Illustration: GENERAL FOUNDRY, WOOLWICH ARSENAL.
Founding is the art of casting metals into various forms by means of
moulds. The products of smelting are of a coarse kind, and have to be
remelted before the process of casting or founding begins. Before any
article can be cast in metal, a pattern must be formed in wood, clay,
or other suitable substance. The floor of the foundry is made up of
sand and powdered charcoal to the depth of several feet, serving to
imbed the moulds which are used, and in several places deep pits full
of the same material are formed for large castings; an iron frame,
corresponding to another like it, and capable of being united to it
by pins and sockets, is used to contain the moulding sand and pattern.
Illustration: GUN FOUNDRY, WOOLWICH ARSENAL.
Let it be supposed, for simplicity, that a cannon ball has to be
cast, one of the frames is filled with moulding sand moist enough
to bear a good impression, and a cannon ball pressed half-way in;
the surface is now dusted over with red ochre (to keep the upper
half from sticking to it), and the other frame applied and united
with the lower one, this is now filled with the sand and beaten or
trodden down firmly. On separating the two and removing the pattern,
there is an impression of half a ball in each half of the mould, and
when these are again put together there is a hollow corresponding
to the pattern used. There are, however, two things more to be
attended to, one is to have an opening for the melted metal to be
poured in, the other an opening for the air to escape, and this is
effected by attaching to the pattern two pieces of wood or iron which
project upwards through the upper half of the mould, and when this
is carefully lifted up two holes appear which on being united to the
lower half lead into the round hollow. When the moulds are ready they
are put into a room heated by means of stoves, and thoroughly dried.
They are then buried in the floor of the foundry, leaving the holes
for pouring the metal exposed, channels being formed in the sand,
so that when the furnace is “tapped” the melted metal may flow down
these and fill the moulds. Some forms are so complicated that the
moulds have to be made in several pieces, and the ingenuity of the
founder is taxed to the utmost to produce those required.