Ordinary building bricks are made of a mixture of clay and sand (to which coal and other foreign substances are sometimes added), which is subjected to various processes, differing according to the nature of the material, the method of manufacture and the character of the finished product.

After being properly prepared the clay is formed in moulds to the desired shape, then dried and burnt.

The Clay. - The quality of a brick depends principally upon the kind of clay used. The material generally employed for making common bricks consists of a sandy clay, or silicate of alumina, usually containing small quantities of lime magnesia and iron oxide. If the clay consists almost entirely of alumina it will be very plastic, but will shrink and crack in drying, warp and become very hard under the influence of heat.

Silica, when added to pure clay in the form of sand, prevents cracking, shrinking and warping, and allows a partial vitrification of the materials. The larger the proportion of sand present the more shapely and uniform in texture will be the bricks. An excess of sand, however, renders the bricks too brittle and destroys cohesion. Twenty-five per cent, of silica is said to be a good proportion.

The presence of oxide of iron in the clay renders the silica and alumina fusible and adds greatly to the hardness and strength of the bricks. Iron also has a great influence upon the color of the bricks (see Section 229), the red color being due to the presence of iron. A clay which burns to a red color will make a stronger brick, as a rule, than one whose natural color when burnt is white or yellow.

"Lime has a twofold effect upon the clay containing it. It diminishes the contraction of the raw bricks in drying, and it acts as a flux in burning, causing the grains of silica to melt, and thus binding the particles of the bricks together. An excess of lime causes the bricks to melt and lose their shape. Again, whatever lime is present must be in a very divided state. Lumps of limestone are fatal to a clay for brickmaking. When a brick containing a lump of limestone is burnt the carbonic acid is driven off, the lump is formed into ' quicklime ' and is liable to slake directly the brick is wetted or exposed to the weather. Pieces of quicklime not larger than pin heads have been known to detach portions of a brick and to split it to pieces. The presence of lime may be detected by treating the clay with a little dilute sulphuric acid. If there is lime present an effervescence will take place."

For the best qualities of pressed brick the clay is carefully selected both for chemical composition and color, and very often two or three qualities of clay from different sources are mixed together to obtain the desired composition.

Clays of especially fine quality are often mixed and shipped to distant portions of the country the same as other raw materials.

2l6. Manufacture. - Handmade Bricks. - Most of the common bricks used in this country, especially in the smaller towns and cities, are still made by hand. The process consists of throwing the clay into a circular pit, where it is mixed with water and tempered with a tempering wheel worked by horse power, until it becomes soft and plastic, and is then taken out and pressed into the moulds by hand.

Unless the clay already contains sufficient sand, additional sand is; added to it as it is put into the pit, and often coal dust or sawdust is added to assist the burning. In some localities screened cinders are mixed with the clay.

In moulding brick by hand the mould is dipped either in water or fine sand to prevent the bricks from adhering to the mould. If dipped in water the process is called "slop moulding," and if in sand the bricks are called "sand struck." The latter method gives cleaner and sharper bricks than those produced by "slop moulding."

After being shaped in the mould the bricks are laid in the sun, or in a dry house, to dry for three or four days, after which they are stacked in kilns and fired.

When the green bricks are dried in the open air they occasionally get caught in a shower, which gives them a pitted effect, that is generally considered undesirable. Unless the edges are much rounded, however, it does not affect the strength of the bricks, and they may be used in the interior of the wall.