The moulds or models intended to receive the deposited metals may be
formed of various materials. For medals and similar small works, moulds of
fusible metal, white wax, stearine, stearic acid, and gutta percha, are
commonly used. The first are formed by dropping or pressing the medals to
be copied upon the melted metal, taking care that the former are quite
cold, and that the surface of the metal is bright or free from oxide. To
make a mould in gutta percha, the material must be softened in warm water,
and then pressed upon the medal by means of a strong screw press. With the
other materials the manipulation is very easy. A ribbon of cardboard or
thick paper is placed round the medal, so as to form a rim; the material,
which has been melted in an earthen vessel, is then poured on, and allowed
to remain until quite cold and hard, when it is cautiously removed. For
large works, moulds of plaster of Paris are usually employed; these
require to be saturated with wax or tallow, by standing them in a shallow
vessel containing these substances in a melted state. For copying seals
and small coins, impressions in ordinary sealing-wax may be used as
electrotype moulds. Non-metallic moulds must be coated with some substance
which has the property of conducting electricity before they can be used
as negative electrodes. The substance commonly employed is plumbago or
black-lead. It must be in the condition of an impalpable powder. It is
rubbed briskly over the surface of the mould (wax, stearine, plaster, &c.)
by means of a strong fine camel-hair brush, till the whole presents the
well-known black-lead polish. The adhesion of the plumbago may be often
promoted by breathing slightly on the mould. To cause it to adhere to
sealing wax impressions, the wax may be slightly moistened with spirits of
wine, or exposed to the vapour of ether. Delicate moulds and objects,
which cannot well be black-leaded, may be covered with a conducting film
of silver, by first dipping them in bisulphuret of carbon holding about
1/20th part of phosphorus in solution, and then, after a few seconds,
immersing them in a weak solution of nitrate of silver, and allowing them
to dry in the light. Metallic moulds require no preparation.
The voltaic apparatus used may now be described. The single-cell
arrangement, used for small works, is formed on the principle of Daniell's
Constant Battery. It consists of a vessel of glass, earthenware, or wood,
containing a smaller cell of thin biscuit ware, or other porous material;
a rod or plate of amalgamated zinc, placed within the porous cell, and a
wire connecting the zinc with the mould to be copied; the latter being
placed in the outer vessel. The annexed figure represents a convenient
form of the single-cell:--
The battery arrangement has many advantages over that described above, and
should always be employed when large objects are to be electrotyped, or
when a number of small moulds are to be operated upon. In this arrangement
the copper solution is electrolysed in a separate vessel, termed the
decomposition cell, and the current generated by one or more cells of a
Daniell's or Smee's battery. This arrangement is shown in the following
engraving:--
[Illustration:
_a._ An oval vessel of salt-glazed earthenware or wood nearly
filled with a saturated solution of sulphate of copper.
_b._ A porous diaphragm, containing the cylinder or plate of zinc
(_c_), and filled with dilute sulphuric acid.
_d._ A small bar of brass or copper fastened to the vessel by the
binding screws (_e, e_), and supporting the plate of zinc
(_c_), by the hook of copper wire (_f_), and the mould (_g_),
by the hook (_h_).
_i._ A small shelf or partition to support crystals of sulphate of
copper, to keep up the strength of the solution.]
[Illustration:
_a._ A constant battery cell.
_b._ Decomposition cell, a cubical vessel made of wood or
earthenware, and filled with a mixture of 1 part of dilute
sulphuric acid (1 acid + 9 water), and 2 parts of saturated
solution of sulphate of copper by measure.
_c, c, c._ Moulds suspended to the brass rod (_f_), and connected
with the zinc or positive element of the battery (_a_), by
means of the screw (_g_.)
_d, d._ Pieces of sheet copper suspended on the brass rod (_h_),
and connected with the zinc end of the battery, by means of
the screw (_i_), employed to keep up the strength of the
cupreous solution in the decomposition cell.]
To connect the moulds with the zinc or positive element, stout copper
wires or strips of thin sheet copper are employed. In the case of a
non-metallic mould, the wire must lead directly to the plumbagoed surface,
or, what amounts to the same thing, the plumbago must be extended to the
point of attachment. The connecting wires, and the backs and edges of
metallic moulds, must be covered with sealing-wax varnish, or other
non-conducting substance, to prevent them receiving the deposit. Before a
mould is placed in the copper solution it is advisable that everything
should be arranged, so that the immersion may occasion immediate voltaic
action. If the connection between the zinc and the mould is not effected
until after the immersion, the solution may act chemically on the surface
of the mould, and cause the deposit to appear dark and dirty. When a mould
has remained in the solution long enough to receive a complete coating of
copper, it may be lifted out with impunity for examination. If everything
is going on well, the deposited metal will present a brilliant, light,
copper-coloured surface. When sufficiently thick, the deposit is removed
with care, washed and placed to dry. Electrotype medals may be polished
with wash-leather and the plate brush, or bronzed. Various natural objects
such as insects, fruits, &c.; small works of art, such as busts and
statuettes; chemical vessels, particularly glass flasks and retorts; and
numerous classes of articles, may be rendered less fragile by coating them
with copper by the electrotype process.