e marls, which are found in connexion
with chalk or limestone formations only. These ordinary brick clays vary
considerably in composition, and many clays, as they are found in nature,
are unsuitable for brickmaking without the addition of some other kind of
clay or sand. The strongest brick clays, _i.e._ those possessing the
greatest plasticity and tensile strength, are usually those which contain
the highest percentage of the hydrated aluminium silicates, although the
exact relation of plasticity to chemical composition has not yet been
determined. This statement cannot be applied indiscriminately to all clays,
but may be taken as fairly applicable to clays of one general type (see
CLAY). All clays contain more or less free silica in the form of sand, and
usually a small percentage of undecomposed felspar. The most important
ingredient, after the clay-substance and the sand, is oxide of iron; for
the colour, and, to a less extent, the hardness and durability of the burnt
bricks depend on its presence. The amount of oxide of iron in these clays
varies from about 2 to 10%, and the colour of the bricks varies accordingly
from light buff to chocolate; although the colour developed by a given
percentage of oxide of iron is influenced by the other substances present
and also by the method of firing. A clay containing from 5 to 8% of oxide
of iron will, under ordinary conditions of firing, produce a red brick; but
if the clay contains 3 to 4% of alkalis, or the brick is fired too hard,
the colour will be darker and more purple. The actions of the alkalis and
of increased temperature are probably closely related, for in either case
the clay is brought nearer to its fusion point, and ferruginous clays
generally become darker in colour as they approach to fusion. Alumina acts
in the opposite direction, an excess of this compound tending to make the
colour lighter and brighter. It is impossible to give a typical composition
for such clays, as the percentages of the different constituents vary
through such wide ranges. The clay substance may vary from 15 to 80%, the
free silica or sand from 5 to 80%, the oxide of iron from 1 to 10%, the
carbonates of lime and magnesia together, from 1 to 5%, and the alkalis
from 1 to 4%. Organic matter is always present, and other impurities which
frequently occur are the sulphates of lime and magnesia, the chlorides and
nitrates of soda and potash, and iron-pyrites. The presence of organic
matter g