such a manner as to form a
resisting body; and thus is performed with a temperature lower in
proportion as the necessary elements are more abundant."(23)

When the earth of the locality where tiles are to be made is not
sufficiently strong for the purpose, and plastic clay can be cheaply
obtained from a distance, a small quantity of this may be used to give
strength and tenacity to the native material.

The compound must always contain a proper proportion of clay and sand. If
too little _clay_ is used, the mass will not be sufficiently tough to
retain its compactness as it passes through the die of the tile machine;
if too little _sand_, the moulded tiles will not be strong enough to bear
handling, and they will crack and warp in drying and burning. Within the
proper limits, the richer earths may be moulded much thinner, and tiles
made from them may, consequently, be made lighter for transportation,
without being too weak. The best materials for tempering stiff clays are
sand, pounded brick or tile, or _scoria_, from smelting furnaces.

*Preparation Of Earths.*--The clay from which tiles are to be made, should
be thrown out in the fall, (the upper and lower parts of the beds being
well mixed in the operation,) and made into heaps on the surface, not more
than about 3 feet square and 3 feet high. In this form, it is left exposed
to the freezing and thawing of winter, which will aid very much in
modifying its character,--making it less lumpy and more easily workable.
Any stones which may appear in the digging, should, of course, be removed,
and most earths will be improved by being passed through a pair of heavy
iron rollers, before they are piled up for the winter. The rollers should
be made of cast iron, about 15 inches in diameter, and 30 inches long, and
set as close together as they can be, and still be revolved by the power
of two horses. The grinding, by means of these rollers, may add 50 cents
per thousand to the cost of the tiles, but it will greatly improve their
quality.

In the spring, the clay should be prepared for tempering, by the removal
of such pebbles as it may still contain. The best way to do this is by
"washing," though, if there be only a few coarse pebbles, they may be
removed by building the clay into a solid cone 2 or 3 feet high, and then
paring it off into thin slices with a long knife having a handle at each
end. This paring will discover any pebbles larger than a pea that may have
remaine