By this procedure, the mould is provided with a surface of eminent conducting quality, by which the uniformity of the metallic deposit over its whole surface is insured. Without such preparation, the deposition of copper would commence at the suspension wires, and would gradually spread from these over the surface of the mould. Those portions nearest the wires would, therefore, receive the heaviest deposit, and those farthest therefrom, the thinnest. Gilt and silvered black-lead, prepared in the manner described on page 454, are occasionally used for this purpose. The coppering process of Mr. Silas P. Knight is, however, preferable, and is in general use for this purpose. The process in question effects in a few seconds the coating of the mould with a thin and uniformly distributed film of copper. It is performed in the following manner: After stopping out those portions of the mould that are not to receive a deposit, it is laid in a shallow trough, and a stream of water turned upon it from a rose jet to remove any particles of black-lead that may remain in the lines or letters. The workman then ladles out of a conveniently placed vessel some sulphate of copper solution, pours it upon the face of the mould, dusts upon it from a pepper-box some inpalpably fine iron-filings, and brushes the mixture over the whole surface, which thus becomes coated with a thin, bright, adherent film of copper. Should any portion of the surface, after such treatment, remain uncoppered, the operation is repeated. The excess of the copper solution is washed off, and the mould is then ready for the bath. The use of tinning in place of coppering * In experimenting with the Knight process, the writer accidentally made the observation that chemically pure iron, in the finest possible state of division (as prepared, for example, by the reduction of the oxide of iron by hydrogen) will not produce an adherent copper film has also been recommended, in accordance with the patented process of Adams. (For description, consult R. Hoe & Co.'s Catalogue, page 91.) Before the moulds are placed in the depositing vat, it is important to wash them thoroughly, which is done generally by directing upon their faces a strong stream of water from a rubber hose and pipe. When the moulds have been thoroughly washed out, they should be placed at once in the depositing vat, and connected. The depositing solution is the same as that previously described and used for obtaining thick electro-deposits (i. e., a saturated solution of sulphate of copper, acidulated with sulphuric acid, vide Chapter L.), and either the battery, or what is now commonly the case, the plating machine, is used to effect the deposit. The vat is provided with a series of large copper anodes, between which the cases containing the prepared moulds are suspended in the solution back to back, so that the moulds face the anodes. When the metallic connections are made, the mode of doing which has been repeatedly described in other portions of this work, the circuit is completed, and upon the mould. In a number of trials with such chemically pure iron, the copper was invariably reduced as a non-adherent powder. The most probable explanation of this curious fact appears to be, that the reducing action of the iron on the copper salt is so energetic, that the solution of the iron and the precipitation of the copper are instantaneous, no time being afforded the copper to come in contact, in the act of precipitation, with the black-leaded surface. Where, on the contrary, iron filings are used, as is the case in practice, the particles of iron, though so minute as not to injure the face of the mould, are enormously larger, comparatively speaking, than those of the reduced iron, their action on the copper salt is much slower, and, as the precipitation of the copper is slowly taking place on the iron particles, as they are moved by the brush to and fro over the black-leaded surface, enough of it is transferred and adheres to the latter surface to give it a continuous metallic coating. the copper at once commences to be deposited on the moulds. The time required to obtain a shell of copper of proper thickness will vary according to the thickness desired and the strength of the battery current. For ordinary book or job work, the copper shell should be about as thick as good book paper, which will require an immersion of from three to five hours. For newspaper headings, and such other blocks as will be subjected to hard usage, a much thicker shell is made, requiring a correspondingly longer immersion in the bath. To give an idea of the rate at which this class of work may be done, we may state that the Smee battery (zincsilver couple with sulphuric acid as an excitant), of 26 pairs, 12 12 inches, will copper from 6 to 6 square feet of prepared moulds in four hours. In this branch of galvanoplastic work, as in electroplating, the battery has, however, at least in the United States, been largely superseded by the dynamo-electric machine, and with the result of greatly increasing the capacity of the establishments, and of dispensing with the trouble and care which even the simplest batteries demand. As every such establishment requires to use steam-power in some portion of its work, and generally has at command more power than is actually required, the taking off of one, two, or even three horsepower additional for running a plating machine, in some cases, makes no appreciable difference in the expense account. A comparison of the relative economy of battery- and machine-plating is given in our chapter on the dynamo-electric machine. The Shell. When the mould has received a suitable deposit it is removed from the bath, and the copper shell removed from the wax. To do this the mould is placed in an inclined position, and a stream of hot water is poured over the copper surface. This artifice softens the wax sufficiently to allow the shell of copper to be stripped off. This may be done by taking hold of one corner of the shell, and quickly lifting it as the hot water flows over it. In removing the shell, care should be taken to keep it straight, as otherwise it will be difficult to back and finish it properly. The film of wax which always adheres to the copper, the quantity of which depends on the skill of the operator, is removed by laying the shell upon a wire-screen, and pouring a hot lye of potash over its surface from a ladle, until the adhering particles of wax have been dissolved, allowing the potash to run back again into the pot from which it was dipped. The shell is then thoroughly rinsed off in cold water. The tinning of the back of the shell is the next operation, and has for its object to strengthen the union between the shell and the backing metal. For this purpose the back of the shell is cleansed by brushing with "soldering fluid" (made by allowing muriatic acid to take up as much zinc as it will dissolve and diluting with about of water, to which sometimes some sal-ammoniac is added). Then, the shell, face down, is heated by laying it upon an iron soldering plate, floated on a bath of melted stereotype metal, and, when hot enough, melted solder (half lead and half tin) is poured over the back, which gives it a clean, bright metallic covering. Or, the shell, is placed downward in the backing pan, brushed over the back with the soldering fluid, alloyed-tinfoil spread over it, and the pan floated on the hot backing metal until the foil melts and completely covers the shell. When the foil is melted the backing pan is swung on to a levelling stand, and the melted backing metal is carefully poured on the back of the shell from an iron ladle, com mencing at one of the corners, and gradually running over the surface until it is covered with a backing of sufficient thickness. The accompanying cut (Fig. 153) represents Fig. 153. YORK a convenient form of backing pan and stand for this purpose. Or, after tinning the shell, as above described, it is allowed to take the temperature of the backing metal on the floating iron plate. The plate is then removed from the molten metal, supported in a level position on a table having projecting iron pins on which it is rested, and the molten stereotype metal is ladled carefully to the proper thickness on the back of the tinned shell. process is called "backing." The thickness of the metal-backing is about an eighth of an inch, enough to give the necessary rigidity to the shell to enable it to with This |