These were succeeded by iron crucibles, especially for melting
silver, and these have now been generally replaced by graphite
(plumbago) crucibles made of a mixture of clay and graphite.
Good graphite crucibles can be used many times in succession if
they are heated gradually each time, but they are usually discarded
after about fifteen or twenty meltings. At the Royal Mint gold is
melted in crucibles about 10 in. in height and 812 in. in diameter at
the widest part. The charge is from 1200 to 1300 oz. (37·3 to 40·5
kilograms) of metal. The furnace is 12 in. square and 2 ft. deep
from the fire-bars to the cover. An old crucible is cut off about 2 in.
from the bottom and the bottom piece is inverted and placed on the
fire-bars as a support for the crucible. The “muffle,” a graphite
cylinder 6 in. in height, is placed on the crucible to allow room for
long bars to be melted in the crucible and to prevent the surrounding
coke from falling into it. The flue, of about 5 in. square, communicates
with a stack 60 ft. high. In many mints the flues pass into
condensing chambers where volatilized gold and silver are recovered.
The crucible is at a red heat when the gold is charged in, the copper
being added last, and a graphite lid put on the crucible to check loss
by volatilization. The charge is completely melted in about half
an hour, and it is then thoroughly mixed by stirring with a graphite
rod. The crucible is then lifted out by circular tongs suspended
in such a way that two men can take part in the operation. The
contents are poured by hand into moulds which are contained side
by side in an iron carriage running on wheels, fig. 1, OP. The
molten gold, which is of a pale green colour, solidifies at once in the
iron moulds, and the bars can be taken out immediately. Bars
from which sovereigns are to be coined are 22 in. long, 158 in. wide
and 12 in. thick, and about seven such bars are cast from one pot.
The rough edges of the bars are removed by a circular revolving file,
and the hollow ends are cut off. Pieces are cut out for assay, and
the bars are then ready for rolling. The amount of gold melted in
an ordinary day’s work is two tons to two and a half tons, of the
value of £250,000 to £300,000 For silver larger crucibles are used,
containing about 5000 oz. troy (155 kilograms). They are heated
in circular furnaces 21 in. in diameter and lifted out with circular
tongs suspended from a travelling crane which is worked by electricity.
The crucible is placed in the pouring cradle, which has
been in use since 1816, and is shown in fig. 1. Here A is the iron
cover surrounding the furnaces, B is the revolving lid of a furnace,
and C is the flue, common to two furnaces and leading to the stack.
The handle D, acting through the gear wheels E, F, G and H, turns
the cogwheel K, which moves the curved rack of the cradle and tips
the crucible M. The molten metal is poured into the moulds N,
which are carried on wheels running on rails Q. The parts of the
range of moulds are brought tightly together and held in position
by the bars O and the screw P, and when one mould is filled the
carrier is moved forward on its rails by wheels worked by a handle
also shown in the figure. In some other mints still larger crucibles
are used, containing various amounts up to about 1000 kilograms
or over 30,000 oz. In foreign mints the molten metal is generally
transferred from the crucible to the moulds by dipping crucibles or
iron ladles covered with clay. Gas is used as fuel for the melting
furnaces at Philadelphia. It is cleaner than coke and is said to
save time and to reduce the loss of the precious metals. At Denver
and Ottawa the fuel used is “first distillate” oil, which is found
to be cheaper than either naphtha or gas. The oil is pumped from
buried tanks and warmed to about 90° F. before it reaches the
burners at the furnaces. At the Denver mint the crucibles are used
for from twelve to fifteen meltings with oil fuel, whereas they were
soon destroyed when gas was employed. A charge of 6000 oz. of gold
is melted in about an hour. The melting losses amount to about
0·2 per 1000 of gold and 0·6 per 1000 of silver in the Royal Mint.
The losses are caused by volatilization, by the absorption of metal
by the crucible, stirring rod, &c., and by occasional projection of
particles from the pot into the furnace. The ash-pit is lined with
iron plates to facilitate the recovery of metal accidentally spilt.
All crucibles and other materials which might contain precious
metal are ground up and washed in a pan, and the pannings together
with a selection from the floor sweepings are remelted. The residues
(the Mint “sweep”) are sold to refiners or ore-smelters.
Fig. 1.—Furnace Apparatus.
Rolling.—The cast bars are reduced to the thickness of the coin by repeated passages between rolls. These are cylinders of cast iron or steel from 6 in. to 15 in. in diameter set parallel to one another with a small interval between, and revolved by electric or steam power. They are divided into breaking-down and finishing rolls, the latter being of smaller diameter than the former. The power is usually transmitted through toothed wheels, each roll being driven independently in some cases, while sometimes power is applied to the lower roll only, the upper roll being coupled to it. The
