To overcome the tendency of a solid brick wall to transmit heat, cold, or dampness, hollow walls are often used. A given number of bricks, if built as a hollow wall, will make a more stable wall than a solid wall built from the same number; besides the gain of the air space. Nearly all building laws require become burned to varying degrees of hardness. "Arch brick" are those which, from being near fire, become burned to a great degree of hardness and are often warped, vitreous, brittle, and weak. "Red bricks" are those which are burned with an even and moderate temperature and compose the bulk of the kiln. Salmon, or soft bricks are those which are found at the top of the kiln and are usually under-burnt. They are good only for inside courses and for lining chimneys. Face bricks, in which general term are included pressed bricks, moulded bricks, etc., are made or re-pressed in a dry press machine. Face bricks are more expensive to use than common bricks and are generally used for facing exterior walls, for fireplaces, and for moulded work. "Enameled bricks" are bricks whose face is covered with a coat of enamel of selected color. The true enameled brick is a very expensive article, so that most of the bricks called by this name are glazed bricks, the difference being that the true enamel is fused into the clay and is opaque in itself, while the glaze is formed by coating the surface with a colored film and covering this with the transparent glazing. This film is apparent on chipping off the glazing, while the true enamel shows no line between the body of the brick and the surface.

Fig. 108. Diagonal Bond, (Plan.)

Fig. 108. Diagonal Bond, (Plan.)

Fig. 109. English Bond.

Fig. 109. English Bond.

Fig. 110. Flemish Bond.

Fig. 110. Flemish Bond.

An important consideration is the bonding of the two portions of a hollow wall so that each shall help sustain the other. The usual method has been to do this by means of withes or headers of brick extending across the air space, as in Fig. 112, but these permit the moisture to pass from the outer to the inner shell, and also allow the mortar dropped from the higher portions of the wall to collect and partially fill the space; a more effective method of bonding is by means of a metal tie, either of steel wire or of iron, and these should be made with a dip in the center to allow any moisture which may come from the outer wall to drop off and not communicate with the inner wall. (Fig. 113.) These ties should be either galvanized or dipped in hot asphalt. It is a good plan to provide for a circulation of air through the space between the walls, by leaving openings in the basement and in the attic where possible. Hollow bricks are sometimes used for the inside course of exterior walls, but, while they are partially effective in excluding moisture, they do not fill the place of a hollow wall Common Bricks. We have made free mention of "common bricks," "face bricks," etc. Let us pause to consider just what is meant by the terms. Common bricks include all rough impressed bricks which have had no special care taken in their manufacture. These, according to their position in the kiln.

Fig. 111. Joint of New and Old Work.

Fig. 111. Joint of New and Old Work.

Fig. 112. Hollow Wall.

Fig. 112. Hollow Wall.

Fig. 113. Metal Wall Ties.

Fig. 113. Metal Wall Ties.

Both glazed and enameled bricks are impervious to moisture and are excellent for the facing of halls, courts, or wherever a light, clean and waterproof surface is desired. In addition to these, special bricks are made for special purposes, as fire bricks for furnace linings, etc., which are open-grained or porous to admit of a rapid loss of heat, and paving bricks which are burned to vitrification to withstand the wear of travel and the action of frost.

Sizes

The sizes of bricks vary with the maker and with local customs. Common bricks in New England average about 2 1/4 X 3 3/4 X 7 3/4 inches; in the Western States about 2 1/2 X 4 1/8 X 8 1/2 inches. Face bricks are more uniform in size and average about 2 3/8 X 4 1/8 X 8 3/8 inches. Pressed bricks are also made in a thinner pattern 1 1/2 X 4 X 12 inches. This style is known as the Roman brick.