ularly important where bricks are made from tempered clay,
_i.e._ clay in the wet or plastic state; where bricks are made from shale,
in the semi-plastic condition, weathering is still of importance.

The lime clays or "marls" of class (2), which contain essentially a high
percentage of chalk or limestone, are not so widely distributed as the
ordinary brick-clays, and in England the natural deposits of these clays
have been largely exhausted. A very fine chalk-clay, or "malm" as it was
locally called, was formerly obtained from the alluvium in the vicinity of
London; but the available supply of this has been used up, and at the
present time an artificial "malm" is prepared by mixing an ordinary
brick-clay with ground chalk. For the best London facing-bricks the clay
and chalk are mixed in water. The chalk is ground on grinding-pans, and the
clay is mixed with water and worked about until the mixture has the
consistence of cream. The mixture of these "pulps" is run through a grating
or coarse sieve on to a drying-kiln or "bed," where it is allowed to stand
until stiff enough to walk on. A layer of fine ashes is then spread over
the clay, and the mass is turned over and mixed by spade, and tempered by
the addition of water. In other districts, where clays containing limestone
are used, the marl is mixed with water on a wash-pan and the resulting
creamy fluid passed through coarse sieves on to a drying-bed. If necessary,
coarse sand is added to the clay in the wash-pan, and such addition is
often advisable because the washed clays are generally very fine in grain.
Another method of treating these marls, when they are in the plastic
condition, is to squeeze them by machinery through iron gratings, which
arrest and remove the pebbles. In other cases the marl is passed through a
grinding-mill having a solid bottom and heavy iron rollers, by which means
the limestone pebbles are crushed sufficiently and mixed through the whole
mass. The removal of limestone pebbles from the clay is of great
importance, as during the firing they would be converted into quicklime,
which has a tendency to shatter the brick on exposure to the weather. As
before stated, these marls (which usually contain from 15 to 30% of calcium
carbonate) burn to a yellow colour which is quite distinctive, although in
some cases, where the percentage of limestone is very high, over 40%, the
colour is grey or a very pale buff. The action of lime in bleaching the
fer