ELECTROPLATING AND ELECTRO TYPING Electroplating.—This process con sists in covering, by means of an elec tric current, the surface of articles composed of cheaper metals with more costly metals, as gold, silver, platinum, nickel, copper, etc. The process is the same as that employed in electrotyping.
German silver is perhaps the best metal to use as a basis in electropla ting; but iron, zinc, pewter, and other metals may also be plated. Articles to be plated must first be cleaned and scoured to free them from rust or other impurities, then dipped in a solution of nitrate of mercury to give them a thin film of mercury, which causes the plating to adhere. They are then suspended in a bath con taining a suitable solution, and treat ed by means of the electric current as in electrotyping. After being re moved, they are brushed and bur nished.
Electrotyping. — This is a process of making the cast or image of an object by gradually depositing metal from a solution by means of an elec tric current. Electroplating is done on the same principle.
Electrotyping or electroplating re quires very little apparatus, and can be done very cheaply. It is a prac tical, useful, and instructive art or amusement. The principle is very simple. If, for example, two strips of clean platinum are suspended in a solution of sulphate of copper, no ac tion takes place; but if these plati num plates be connected with the op posite poles of an electric battery the sulphate of copper solution will be decomposed, and the metallic copper will be deposited upon the plate or surface by which the electric current leaves the solution. This is the nega tive pole of the electric battery. By reversing the direction of the current the copper will be sent back to the other plate, and it can thus be sent backward and forward indefinitely if the current be continually reversed.
By adding more sulphate of cop per to the solution and continuing the electric current, a metallic film of copper may be made of any required thickness, which may afterwards be peeled off the platinum plate if de sired.
The best conditions for copperpla ting or electrotyping are a rather strong solution of sulphate of copper slightly acidulated with sulphuric acid and at a temperature of not less than 60°, with an electric current of low intensity. Gold and other metals may
be substituted for copper by suitable solutions, as described under electro plating.
If an object having raised surfaces, as a coin or medal, be substituted for the strip of platinum at the negative pole of the battery, it will be covered with metal to any desired depth. This may afterwards be stripped off, thus giving an exact inverted impression of the object, corresponding to a die.
If this die or a cast taken from the object by means of plaster of Paris or wax be substituted for the plati num plate at the negative pole of the battery, the depressions in its surface will be filled. Thus a solid casting or image of the object will be secured, and this is the usual method of elec trotyping.
Hence, to make a cast of an object by electrotyping, it is first necessary to make an impression of the object in wax or plaster of Paris. If plaster of Paris is used, the mold must be soaked in oil, tallow, or melted wax so that it will be impervious to water. The surface of the mold must then be brushed with a coating of black lead to make it a conductor of elec tricity. Or if a cast of the object itself is to be made, only half of its surface can be electrotyped at one time, and the remainder must be cov ered with sealing wax, varnish, or shellac.
Molds for Electrotyping.—To make a mold for electrotyping, lay the mod el on a flat surface and cover it with a coat of sweet oil or olive oil. Mix a little plaster of Paris with water to a thin paste and pour it over the model. Before the plaster of Paris sets, insert in it a wire ring from which the wire extends an inch or two to form a hook from which to suspend the mold in the solution. When the plaster sets, carefully re move the model and paint the back of the mold and the edges, except where it• is desired to have the metal deposited, with melted sealing wax, varnish, or shellac. Now brush black lead thoroughly into the mold so as to cover every portion of the surface and extend upward to , the wire on which the mold is to be suspended.