The following solution for depositing a black nickel coating on metal surfaces is given by the Brass World. The solution consists of the following constituents: water, 1 gallon; double nickel salts, 8 ounces; ammonium sulphocyanate 2 ounces; zinc sulphate, 1 ounce. If the zinc sulphate is not in the form of white crystals, but is white and dry, then only ½ ounce should be used. The double nickel salts are dissolved in the water, and then the ammonium sulphocyanate is added. After this has been done, the zinc sulphate is introduced. The solution is used at its ordinary temperature, but in winter should not be allowed to get colder than 60 degrees F., and works best at about 80 degrees F. Ordinary nickel anodes are employed, with a surface several times that of the work to be plated. The work is cleaned carefully, preparatory to the plating. The black nickel deposit may be put directly on steel, brass, copper, German silver, or bronze, but it is preferable to first flash the work in a hot copper solution, then in a white nickel solution, and finally deposit the black nickel. For cheap work, the copper and white nickel deposits may be dispensed with, but the black nickel is less apt to peel off if put on the white nickel. The black nickel is deposited with a weak current. Best results are obtained with a current from ½ to ¾ of a volt.
The deposition should be allowed to stand for an hour or more if a heavy deposit is desired. When the article comes from the black nickel solution, it will be found that it is of a gray or brown shade. While this disappears to a considerable extent when lacquered, the color is not a dead black. By using a dip consisting of 1 gallon of water, 12 ounces of iron per-chloride, and 1 ounce of muriatic acid, a dead black color is produced. All nickel deposits should be lacquered after dipping.
The following causes of difficulties should be guarded against: If the black nickel deposit has spear-shaped markings on it and is partly white, too high a voltage has been used. If the deposit flakes off after standing for some time, too strong a current has been used, or the work has not been clean. If the deposit is too heavy, it is also apt to flake off. If the deposit is still brown or gray after it comes from the dip, the dip is old, or the article has not remained in the dip long enough. If, although the voltage is right (less than one volt), the deposit is streaked, the bath has become acid; add carbonate of nickel (plastic) to neutralize the acid. Use plenty of anode surface and old nickel anodes if possible. If the edges of the deposit are removed in the dip, the dip is too hot, or the black nickel was not deposited a sufficiently long time. If the surface is iridescent after lacquering, the lacquer is too thin.
To make an imitation gun-metal finish by electrical process take ¾ pound of the double nickel salts to a gallon, and dissolve in boiling water. After the solution has cooled, add ammonia until it is slightly alkalized, then add sulphuret cyanide of potassium, about ½ ounce to a gallon. If a darker finish is required add more sulphuret. This will work excellently on all metals and they will come from the solution with a very high luster. If the work has been buffed and dipped before plating, it will require no further finishing, and should then be lacquered. It should be run with a very mild current from three to four minutes. J. L. Lucas.