The three ingredients thus prepared are ready for manufacturing into
gunpowder. They are, 1. Separately ground to a fine powder, which is
passed through sorted silk sieves or bolting machines; 2. They are mixed
together in the proper proportions, which we shall afterwards discuss;
3. The composition is then sent to the gunpowder mill, which consists
of two edge-stones of a calcareous kind, turning by means of a
horizontal shaft, on a bed-stone of the same nature; incapable of
affording sparks by collision with steel, as sand-stones would do. On
this bed-stone the composition is spread, and moistened with as small a
quantity of water as will, in conjunction with the weight of the
revolving stones, bring it into a proper body of cake, but by no means
into a pasty state. The line of contact of the rolling edge-stone is
constantly preceded by a hard copper scraper, which goes round with the
wheel, regularly collecting the caking mass, and bringing it into the
track of the stone. From 50 to 60 pounds of cake are usually worked at
one operation, under each millstone. When the mass has been thoroughly
kneaded and incorporated, it is sent to the corning-house, where a
separate mill is employed to form the cake into grains or corns. Here it
is first pressed into a hard firm mass, then broken into small lumps;
after which the corning process is performed, by placing these lumps in
sieves, on each of which is laid a disc or flat cake of lignum vitæ. The
sieves are made of parchment skins, or of copper, perforated with a
multitude of round holes. Several such sieves are fixed in a frame,
which, by proper machinery, has such a motion given to it as to make the
lignum vitæ runner in each sieve move about with considerable velocity,
so as to break down the lumps of the cake, and force its substance
through the holes, in grains of certain sizes. These granular particles
are afterwards separated from the finer dust by proper sieves and
_reels_.
The corned powder must now be hardened, and its rougher angles removed,
by causing it to revolve in a close reel or cask turning rapidly round
its axis. This vessel resembles somewhat a barrel-churn, and is
frequently furnished inside with square bars parallel to its axis, to
aid the polish by attrition.
The gunpowder is finally dried, which is now done generally with a steam
heat, or in some places by transmitting a current of air, previously
heated in another chamber, over canvas shelves, covered with the damp
grains.