This process of burning passes through three main stages which
require very skillful attention on the part of the burner. First,
the water chemically combined with the material must be driven
off; then the various impurities of the clay must be burnt out or
oxidized; and finally, the ware, except in case of fire clays, must
be brought to the point of incipient vitrification. Throughout the
whole process there is danger of distortion or discoloration in
the ware unless the fires are skilfully handled. Properly done,
the brick come out of the kiln in their beautiful, natural colors,
due to the constitution of the clay or the various metallic oxides
contained in it. To enhance these effects, different clays are
sometimes mixed in going through the machines, certain ores may
be added to modify the color, the brick surfaces may be scored in
various ways, or the ware may be set in the kiln so as to avoid or
get the flash of the fire. So that when you specify a fine face
brick, you are getting a product which Nature has taken long to
create and to which man has devoted his best scientific knowledge
and inventive art.
[Illustration: The Philadelphia Carpenters used Brick Two Centuries
Ago]