s
produce a more evenly fired product than the intermittent kilns usually do,
and, of course, at much less cost for fuel. Gas firing is now being
extensively applied to continuous kilns, natural gas in some instances
being used in the United States of America; and the methods of construction
and of firing are carried out with greater care and intelligence, the prime
objects being economy of fuel and perfect control of firing. Pyrometers are
coming into use for the control of the firing temperature, with the result
that a constant and trustworthy product is turned put. The introduction of
machinery greatly helped the brickmaking industry in opening up new sources
of supply of raw material in the shales and hardened clays of the
sedimentary deposits of the older geologic formations, and, with the
extended use of continuous firing plants, it has led to the establishment
of large concerns where everything is co-ordinated for the production of
enormous quantities of bricks at a minimum cost. In the United Kingdom, and
still more in Germany and the United States of America, great improvements
have been made in machinery, firing-plant and organization, so that the
whole manufacture is now being conducted on more scientific lines, to the
great advantage of the industry.

_Blue Brick_ is a very strong vitreous brick of dark, slaty-blue colour,
used in engineering works where great strength or impermeability is
desirable. These bricks are made of clay containing front 7 to 10% of oxide
of iron, and their manufacture is carried out in the ordinary way until the
later stages of the firing process, when they are subjected to the strongly
reducing action of a smoky atmosphere, which is produced by throwing small
bituminous coal upon the fire-mouths and damping down the admission of air.
The smoke thus produced reduces the red ferric oxide to blue-green ferrous
oxide, or to metallic iron, which combines with the silica present to form
a fusible ferrous silicate. This fusible "slag" partly combines with the
other silicates present, and partly fills up the pores, and so produces a
vitreous impermeable layer varying in thickness according to the duration
and character of the smoking, the finishing temperature of the kiln and the
texture of the brick. Particles of carbon penetrate the surface during the
early stages of the smoking, and a small quantity of carbon probably enters
into combination, tending to produce a harder surface and darke