(a) The ordinary potassium cyanide is often preferred to the pure article, on account of its lower price; but the real value and dissolving property of ordinary cyanide are very variable. The following is a general method by which a bath of brass may be prepared with any kind of cyanide. Dissolve together, in 2 gal. water, 8 oz. copper sulphate, and 8-10 oz. zinc sulphate. (6) 4 oz. copper acetate, with 4-5 oz. fused zinc proto-chloride; and add a solution of 30 oz. soda carbonate, which produces a precipitate of copper and zinc carbonates; allow this to settle; then decant the supernatant liquor, and replace it by fresh water two or three times, after as many settlings. Then pour on 2 gal. of water containing, in solution, 30 oz. soda' carbonate, and 15 oz. soda bisulphite; while stirring with a glass or wooden rod, add ordinary potassium cyanide until the liquor is perfectly clear, or until nothing but the greyish-black iron, found in the cyanide, or the brown-red iron oxide in the zinc sulphate, remains in suspension.
An additional quantity of about 1 oz. of ordinary cyanide improves the conducting power of the liquor.
Copper and zinc carbonates, recently prepared, each 4 oz.; soda carbonate, in crystals, soda bisulphite, and potassium cyanide, pure, each 8 oz.; and 1/10 oz. white arsenic; water, about 2 gal. This bath is prepared as follows: - Dissolve in 3 pints water, 5 oz. copper sulphate, and 5 oz. crystallised zinc sulphate, and add a solution of 14 oz. soda carbonate in 1 qt. water. A greenish precipitate of mixed copper and zinc carbonates is formed: stir well, and allow to denosit for several hpurs. The supernatant liquid, holding the useless soda sulphate, is thrown away, and replaced by nearly 2 gal. water, in which are dissolved the bisulphite and carbonate. Dissolve together in the remaining warm water the potassium-cyanide and the arsenious acid, and pour this liquor into the former one, which is rapidly decolourised, and forms the brass bath. Filter if necessary. - Arsenious acid causes the deposit to be. bright, but if in too great a proportion may give a white or steel-grey < colour to the metal. This inconvenience is slight, as the yellow colour soon predominates. The arsenious acid may be replaced by soluble arsenites of potash, soda, or ammonia, but the proportions must be doubled.
The baths for coll plating are generally placed in Wooden tanks lined inside with gutta-percha, which resists their action for a Ions: time. The sides of the tank are also lined with one or more brass sheets joined together, connected with the last carbon or copper of the same battery, the intensity of which is regulated by the surface of the articles to be plated. The articles are suspended by copper or brass hooks to stoat rods of the same metal, all connected with the last zinc of the battery.
Dissolve together in 14 pints pure water: - 7 oz. soda bisulphite, 17 oz. potassium cyanide No. 2, 34 oz. soda carbonate. To this solution add the following, made.in 3 1/2 pints water: - 4 1/2 oz. copper acetate, 3 1/2 oz. neutral zinc protochloride. The two liquors become colourless when mixed. Ammonia must not be used for brass plating baths for iron, especially for solutions worked in the cold.
(e) A colour resembling brass is given to small articles of iron or steel by a long stirring in a suspended tub, containing 1 qt. water, and of copper sulphate and tin protochloride crystallised, about 1/5 oz. each. The shades are modified by varying the proportions of the two salts.
(/) For Lead and Pewter. - Lead and pewter should be cleansed in a solution of about 4 oz. nitric acid to 1 gal. water, in which they remain for 1/2 hour. Pewter is more easily coated with brass than lead, but the same bath may be used for either. They are then rinsed, scoured with sand, and rinsed again. A good battery power and a large surface of anode are necessary, especially at the beginning of the deposit. The proper temperature of the bath for brassing lead, pewter, and tin is about 90° F. Stirring articles in a brass bath has a tendency to cause the deposition of copper alone.
Pure water, 4 1/2 gal.; soda bisulphite, 24 1/2 oz.; potassium cyanide, No. 2, 35 oz. Add the following solution: - Water, 9 pints; copper acetate, and zinc protochloride, each 12 1/2 oz.; ammonia, 14 oz. The filtered bath is colourless, and gives, under the action of the battery, a brass deposit of a very fine shade, varying from red to green, by increasing the proportion of copper, or that of zinc. The anode is of brass.
In the disposition of the baths for brass plating, it is always necessary to have all the articles suspended at about equal distances from the anodes; the bath may be subdivided by several anodes forming partitions, so that each loaded rod is between two anodes, or smaller separate baths may be employed. , The anodes should be removed when the bath is not at work. In order that the brass plating of zinc and copper may be lasting, the deposit must not be too thin, and must be scratch-brushed, rinsed in water rendered slightly alkaline by quicklime, and thoroughly dried in a stove. But generally the articles are brass plated by remaining in the bath for 10-25 minutes. Cast and wrought iron, lead and its alloys, require brass solutions richer in the metals than when depositing upon zinc or its alloys. The battery power should also be greater. .
The losses of the solution are to be repaired by additions of copper and zinc salts, and arsenious acid dissolved in potassium cyanide. The operator will determine the needed substances from the rapidity of the deposit, its colour, and so on. If the deposit is too slow, try whether the bath will absorb the salts of copper and zinc, without the addition of cyanide. If the coat of brass has an earthy and ochreous appearance, and especially if the liquor is blue or green, add potassium cyanide until perfect decolourisa-tion takes place. If the deposit is dull and unequal, add a small quantity of arsenious acid dissolved in cyanide. If the deposit is too red, add the salt of zinc, alone, or dissolved in cyanide. If the deposit is too white, or of a greenish-white colour, add the salt of copper, alone, or dissolved in cyanide. When the bath after long use has become overloaded with salts, the specific gravity is too great for the easy passage of the electric current, the liquor must be diluted with water until it works satisfactorily.
The specific gravity of a brass bath may vary from 5o to 12° B. The pieces, before brass plating, must be perfectly cleansed in the same manner as zinc or iron; if the brass deposit is irregular, remove the objects from the bath, rinse, scratch-brush, and put again into the bath until the colour and the thickness of the deposit are satisfactory. Scratch-brush again, and, if necessary, rinse in hot water, dry in warm sawdust of white wood, and put in the stove-room. The last three operations are indispensable for hollow pieces.
The difficulty in brass plating, especially with small baths, is in keeping uniform the colour of the deposit, as the galvanic current, having simultaneously to decompose two salts each offering a different resistance, must, according to its intensity, vary the composition and the colour of the deposited alloy. It will be found that a feeble current principally decomposes the copper salt, and results in a red deposit; whilst too great intensity in the current decomposes the solution of zinc too rapidly, and the deposit is a white or bluish-white alloy. This is the case more especially with newly-prepared baths, and is an indication of irregularity in the conducting power of the bath, which, however, becomes more regular after being used for some time. The inconvenience of a red deposit may be remedied by increasing the number of the elements of the battery, or employing stronger acids, or decreasing the number and the surfaces of the objects to be plated; the other inconvenience of white deposits will disappear by diminishing the- number of elements, or by increasing the surfaces to be covered. The deposit may also be modified by substituting for the brass anode, either a sheet of pure copper, or one of zinc, or by simply hooking one of these sheets to the brass anode.
A bath of pure copper will be transformed into one of brass by the use of a zinc anode; and a bath of brass will become one of copper by the aid of a copper anode.