From
the melting rooms through the corridor we reach the rolling
room. The upright engine, on the right, of one hundred and
sixty horse power, supplies the motive force to the rolling
machines, four in number. Those on the left, are massive and
substantial in their frame-work, with rollers of steel,
polished by service in reducing the ingots to planchets for
coining. The first process or rolling is termed breaking down;
after that it requires to be passed through the machine until
it is reduced to the required thinness--ten times if gold,
eight if silver, being annealed in the intervals to prevent
breaking. The rollers are adjustable and the space between
them can be increased or diminished at pleasure, by the
operator. About two hundred ingots are run through per hour on
each pair of rollers.
The
pressure applied is so intense that half a day's rolling
heats, not only the strips and rollers, but even the huge iron
stanchions, weighing several tons, so hot that you can hardly
hold your hand on them.
When
the rolling is completed the strip is about six feet long, or
six times as long as the ingot.
It
is impossible to roll perfectly true. At times there will be a
lump of hard gold, which will not be quite so much compressed
as the rest. If the planchets were cut from this place, it
would be heavier and more valuable than one cut from a thinner
portion of the strip. It is, therefore, necessary to
"draw" the strips, after being softened by
annealing.
Annealing
Furnaces
These
are in the same room, to the right facing the rollers. The
gold and strips are placed in copper canisters, and then
placed in the furnaces and heated to a red heat; silver strips
being laid loosely in the furnace. When they become soft and
pliable, they are taken out and allowed to cool slowly.
The
Drawing Benches
These
machines resemble long tables, with a bench on either side, at
one end of which is an iron box secured to the table. In this
are fastened two perpendicular steel cylinders, firmly
supported in a bed, to prevent their bending or turning
around, and presenting but a small portion of their
circumference to the strip. These are exactly at the same
distance apart that the thickness of the strip is required to
be. One end of the strip is somewhat thinner than the rest, to
allow it to pass easily between the cylinders. When through,
this end is put between the jaws of a powerful pair of tongs,
or pincers, fastened to a little carriage running on the
table. The carriage to the further bench is up close to the
cylinders, ready to receive a strip, which is inserted
edgewise. When the end is between the pincers, the operator
touches a foot pedal which closes the pincers firmly on the
strip, and pressing another pedal, forces down a strong hook
at the left end of the carriage, which catches in a link of
the moving chain. This draws the carriage away from the
cylinders, and the strip being connected with it has to
follow. It is drawn between the cylinders, which operating on
the thick part of the strip with greater power than upon the
thin, reduces the whole to an equal thickness. When the strip
is through, the strain on the tongs instantly ceases, which
allows a spring to open them and drop the strip. At the same
time another spring raises the hook and disengages the
carriage from the chain. A cord fastened to the carriage runs
back over the wheel near the head of the table, and then up to
a couple of combination weights on the wall beyond, which draw
the carriage back to the starting place, ready for another
strip.