This liquid forms the moulding mixture, and it is allowed to flow round the object to be copied, contained in a suitable box, whose sides have been slightly oiled. The object to be copied should also be oiled. After some hours, when the glue mixture has set, it will be found to be highly elastic, so that it may be pulled away from the mould, and afterwards resume very nearly its original form.
One drawback to the use of these moulds lies in the fact that the gelatine will rarely stand the plating solution without undergoing change, but this may be partially obviated by dipping it for a few seconds in a 10 per cent solution of bichromate of potash, exposing it to the sunlight for a few minutes, and then rinsing it.
In order to render the surface conducting, it is washed over with a solution of a gold or silver salt, and the latter reduced in situ to metal by a suitable reagent. A solution of phosphorus is the most usual one (see Gore, Electro-metallurgy, p. 216). Such a mould may be copper-plated in the sulphate bath, connection being made by wires suitably thrust into the material.
Plaster of Paris moulds require to be dried and waxed by standing on a hot plate in melted wax before they are immersed in the plating bath. In this case the surface is best made conducting either by silvering it by the silvering process used for mirrors, or by brushing it over with good black lead rendered more conducting by moistening with an ethereal solution of chloride of gold and then drying in the sun.
The brushing requires a stiff camel's-hair pencil of large size cut so that the hairs project to a distance of about a quarter of an inch from the holder. The brushing must continue till the surface is bright, and is often a lengthy process.
The same process of blackleading may be employed to get a coat of deposited metal which will strip easily from the cathode.
In all cases where extensive deposits of copper are required, the growth takes place too rapidly at the corners. Consequently it is often desirable to localise the action of the deposit. A "stopping" of ordinary copal varnish seems to be the usual thing, but a thin coat of wax or paraffin or photographic (black) varnish does practically as well.
I do not propose to deal with the subject of electrotyping to any extent, for if practised as an art, a good many little precautions are required, as the student may read in Gore's Electro-metallurgy. The above instructions will be found sufficient for the occasional use of the process in the construction of apparatus, etc. There is no advantage in attempting to hurry the process, a current density of about ten ampères per square foot being quite suitable and sufficiently low to ensure a solid deposit.
[§ 143. Blacking Brass Surfaces. —]
A really uniform dead-black surface is difficult to produce on brass by chemical means. A paste of nitrate of copper and nitrate of silver heated on the brass is said to give a dead-black surface, but I have not succeeded in making it act uniformly. For optical purposes the best plan is to use a paint made up of "drop" black, ground very fine with a little shellac varnish, and diluted for use with alcohol. No more varnish than is necessary to cause the black to hold together should be employed.