ives the wet clay a greater plasticity, probably because it forms a
kind of mucilage which adds a certain viscosity and adhesiveness to the
natural plasticity of the clay. In some of the coal-measure shales the
amount of organic matter is very considerable, and may render the clay
useless for brickmaking. The other impurities, all of which, except the
pyrites, are soluble in water, are undesirable, as they give rise to
"scum," which produces patchy colour and pitted faces on the bricks. The
commonest soluble impurity is calcium sulphate, which produces a whitish
scum on the face of the brick in drying, and as the scum becomes
permanently fixed in burning, such bricks are of little use except for
common work. This question of "scumming" is very important to the maker of
high-class facing and moulded bricks, and where a clay containing calcium
sulphate must be used, a certain percentage of barium carbonate is nowadays
added to the wet clay. By this means the calcium sulphate is converted into
calcium carbonate which is insoluble in water, so that it remains
distributed throughout the mass of the brick instead of being deposited on
the surface. The presence of magnesium salts is also very objectionable, as
these generally remain in the burnt brick as magnesium sulphate, which
gives rise to an efflorescence of fine white crystals after the bricks are
built into position. Clays which are strong or plastic are known as "fat"
clays, and they always contain a high percentage of true "clay substance,"
and, consequently, a low percentage of sand. Such clays take up a
considerable amount of water in "tempering"; they dry slowly, shrink
greatly, and so become liable to lose their shape and develop cracks in
drying and firing. "Fat" clays are greatly improved by the addition of
coarse sharp sand, [v.04 p.0519] which reduces the time of drying and the
shrinkage, and makes the brick more rigid during the firing. Coarse sand,
unlike clay-substance, is practically unaffected during the drying and
firing, and is a desirable if not a necessary ingredient of all brick
clays. The best brick-clays feel gritty between the fingers; they should,
of course, be free from pebbles, sufficiently plastic to be moulded into
shape and strong enough when dry to be safely handled. All clays are
greatly improved by being turned over and exposed to the weather, or by
standing for some months in a wet condition. This "weathering" and "ageing"
of clay is partic