When the vapours have disappeared, the pieces, even after washing, remain of a dull black, on account of the formation of a basic copper salt which is not soluble in water. This last mode of operating, called blacking by aquafortis, is preferred by a few gilders, varnishers, and colour fixers, who find it economical to allow the production of nitrous vapours while the pieces are draining on top of the vessel which contains the acids. Any subsequent operation is to be prefaced by a rinsing in fresh water. When small objects, such as pins, caps, or eyelets, are to be dipped, they are put into a stoneware pot, with a small quantity of aquafortis, and then rapidly shaken and stirred. In this case the acid is entirely used up with the production of abundant vapours, and the objects remain blackened, and ready for a further cleansing. Care must be taken in the choice of aquafortis. Three kinds of nitric acid at 36° B, are to be found in the trade. "One is perfectly white, another is straw-yellow, and another is more or less dark-red. The white acid, without nitrous gas, does not cleanse well, especially when freshly used. The red acid acts too powerfully and pits the copper. The straw-yellow acid is preferred to the others.
Nitric acid at 40° B. is too energetic and costly; however, certain operators who have to cleanse large quantities of copper wares prefer it on account of the rapidity of the operation. The acid is spent when its action on copper goods becomes too slow, and when the objects removed from the bath are covered with a kind of bluish-white film. Such acid is preserved for the preceding operation, namely, dipping in old aquafortis; or for dipping in the whitening bath. Very good aquafortis may cleanse imperfectly when the temperature is too low or too high. This accounts for the difficulty of cleansing in frosty weather, or during the great heat of summer.
There is an excellent way of obtaining a bright lustre on surfaces which have been dulled or slightly pitted by defective cleansing, or by passage through acids for removing gold or silver. Place them for a few minutes in a bath composed of 1 part old aquafortis, nearly spent, 6 hydrochloric acid, 2 water. The pieces, when removed from the bath, are entirely black, and must be thoroughly rinsed in water to remove the kind of black mud which covers them. They are then cleansed and dipped again. This bath will be found useful by electro-gilders. It is also convenient for removing the sand adhering to the castings of copper alloys. Large pieces may remain in the bath for 20-30 minutes, as this mixture acts very slowly on copper and its alloys.
These acids are of two kinds, according to the object in view. If the pieces are to have a bright lustre, they are stirred for 1 or 2 seconds in a liquid, prepared the day before, and cold, made of 100 parts nitric acid at 36° B., 100 sulphuric acid at 66° B., 1 common salt. In preparing this bath, nitric acid is first put into the vessel, and then sulphuric acid, which is much denser, and would not mix readily if it were pat in first. At the time of mixing, especially when the salt is added, considerable heat and a quantity of acid and injurious fumes are produced, so that it is prudent to operate in the open air, or under a good chimney-hood with a movable glass sash. As these acids must be employed cold, it is necessary to prepare them in advance. Copper articles, after this dipping, are lighter coloured and much brighter than after the passage through aquafortis. They may then be considered as completely cleansed, and must be immediately rinsed in plenty of clean water. The above acids are too energetic for small articles, such as pins or hooks, which are generally cleansed in stoneware colanders.
As the small articles stop up the perforations, the acid cannot run out so quickly as desired, and begins to heat and give off fumes, the pieces blackening before they can be rinsed. Therefore, for small pieces, add to the above mixture one-eighth of its volume of water. Place the articles in a stoneware pot; stir rapidly with a small quantity of bitters, as the last mixture is termed, and then the whole is plunged into a quantity of fresh water as soon as the acid has sufficiently acted. This method is not economical, as the acid is lost; but the dipping liquors do not become heated. Whitening bath consists of old aquafortis, sulphuric acid, common salt, and uncalcined soot. Pour into a large stoneware vessel a certain quantity of old aquafortis from previous dippings, and then add twice the volume of sulphuric acid at 66° B. The mixture is allowed to cool off until the next day. The copper nitrate of the old aquafortis becomes converted into sulphate, which, by cooling, crystallises against the sides of the vessel. Decant the liquid portion into another vessel, and then add 2-3 per cent. of common salt, and as much of calcined soot. This mixture is much less energetic than the compound acids for a bright lustre, and often replaces them advantageously.
The crystallised copper sulphate is collected and sold. This bath is strengthened, when necessary, by the addition of stronger aquafortis and oil of vitriol. To replace the portion used up during the day, equal quantities of old aquafortis and oil of vitriol are added at the end of the day. The next morning the liquors are decanted, and the copper sulphate is gathered. Soot and common salt in sufficient proportions are then added. In this manner a perpetual and cheap whitening lath is prepared.
. If it is desired to give the objects a dead lustre, they are, after dipping in aquafortis and rinsing, plunged into a bath, prepared previously, composed of 200 parts nitric acid at 36° B., 100 sulphuric acid at 66° B., 1 common, salt, 1-5 zinc sulphate. Copper articles may remain 5-20 minutes in the cold bath, and the dead lustre will be the more apparent the longer the immersion has been. From this bath, after a long rinsing, the objects have an earthy appearance. This dulnessis removed by a rapid passage of the pieces through the compound acids for a bright lustre, and by an immediate rinsing. If they remain too long in the latter acids, the dead lustre will disappear, and the operation for dead lustre will have to be repeated. If a bath for the bright lustre is not at hand, the objects, after rinsing, may be rapidly passed through the dead-lustre bath, which will remove the dul-ness of the lustre caused by too long immersion. After long use, the com-pound acids for a bright lustre may be employed in a certain measure for a dead-lustre bath.
The mode of operation remains the same.
This operation consists in plunging the cleansed articles for 1-2 seconds into a solution of 2 1/5 gal. water, 1/3 oz. nitrate of binoxide of mercury, 2/3 oz. nitric or sulphuric acid. When nitrate of binoxide of mercury is poured into the water, a thick cloud is formed, of a yellowish-white colour, which subsequently disappears. Stir the mixture before using it. The proportion of mercury salt above-named must be modified, according to the size of the pieces, and the nature of the alloy. Thus leas mercury will be used for light pieces of jewellery which need a very thin deposit. On the other hand, more mercury is required for heavy objects, such as table ornaments, which should receive a thick deposit of gold or silver. The latter must come from the mercurial solution with a perfectly white and bright appearance, looking like silver, whilst the colour of the light articles is scarcely changed. After a perfect cleansing, the pieces will, after passing through a strong mercurial solution, be perfectly white and bright. But there will be a cloudy appearance, or various shades of colour, if the cleansing has not been properly done. The amalgamating bath becomes spent by use; it may be revived by the addition of a few drops of mercury nitrate; but it is better to prepare a fresh one.
No intervals must be allowed between the various operations of cleansing. The dipping baths are ordinarily held in vessels of glass, stoneware, porcelain, or of any other material which resists the corrosion of acids. Common earthenware and that with a lead glaze must be carefully avoided. The dipping pots must be rather high, and be furnished with a cover, in order to prevent evaporation. Those with ground edges may be covered with a pane of glass. Wide open-mouthed earthen pans are very good for rinsing. A large hood, communicating with a chimney, and closed with a sliding glass sash, should contain the following apparatus for complete cleansing operations: - A furnace and separate pans for first dipping, old aquafortis, aquafortis and Boot, compound acids for dead lustre, compound acids for bright lustre, solution of mercury nitrate, acids to dissolve gold from old pieces; acids to dissolve silver from old pieces; with two large pans for rinsing with a constant flow of water. If the draught of the chimney is not sufficient, a small fire may be kindled under the hood. A gas-burner is often sufficient.
The pot of mercury nitrate, with two rinsing pans, may be placed near the plating bath. .. Cleansing iron, - Cast iron is cleansed by being immersed for 2-3 hours in water containing 1 per cent. sulphuric acid; the metal is afterwards rinsed in cold water, and scoured with sharp sand and a fibre brush, or a coarse rag; then put again in the acid pickle, rinsed, and plunged into the plating bath. If more than 1 per cent. of sulphuric acid is added to the water, the length of the immersion must be shortened, otherwise the cast iron will be deeply corroded, and the carbon of the metal, which is insoluble in the pickle, will with great difficulty be removed by the friction of the sand. Cast iron does not gild or silver well, by a direct deposit of the precious metals. Copper or brass deposits are better, although far from perfect; but if cast iron is tinned, the coat is adherent, and will afterwards receive copper, brass, gold, or silver, if desired. If it is desired to keep cast iron cleansed for some time before plating it, it is necessary to preserve it in a liquor rendered alkaline by caustic lime, potash, or soda, or their carbonates; but caustic lime-water is the cheapest and most easy method, and cast iron which has remained in it for a few hours will not rust after a long exposure to a damp atmosphere.
The cleansing of wrought iron is effected in the same manner as cast iron, but it will bear a stronger pickle and a longer immersion, i.e. ordinary wrought iron covered with a film of black magnetic scale or of red rust. Whitened, filed, or polished iron must be treated like steel.