into
chemical combination with the other substances present.

It is evident that the best method of firing bricks is to place them in
permanent kilns, but although such kilns were used by the Romans some 2000
years ago, the older method of firing in "clamps" is still employed in the
smaller brickfields, in every country where bricks are made. These clamps
are formed by arranging the unfired bricks in a series of rows or walls,
placed fairly closely together, so as to form a rectangular stack. A
certain number of channels, or firemouths, are formed in the bottom of the
clamp; and fine coal is spread in horizontal layers between the bricks
during the building up of the stack. Fires are kindled in the fire-mouths,
and the clamp is allowed to go on burning until the fuel is consumed
throughout. The clamp is then allowed to cool, after which it is taken
down, and the bricks sorted; those that are under-fired being built up
again in the next clamp for refiring. Sometimes the clamp takes the form of
a temporary kiln, the outside being built of burnt bricks which are
plastered over with clay, and the fire-mouths being larger and more
carefully formed. There are many other local modifications in the manner of
building up the clamps, all with the object of producing a large percentage
of well-fired bricks. Clamp-firing is slow, and also uneconomical, because
irregular and not sufficiently under control; and it is now only employed
where bricks are made on a small scale.

Brick-kilns are of many forms, but they can all be grouped under two main
types--Intermittent kilns and Continuous kilns. The intermittent kiln is
usually circular in plan, being in the form of a vertical cylinder with a
domed top. It consists of a single firing-chamber in which the unfired
bricks are placed, and in the walls of which are contrived a number of
fire-mouths where wood or coal is burned. In the older forms known as
_up-draught_ kilns, the products of combustion pass from the fire-mouth,
through flues, into the bottom of the firing-chamber, and thence directly
upwards and out at the top. The modern plan is to introduce the products of
combustion near the top, or crown, of the kiln, and to draw them downwards
through holes in the bottom which lead to flues connected with an
independent chimney. These _down-draught_ kilns have short chimneys or
"bags" built round the inside wall in connexion with the fire-mouths, which
conduct the flames to the upper