ring and working of the clay are effected by the use of machinery,
especially when the harder clays and shales are used. The machines used in
the preparation of such clays are grinding-mills and pug-mills. The
grinding-mills are either a series of rollers with graduated spaces
between, through which the clay or shale is passed, or are of the ordinary
"mortar pan" type, having a solid or perforated iron bottom on which the
clay or shale is crushed by heavy rollers. Shales are sometimes passed
through a grinding-mill before they are exposed to the action of the
weather, as the disintegration of the hard lumps of shale greatly
accelerates the "weathering." In the case of ordinary brick-clay, in the
plastic condition, grinding-mills are only used when pebbles more than a
quarter of an inch in diameter are present, as otherwise the clay may be
passed directly through the pug-mill, a process which may be repeated if
necessary. The pug-mill consists of a box or trough having a feed hole at
one end and a delivery hole or nose at the other end, and provided with a
central shaft which carries knives and cutters so arranged that when the
shaft revolves they cut and knead the clay, and at the same time force it
towards and through the delivery nose. The cross section of this nose of
the pug-mill is approximately the same as that of the required brick (9 in.
x 41/2 in. plus contraction, for ordinary bricks), so that the pug delivers a
solid or continuous mass of clay from which bricks may be made by merely
making a series of square cuts at the proper distances apart. In practice,
the clay is pushed from the pug along a smooth iron plate, which is
provided with a wire cutting frame having a number of tightly stretched
wires placed at certain distances apart, arranged so that they can be
brought down upon, and through, the clay, and so many bricks cut off at
intervals. The frame is sometimes in the form of a skeleton cylinder, the
wires being arranged radially (or the wires may be replaced by metal
disks); but in all cases bricks thus made are known as "wire-cuts." In
order to obtain a better-shaped and more compact brick, these wire-cuts may
be placed under a brick press and there squeezed into iron moulds under
great pressure. These two processes are now generally performed by one
machine, consisting of pug-mill and brick press combined. The pug delivers
the clay, downwards, into the mould; the proper amount of clay is cut off;
and the