Articles joined with solder in which tin is an ingredient must not remain any length of time in this liquor, or the solder will be dissolved and the copper blackened ; nearly every metallic article is dipped in this solution, and afterwards thoroughly rinsed or swilled in water, and pewter, lead, tin, and Britannia metal can be transferred without further preparation to the depositing bath.
Cast iron may be cleansed by immersing in a dilute solution of sulphuric acid and water, the time of immersion entirely depending upon the proportion of acid used ; thus in water con taining part of acid it may remain several hours, the metal being afterwards rinsed, scoured, again plunged into the pickle, and rinsed before submitted to deposition. Spots containing silica frequently occur upon the surface of cast-iron articles, and this can be removed with hydro fluoric acid, sufficient care being exercised in its use to avoid the injurious fumes and contact with metals for which it is a solvent.
Wrought iron with an untouched surface may be treated in the same way as cast iron, but will bear a stronger pickle and longer immersion. Steel and polished iron are first scoured, boiled in the potash solution, and then passed through a bath composed of one gallon of water, a pint of sulphuric acid, with a few ounces of either nitric or hydrochloric acid or a mixture of both ; if nitric) acid is used, the sulphuric acid may previously have a few ounces of zinc dissolved in it, with advantage. Articles of iron or steel thus cleansed may be kept for some time in proper condition for work by immersing them in liquor rendered alkaline by caustic lime, potash, or other alkali.
Silver is heated and plunged into a boiling pickle of water and dilute sulphuric acid, then rinsed in clean water, the operations being repeated as many times as they are needed ; nitric instead of sulphuric acid may be used for the pickle, and if strong the articles may be dipped cold ; in this case the water must be distilled, free from chlorine or hydrochloric acid, otherwise the goods will be covered with a bluish-wbite film of chloride of silver. This latter method is not applicable to articles having parts of either iron or zinc, in such cases the plan is to dip in alkali and polish afterwards with powders or scratch-brush ; cleansed silver may be placed in the deposit bath directly, but it is usual before doing so to scratch-brush it.
Copper, German silver, and brass, the three metals of which the bodies of most objects to be electroplated are made, are cleansed in a series of liquids; if they are very dirty they are boiled in alkali, rinsed, and then dipped in the acids. They frequently receive a preliminary cleansing by being heated and then dipped in dilute sulphuric acid and water, but this will not serve of course for articles which are united by solder. Articles which have been cleansed by alkali must be washed in clean water before being put into the dipping bath or pickle ; in fact it is advisable to thoroughly and rapidly rinse in fresh water all articles before and after any cleansing operations.
The various dippings which complete the cleansing of these articles follow each other in rapid succes sion, and to effect this the pans or troughs are arranged in rotation. The first dip may be composed of old aquafortis, that is nitric acid or dipping liquid which has been already weakened by preceding dippings ; the goods may than be run through dipping liquid composed of 4 parts sulphuric, acid, 4 parts water, 2 parts nitric acid, and part hydrochloric acid. Articles that have been first cleansed by heating may be soaked in old aquafortis until, after rinsing, they have a uniform metallic lustre ; they may then be dipped in strong aquafortis for a few moments and rapidly rinsed. The volume of acid should be at least 30 times that of the article to be cleaned in order to prevent too great an elevation of temperature and as rapidly weaken that of the acid. It should be here remarked that care must be taken in the choice of nitric acid, since only the straw coloured cleans well ; the white acid is not strong enough, whilst the red acts too powerfully and pits copper.
The acid will be spent when its action on copper goods becomes too slow ; it is then employed for the first operation, dipping in old aquafortis, or for whitening baths. Extreme temperatures have considerable effect upon aquafortis, which cleans imperfectly when either very hot or extremely cold. To impart a bright lustre, which is particularly required in metal employed for gilding, the goods may be dipped in weak aquafortis until a black coating is formed, then in old aquafortis or strong pickle as it is called, and afterwards into strong aquafortis, afterwards again and again alternately into strong aquafortis and water. Amongst the many mixtures for obtaining a bright lustre, a very good one is composed of old aquafortis nearly spent 1 part, hydrochloric acid 6 parts, water 2 parts ; the articles are immersed a few minutes, rinsed to remove the black coating which covers them, elegted, and dipped again ; this bath will be found useful for gilding metal and also for copper castings. If it is desired to obtain a dead lustre, the articles are, after dipping in aquafortis and rinsing, plunged iuto a cold bath containing 1 part sulphuric acid to 2 parts of nitric acid and a little salt ; to this the French operators add a little sulphate of zinc. The immer sion in this bath will be at the discrdtion of the operator, the time usually occupied being from 5 to 20 minutes: from this bath, after a long rinsing, the piece should be rapidly passed through the bath for producing a bright lustre and again immediately rinsed ; this latter operation is to prevent an earthy dullness which is otherwise produced if the dead lustre bath alone is employed. It may be here observed that old aquafortis may be considerably revived by the addition of strong sulphuric acid and common salt.