskin, and pressed with force between the hands. When no more mercury can be squeezed through, the bag may be opened and the lump of brittle amalgam removed, and preserved in a well-cleaned and stoppered bottle until more has been accumulated.