ric oxide and producing a yellow instead of a red brick, has not been
thoroughly investigated, but it seems probable that some compound is
produced, between the lime and the oxide of iron, or between these two
oxides and the free silica, entirely different from that produced by oxide
of iron in the absence of lime. Such marls require a harder fire than the
ordinary brick-clays in order to bring about the reaction between the lime
and the other ingredients. Magnesia may replace lime to some extent in such
marls, but the firing temperature must be higher when magnesia is present.
Marls usually contract very little, if at all, in the burning, and
generally produce a strong, square brick of fine texture and good colour.
When under-fired, marl bricks are very liable to disintegrate under the
action of the weather, and great care must be exercised in burning them at
a sufficiently high temperature.

_Brickmaking_.--Bricks made of tempered clay may be made by hand or by
machine, and the machines may be worked by hand or by mechanical power.
Bricks made of semi-plastic clay (_i.e._ ground clay or shale sufficiently
damp to adhere under pressure) are generally machine-made throughout. The
method of making bricks by hand is the same, with slight variation, the
world over. The tempered clay is pressed by hand into a wooden or metal
mould or four-sided case (without top or bottom) which is of the desired
shape and size, allowance being made for the shrinkage of the brick in
drying and firing. The moulder stands at the bench or table, dips the mould
in water, or water and then sand, to prevent the clay from sticking, takes
a rudely shaped piece of clay from an assistant, and dashes this into the
mould which rests on the moulding bench. He then presses the clay into the
corners of the mould with his fingers, scrapes off any surplus clay and
levels the top by means of a strip of wood called a "strike," and then
turns the brick out of the mould on to a board, to be carried away by
another assistant to the drying-ground. The mould may be placed on a
special piece of wood, called the stock-board, provided with an elevated
tongue of wood in the centre, which produces the hollow or "frog" in the
bottom of the brick.

Machine-made bricks may be divided into two kinds, plastic and
semi-plastic, although the same type of machine is often used for both
kinds.

The machine-made plastic bricks are made of tempered clay, but generally
the tempe