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This process, therefore, differs from that with hyposulphite. The latter acts by dissolving the iodide of silver, the former by decomposing it. It is necessary not to leave the zinc in contact too long, or it deposits stains, and in large plates the contact should be made at the four corners successively, to avoid this accident. Gilding Solution.
A concentrated solution of iodine is prepared by putting into a common bottle two thimblesful of hyposulphite of soda and a rather larger quantity of iodine, so that there may be more than sufficient. After shaking it a short time, the water is rapidly and strongly colored. The solution should be poured into a bottle with a ground stopper, and when cool used for iodizing.
Having fitted up the apparatus, about two ounces of hyposulphite of soda are placed in the bottle A, while the bottle B is about three-fourths filled with water distilled or melted ice water is to be preferred; some sulphuric acid about two ounces is now diluted with about twice its bulk of water, by first putting the water into a dish and pouring in the acid in a steady stream, stirring meanwhile.
Such gilding is continually deteriorating, which with good chloride and distilled water may be prevented. Distilled water should also be used for the hyposulphite. and for cleaning plates. Any good, clear water may be afterwards used for washing off, with equally good results. I am very rarely troubled with specs, and deem this as the main reason.
This came from a small laboratory in one corner of the room the possession of which, together with the free chattery and smattery use of such words as "carbonate," "hyposulphite," "phosphate," and "affinity," were enough to convince even the most sceptical that Dr Skinner had a profound knowledge of chemistry.
Gilded articles are most readily cleansed with a solution of 5 grains borax in 100 parts water, by means of a sponge or soft brush. The articles are then washed in pure water, and dried with a soft linen rag. Silverware is cleansed by rubbing with a solution of sodium hyposulphite. Carbon, in one form or another, is the base of all black pigments.
The formula of this substance is NaHSO , as distinguished from what is commonly known as hyposulphite of soda, Na S O . It is produced by the action of zinc-dust on the acid sulphite of soda.
When the hyposulphite of gold is used in gilding, it requires less heat and a longer application, as there is some danger of producing a glossy scum over some parts of the surface of the plate.
The hyposulphite solution should be filtered through sponge every time it is used. The direct rays of light must not enter the camera in conjunction with those reflected from the object; or the picture will be veiled, and the color of the plate changed to a thick green.
The hyposulphite solution should be often filtered through a sponge, and it will answer for a great number of washings. Yet it is observed that the mercury collects in this solution in small globules; these often come in contact with the plate, causing white spots, which spoil the impression. They should be guarded against, and the solution renewed.
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