Galvanoplasty or Electrotype, is the art of cold casting of metals by the agency of electricity. Its applications are extensive. It is used to multiply engravings and photographs; to cover the faces of types with harder metal; to deposit grad, silver, and alloys on other metals, etc. The process depends upon the fact that an electrical current passed through a metallic solution properly prepared, will cause a decomposition of the solution; the metal being deposited upon any conducting body attached to the negative pole (cathode) of a voltaic cell or battery. This is the pole attached to the zinc plate in all cases.
The term battery is properly applied to several voltaic cells united. Frequently, however, it is used to designate a single cell. The forms usually employed in practice are Smee's, Daniell's, and the nitric acid battery. In order to avoid confusion, the following points must be well understood. In all the batteries named, there are two plates and at exciting fluid. One of these plates is of zinc, which must be amalgamated by dipping it into weak sulphuric acid and rubbing the surface with mercury; or better still, immersing the whole plate in a bath of mercury. This must be repeated from time to time, when the battery is in use. This zinc plate is alone acted on by the exciting fluid. It is galled the positive plate. Attached to it is a binding screw, by which a wire may be connected with the plate. This screw, or the end of the attached wire, is called the pole or electrode. The name of the pole is opposite to that of the plate. The positive pole or anode being attached to the negative plate, and the negative pole or cathode to the positive (zinc) plate.
The Decomposing Cell
Usually the liquid to be decomposed (electrolyte) is kept in a separate vessel, and the current conveyed to it by wires. To the anode is usually attached a piece of metal of the same character as that to be deposited. This is gradually eaten away while the deposition is going on, on the cathode, and the solution thus kept of uniform strength. The current may be regulated by altering the distance between the poles. With the same battery power, the amount of electricity passing will be less as the distance of the poles in the electrolyte is greater. Too powerful a current must be avoided, as it renders the coating brittle and non-adherrent. It should not be strong enough to cause bubbles of gas to arise from the object. A large number of objects can be plated by one battery if they are suspended on copper rods, the ends of which are connected with the pole.
Consists of two plates of amalgamated zinc, separated by a piece of baked and varnished wood and between them a plate of silver having deposited on it by the electric current finely divided platinum; so as to roughen it and prevent the adhesion of hydrogen. The silver plate is fixed in the wood separating the zinc plates, to the zinc and to the silver plates are attached binding screws for the wires. The exciting fluid is dilute sulphuric acid; 1 part of acid to 20 of water, is strong enough. When more intensity is required several cells are joined by passing wires from the anode of one cell to the cathode of the next. This form of battery is generally preferred on account of its simplicity, constancy, and ease of management.
In delicate operations, as in copying engraved plates, where great constancy is required, this form of cell is employed. It consists of a plate of amalgamated zinc, one of copper, generally of cylindrical form separated by a cell of porous earthenware (a flower-pot with the hole closed by a cork, makes a very good porous cell). The plates and cell are enclosed in a glass or earthenware vessel; the zinc is excited by dilute sulphuric acid; the copper is kept immersed in saturated solution of sulphate of copper (blue-stone). The solution of copper is gradually decomposed; the copper being deposited in the copper plate. Hence there should always be a quantity of crystals of the sulphate at the bottom of the cell, and the solution should be stirred from time to time; or the crystals may be suspended in a basket near the top of the solution
Nitric Acid Batteries
When great intensity is required as in the deposition of copper on iron, and of certain alloys the decomposition of fused chlorides for the purpose of obtaining certain metals, these batteries are used. In all cases the positive plate is of amalgamated zinc excited by dilute sulphyric acid, which may be as strong as 1 in 10 with 1-10th of nitric acid. This is separated by a porous cell from the negative plate, which may be of platinum (Grove), carbon (Bunsen), or passive iron (Callan). The negative plate is immersed in strong nitric acid. Iron may be rendered passive by dipping it once or twice into strong nitric acid, and then washing with water and carefully drying.
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