If low spaces have been used, and the forme has not been floated prior to moulding, the work of the "builder" is greatly augmented. The removal of the wax spaces becomes a long and tedious job, and the risk of spoiling the mould is considerably increased. A sharp flat knife, slightly warmed, should be used for the purpose, and the pieces cut away uniformly. But it is never advisable to take a mould from a low-spaced forme (except by Clay's process), as floating takes less time than removing the wax from the spaces.
Wax being a non-conductor of electricity, it is necessary to cover the surface of the mould with some substance that is not only a conductor, but can be applied without distorting or damaging the wax. Graphite or "blacklead," as it is more commonly called, is not only an excellent conductor, but can also be easily and safely deposited on the surface of the mould.
The graphite must be perfectly pure, well ground, and free from grit. If it is not, the surfaces of the moulds are liable to be scratched or otherwise injured in the brushing. The best plan is to sift it through a very fine gauze sieve prior to using. No metallic deposit will take place except on the spots coated by the blacklead. Brushing the mould with blacklead prior to making the mould, somewhat assists the operation, but the pressure forces up to the surface a quantity of wax which must also be coated.
The conduct of the operation of black-leading has been already described on p. 210.
The blacklead is entirely removed from the surface, which should be gently polished by the goats'-hair brush, until it attains a bright metallic lustre.
The mould is next "stopped out," by brushing liquid wax on those portions of the frame and wax upon which no deposition is intended to take place. With a brush dipped in the liquid wax, thoroughly cover the back and sides, and that portion of the front that is not impressed. It is also necessary to exclude all the minute air-bubbles that are probably attached to portions of the face of the mould. If this were not done effectually, the shell would be ruined by minute perforations. It is performed by placing the mould face upwards in a pan, covering it with about 3 in. of water, and directing streams of water on to the mould through the rose. The operation also removes any remaining particles of graphite. It is a good plan to add a little alcohol to the water, as it has a great affinity for the wax, and in the action of rising to the surface displaces the air.
The mould is now ready for immersion in the battery trough.
A full account of the electro-depositing process will be found in 'Workshop Receipts,' First Series, pp. 170-235.
A substantial electrotype or shell should be obtained in 10-15 hours. To test the thickness of the shell, the mould may be lifted from the solution, and a corner of the copper slightly raised from the wax by a knife. The usual thickness is about 1/32 in. When properly backed, any number of impressions may be obtained from such a plate. A thin shell is more secure on its backing than a thick one; being light in texture, it is better able to adapt itself to the metal. If, when the mould is taken out, the deposit is found not to be sufficiently thick, rinse it well in water and replace it till done; then wash it in cold water, lay it on the inclined board near the sink, and pour hot water on the back, when the copper will immediately disengage from the wax. If, on holding the plate up to the light, many holes are seen, the defect may generally be attributed to faulty blackleading, and a new electro will have to be taken; if there are but few, the plate may be repaired by the picker when finishing.
A number of shells may be backed at one time, and sufficient metal should be in readiness. As the backing metal and the copper shell have no affinity for each other, it is necessary to employ a medium to unite them. Granulated tin, mixed with lead, is generally employed. Take equal parts of lead and tin, and when in a molten state pour the metal through a fine gauze net, allowing it to fall into a pail of water.
The backing-metal is made by melting together 91 parts lead, 5 of antimony, and 4 of tin. Less antimony is used than for stereotype plates, because it has a great affinity for tin, and has a tendency, if in too great a proportion, to take it up from the shell. The "peeling" of the shells is sometimes owing as much to this cause as to an insufficiency of granulated tin. Backing-metal can be bought ready made.
The backing-pans are allowed to rest on the surface of the metal pot until they are hot, before placing the shells in position. Meantime the latter are arranged on the iron surface, and any tendency to curl at the sides or ends may be cured by laying stereo-clumps on the edges. With a stiff brush cover the back with muriatic acid, and sprinkle so much granulated tin as will, when melted, cover the entire surface. To muriatic acid used for soldering must be added 1/3 water, and as much zinc (amalgamated) as it will take up; also a little borax or sal-ammoniac.
Lowering the shells in the backing-pan on to the surface of the metal, the tin soon melts, covering the whole of the shell with an even film. Wherever it fails to touch, a piece of solder must be applied, as the metal will not adhere in the bare places. The copper must not be allowed to become superheated, or it will oxidise.
The shell being properly covered with tin, skim the dross from the surface of the metal, and test the latter as described on p. 220. Pour metal on the shells, commencing at one corner, and gradually advancing over the entire surface, until the necessary thickness is attained. Set the pan aside to cool, when the plates can be taken out and placed in racks ready for finishing. Any small pieces of metal found between the lines, or between the hollows of the letters, may be easily removed with a bodkin.