The term brick is usually applied to the product resulting from burning moulded prisms of clay in a kiln at a high temperature.

Common brick is not extensively used in engineering structures, except in the construction of sewers and the lining of tunnels. Brick is easily worked into structures of any desirable shape, easily handled or transported, and comparatively cheap. When well constructed,

Table I. Physical Properties Of Building Stones

(From Merrill's "Stone for Buildines and Decoration."')

brick masonry compares very well in strength with stone masonry, but is not so heavy as stone. Brickwork is but slightly affected by changes of temperature or humidity.

Brick is made of common clay (silicate of alumina), which usually contains compounds of lime, magnesia, and iron. Good brick clay is often found in a natural state. The quality of the brick depends greatly on the quality of the clay used, and equally as much on the care taken in its manufacture.

Oxide of iron gives brick hardness and strength. The red color of brick is also due to the presence of iron. The presence of carbonate of lime in the clay of which brick is made, is injurious, since the carbonate is decomposed during the burning, forming caustic potash, which, by the absorption of water, will cause the brick to disintegrate. An excess of silicate of lime makes the clay fusible, which softens the brick and thereby causes distortion during the burning process. Magnesia in small quantities has but little influence on brick. Sand, in quantities not in excess of about 25 per cent, will help to preserve the form of the brick, and is beneficial to that extent; but in greater quantities than 25 per cent, it makes the brick brittle and weak.

21. Requisites For Good Brick

Good brick should be of regular shape, with plane faces, parallel surfaces, and sharp edges and angles. It should show a fine, uniform, compact texture; should be hard, and, when struck a sharp blow, should ring clearly; and should not absorb more water than one-tenth of its weight. The specific gravity should be 2 or more. Good brick will bear a compressive load of 6,000 pounds per square inch when the sides are ground flat and pressed between plates. The modulus of rupture should be at least 800 pounds per square inch.

22. Absorptive Power

The amount of water that a brick absorbs is very important in indicating the durability of brick, particularly its resistance to frost. Very soft brick will absorb 25 to 30 per cent of their weight of water. Weak, light-red ones will absorb 20 to 25 per cent; this grade of brick is used commonly for filling interior walls. The best brick will absorb only 4 to 5 per cent, but brick that will absorb 10 per cent is called good.

23. Color Of Bricks

The color of brick depends greatly upon the ingredients of the clay; but the temperature of the burning, the moulding sand, and the amount of air admitted to the kiln also have their influence. Pure clay or clay mixed with chalk will produce white brick. Iron oxide and pure clay will produce a bright red brick when burned at a moderate heat. Magnesia will produce brown brick; and when it is mixed with iron, produces yellow brick. Lime and iron in small quantities produce a cream color; an increase of lime produces brown, and an increase of iron red.

24. Size And Weight

The standard size for common brick is 8 1/4 by 4 by 2 1/4 inches; and for face brick, 8f by 4 1/8 by 2 1/4 inches. There are numerous small variations from these figures; and also, since the shrinkage during burning is very considerable and not closely controlled, there is always some uncertainty and variation in the dimensions. Bricks will weigh from 100 to 150 pounds per cubic foot according to their density and hardness, the harder bricks being of course the heavier per unit of volume.