Three pints more water are then added with the bisulphate of soda, and the mixture becomes a dirty yellow colour. Lastly add the remainder of the water and the cyanide of potassium. The electro copper bath must be colourless.
Large pieces of silverware may be coppered in these baths. Very small articles are threaded upon a zinc or iron wire, or placed in a perforated ladle with granules or cuttings of either of these metals. Place the whole for a few minutes in a diluted but very acid solution of sulphate of copper, the zinc or the iron is dissolved, and the copper is deposited upon the silver. When the article is intended to be gilded or silvered, it is immediately passed through the " quirking " solution of nitrate of mercury, rinsed in cold water, and placed in the electro-baths, without drying or scratch-brushing.
To electrotype printing type, the surface is first cleaned by means of turpentine, dried either in the air or with hard wood sawdust, and dusted with fine plumbago. A shallow dish of pewter, termed a chase, made about of an in. deep, and of the requisite size for the work, and provided with ears for hooks to be attached, is filled with a mixture of beeswax and fine plumbago which has been previously mixed hot ; it is allowed to cool until the wax has become nearly set, when the type is inverted on to its surface, and both chase and type are then placed in a press and squeezed gently, the type is lifted off to see if any wax will adhere, it is then dusted with plumbago, replaced in exactly the same position, and squeezed tightly. The squeeze, as the wax mould is then termed, is dusted with plumbago and polished with a soft brush. In large works machines are employed for this purpose, consisting of a closed case containing the plumbago, into which the squeeze is intro duced, and where it is dusted by a series of brushes arranged to move irregularly to and fro by means of eccentrics ; the chase is then hooked and hung in the deposit trough in the usual copper sciatica, a few inches from the anode. When the deposit is sufficiently thick, which will occur in from 8-12 hours, the chase is removed, washed, and heated, the electrotype removed, placed on a tray, brushed over with soldering fluid, and sprinkled with solder, which is rubbed over the interior with a rag ; the eleotro is then filled to a requisite depth with fluid stereo metal, planed to gauge, and mounted type-high on a block of wood.
An older method is to employ guttapercha instead of wax.
Silvering may be practised by simple immersion with very good results, if care is taken to use the ingredients in correct proportions, and if the same care is exercised with respect to the purity of the ingredients and the previous cleansing of the articles to receive the deposit, which is observed in operations whore the battery is employed. The simplest silvering solution is made by mixing into a thin paste 3 parts of soda, 1°25 common salt, and 1 part chloride of silver, with sufficient warm water ; another mixture may be made with the chloride of silver and salt alone ; in either case the paste is applied with a rubber upon the surface until it is properly coated, when it should be washed, dried, and varnished in the usual way. Another solution, which may be either used in
paste or as a bath, consists of 1 part each of cream of tartar and common salt, and part chloride of silver dissolved in boiling water in a common kettle, with the addition of a little alum ; the articles are stirred up in this composition, and as each fresh batch is operated upon, a quantity of paste is added in proportion to the surface to be whitened. This bath improves greatly by use, but should it not produce the desired result, the following solution, used boiling, will be found quite effectual. It is composed of 1 oz. nitrate of silver, 9 oz. oynnide of potassium, and 1 j gal. of water. The foregoing are practically applicable to articles of jewellery, such as earrings, bracelets, chains, buckles, studs, and the like, whilst with a little experience and attention much larger articles may be treated with success. Roseleur recommends a bath made by dissolving in an enamelled oast-iron kettle, 17i oz. of cyanide of potassium in 2 gal. of water ; in a separate vessel 5i oz. of fused nitrate of silver is dissolved in 2 pints of water. This second solution is added gradually to the first and stirred again and again until the precipitate produced is dissolved ; if necessary the whole may be filtered. It is then brought to a boiling point and is ready for use. This bath cannot be renewed, and when exhausted should he added to the waste and a new one formed. Another simple immer sion process which produces excellent results is made by dissolving common soda in water, the proportions being 1 lb. of the former to a pint of the latter, and pouring a little mercury into the bottom of the vessel ; the end of a glass tube is now allowed to dip into the mercury; either sulphurous acid gas or sulphuretted hydrogen is passed through the tube, the mercury merely preveuting the formation of crystals ; the passage of the gas is continued until the liquid slightly reddens blue litmus paper, when the whole is put on one side for a day. The liquid portion is then poured off, stirred, and again tested ; if it turns the litmus paper red, soda must be added ; if blue, there is too much alkali and more gas must be passed through the liquid. The solution should mark from 22° to 26° Blume, and must not touch iron, zinc, tin, or lead. When required for use, a portion of the liquid is taken and to it is added a solution of nitrate of silver in distilled water ; a precipitate will be formed, and the silver solution is added so long as this readily disappears, the condition of the bath being preserved by additions of either gas or soda and silver, when necessary. The articles are simply moved about in the bath until the required coating is obtained; it will give an excellent result upon either brass or copper.