om the die, corresponds
to the open space, between the plug and the edge of the aperture. The clay
is forced out in a continuous pipe, which is cut to the desired length by
a wire, which is so thin as to pass through the mass without altering the
shape of the pipe. The short lengths of pipe are dried in the air as
thoroughly as they can be, and are then burned in a kiln, similar to that
used for pottery.

*Materials.*--The range of earths which may be used in the manufacture of
tiles is considerable, though clay is the basis of all of them. The best
is, probably, the clay which is almost invariably found at the bottom of
muck beds, as this is finer and more compact than that which is dug from
dry land, and requires but little preparation. There is, also, a peculiar
clay, found in some localities, which is almost like quick-sand in its
nature, and which is excellent for tile-making,--requiring no freezing, or
washing to prepare it for the machine. As a general rule, any clay which
will make _good_ bricks will make tiles. When first taken from the ground,
these clays are not usually adhesive, but become so on being moistened and
kneaded.

It is especially important that no limestone _pebbles_ be mixed with the
clay, as the burning would change these to quicklime, which, in slaking,
would destroy the tiles. The presence of a limey earth, however, mixed
through the mass, is a positive advantage, as in this intimate admixture,
the lime forms, under the heat of the kiln, a chemical combination with
the other ingredients; and, as it melts more readily than some of them, it
hastens the burning and makes it more complete. What is known as _plastic
clay_, (one of the purest of the native clays,) is too strong for
tile-making, and must be "tempered," by having other substances mixed with
it, to give it a stiffer quality.

The clay which is best for brick-making, contains Silica, and Alumina in
about the following proportions:

Silica ... 55 to 75 per cent.

Alumina ... 35 to 25 per cent.

Variable quantities of other materials are usually found in connection
with the clay, in its native condition. The most common of these are the
following:--

Magnesia 1 to 5 per cent.--sometimes 20 to 30 per cent.

Lime 0 to 19 per cent.

Potash 0 to 5 per cent.

Oxyd of iron 0 to 19 per cent.

"These necessary elements give fusibility to earthenware, and, therefore,
allow its constituent substances to combine in