method, however, enables the brickmaker to make use of certain kinds of
clay-rock, or shale, that would be impracticable for plastic bricks; and
the weathering, tempering and "ageing" may be largely or entirely dispensed
with. The plant required is heavier and more costly, but the brickyard
becomes more compact, and the processes are simpler than with the "plastic"
method.

The drying of bricks, which was formerly done in the open, is now, in most
cases, conducted in a special shed heated by flues along which the heated
gases from the kilns pass on their way to the chimney. It is important that
the atmosphere of the drying-shed should be fairly dry, to which end
suitable means of ventilation must be arranged (by fans or otherwise). If
the atmosphere is too moist the surface of the brick remains damp for a
considerable time, and the moisture from the interior passes to the surface
as water, carrying with it the soluble salts, which are deposited on the
surface as the water slowly evaporates. This deposit produces the "scum"
already referred to. When the drying is done in a dry atmosphere the
surface quickly dries and hardens, and the moisture from the interior
passes to the surface as vapour, the soluble salts being left distributed
through the whole mass, and consequently no "scum" is produced. Plastic
bricks take much longer to dry than semi-plastic; they shrink more and have
a greater tendency to warp or twist.

The burning or firing of bricks is the most important factor in their
production; for their strength and durability depend very largely on the
character and degree of the firing to which they have been subjected. The
action of the heat brings about certain chemical decompositions and
re-combinations which entirely alter the physical character of the dry
clay. It is important, therefore, that the firing should be carefully
conducted and that it should be under proper control. For ordinary bricks
the firing atmosphere should be oxidizing, and the finishing temperature
should be adjusted to the nature of the clay, the object being to produce a
hard strong brick, of good shape, that will not be too porous and will
withstand the action of frost. The finishing temperature ranges from 900 deg.
C. to 1250 deg. C., the usual temperature being about 1050 deg. C. for ordinary
bricks. As before mentioned, lime-clays require a higher firing temperature
(usually about 1150 deg. C. to 1200 deg. C.) in order to bring the lime