A brick is a block made of kiln-fired material, usually clay or ground shale. Clay bricks are formed in a moulding (the soft mud method), or in commercial manufacture more frequently by extruding clay through a die and then wire-cutting them to the proper size (the stiff mud process). Brick made from clay that is hardly more than dampened must be formed in molds with a great deal of pressure, usually applied by a hydraulic press. These bricks are known as hydraulic-pressed bricks, and have a dense surface which makes them suitable for facing work. The shaped clay is then dried and fired to achieve the final, desired strength. Usually this is done in a continuously fired kiln, in which the bricks move slowly through the kiln on conveyors, to achieve consistent physical characteristics for all bricks.
Construction and types
Hard-burned brick should be used for face work exposed to the weather, and soft brick for filling, foundations, and the like. The standard brick measures approximately 2.25 x 4 x 8 inches, and has a crushing strength of between 1000 and 3000 lb/in² (7 to 21 million pascals) depending on quality. A highly impervious and ornamental surface may be laid on brick either by salt glazing, in which salt is added during the burning process, or by the use of a "slip," which is a glaze material into which the bricks are dipped. Subsequent reheating in the kiln fuzes the slip into a glazed surface integral with the brick base.
Regardless of size, bricks are often times manufactured with the depth equal to half the length (assuming that the brick is laid horizontally). This allows for several convenient layouts which must necessarily interweave the bricks in any structure, often both at the corners and within the wall depth in order to ensure the greatest possible durability of the structure.
Bricks are typically used to build structures. At one time, it was popular to pave roads with bricks; this is coming back into use as a method of traffic calming.