amount of skill that can only be acquired by long prac tice. It is very essential, in order to avoid the chiselling ("routing," or deepening of open spaces) that would other wise be necessary to perform upon the finished electrotype, for, unless these open spaces are considerably lower than the spaces between the fine lines of the subject they are apt to smut in the printing process. To cut these out with the chisel, or routing machine, from the finished electrotype would be a difficult and dangerous operation: difficult, because of the comparative hardness of the copper surface; and dangerous, because the breaking of the continuity of the copper surface will be liable to cause it to curl up on the edge of the cut, and to gradually destroy its attachment to the stereotype metal with which it is backed up. The proper building up, therefore, of a ridge of wax upon such portions of the mould as require to be depressed in the finished electrotype, avoids the necessity of chiselling with its attendant objections.

The great difficulty in this operation is to prevent the wax from running where it is not wanted, and to perform the work properly requires a quick eye, a steady hand, and long practice. The wax used is cut into strips six or eight inches long, and about half an inch thick. The wax must be perfectly free from moisture to prevent from spattering over the mould, and the building iron must not be too hot.

it

Black-Leading.-When the wax mould has been properly built up it is thoroughly black-leaded. This is necessary in order to give it a conducting surface, upon which the galvanic deposition of the copper may take place. It is hardly necessary to add that this dusting must be very thorough, so that the black-lead shall pene trate into every line and letter of the mould, otherwise the copper deposited on the surface will be an imperfect,

copy of the original, and it will be useless to place the mould in the bath.

It should be noticed here, that the black-lead used in every stage of the electrotyping process must be of the purest description, and in the most minute state of division. An inferior material will injuriously affect the electrical conductivity of the surface of the mould, and the presence of any gritty particles will scratch and

deface it. The manufacturers of this material, therefore, prepare electrotypers' stock with special care. The best for this purpose is prepared from the purest selected Ceylon graphite, which is ground by rolling with heavy iron

balls until it is reduced to a dead-black, infinitely fine powder. It should be separated by air-floating, or by floating in a bath of dilute sulphuric acid, which will dissolve out its mineral impurities.

The operation of black-leading the moulds is performed either by hand, or, more commonly, by machines. The preceding cut (Fig. 151) shows one of the most practical of these machines with its cover removed to exhibit its construction. It has a travelling carriage holding one or more forms, which passes backward and forward under a laterally vibrating brush. Beneath the machine is placed an apron, which catches the powder, which is made to do service over again. (Several modifications of the dry black-leading machine have been devised, and an excellent one is described in The Polytechnic Review, vol. p. 262, 1877.)

iv.

When the mould has been thoroughly dusted, it is removed from the machine, and the superfluous plumbago is blown out of the lines of the mould with a broadnosed bellows, or brushed out, leaving the mould covered with a film of graphite of infinitesimal thickness, but very lustrous. If any loose particles of the dust are permitted by carelessness to remain, the result will be a faulty electrotype.

On account of the dirt and dust caused by the dry process of black-leading, and which cannot be avoided, owing to the impalpably fine condition of the blacklead powder, some electrotypers prefer the wet process, invented by Mr. Silas P. Knight, chief of the electrotyping department of the Messrs. Harper & Brothers, New York. This process is designed to work more quickly and neatly, producing moulds that are thinly, evenly, and perfectly covered, not omitting the dot of an i, nor allowing "bridging" over fine lines. In Mr. Knight's process

the moulds are placed upon a shelf in a suitable receptacle, and a rotary pump forces an emulsion of graphite and water over their surfaces through a travelling finerose nozzle. This process is pronounced to be rapid, efficient, neat, and economical.

Wiring. After the above operation of black-leading has been properly performed, the workman takes one or several stout copper wires, the ends of which, after thorough cleansing, he heats for an instant, and imbeds in the wax on the side of the mould. The surface of this wire is carefully exposed, and, by way of precaution, the place is rubbed with black-lead with the finger, to restore the black-lead surface that may have been disturbed. Trifling as this circumstance of exposing the imbedded wire may appear, the galvanic deposit of the copper on the face of the mould would be impossible were it neglected, as the mass of wax being a non-conductor of electricity, a galvanic circuit could not otherwise be established. The exposure of the wire, therefore, is essential, in order that the surface of the mould may be rendered properly conductive to insure the uniform deposition of copper upon it. To confine the deposit of copper where it is actually desired, and to prevent it from unnecessarily spreading over the edges of the mould, the hot buildingiron is run over it so as to destroy the continuity of the black-lead surface save where the deposit of copper is wanted.

In order that the deposition of copper may be as nearly uniform in thickness as possible over the entire surface of the mould, it becomes necessary where a large surface is to be coated, to provide as much metallic surface as possible on which the deposit of copper may commence and spread. One method of accomplishing this is to attach one or more pieces of metal to the wax on the edges of

the mould, and connect them with the slinging wires by good metallic connections.

A very practical device in this connection is the "electric connection gripper" of Messrs. R. Hoe & Co., of New York, which is shown in Fig. 152.

This arrangement is designed to hold and sustain the

moulding case, and at the same time to make an electric connection with the prepared conducting face of the mould only; consequently leaving the metal case itself entirely out of the current, so that no copper can be deposited on it.

Metallizing the Moulds.-On account of the comparatively inferior conducting character of the black-lead surface of the mould, several methods have been devised and are in use for improving its conductivity by metallization.