CLASSIFICATION OF BUILDING BRICK. Bricks are classified according to (1) the way in which they are moulded; (2) their posi tion in the kiln while being burned; and (3) their form or use.
Brick. One moulded by forcing a prism of stiff clay through a die and afterwards cutting it up into bricks.
Pressed Brick. One moulded by pressing dry or semi-dry clay into a mould.
Brick. Usually a stiff-mud brick which has been subjected to an enormous pressure to render the form more regular and to increase its strength and density. It is doubtful whether the re-pressing increases either the strength or the density. Occasionally in the East, and more formerly than at present, a soft-mud brick, after being partially dried, is repressed, which process greatly improves the form and also the strength and the density. A re pressed brick is sometimes, but inappropriately, called a pressed brick.
Slop Brick. In moulding brick by hand, the moulds are some times dipped into water just before being filled with clay, to pre vent the mud from sticking to them. Brick moulded by this process is known as slop brick. It is deficient in color, and has a compara tively smooth surface, with rounded edges and corners. This kind of brick is now seldom made.
Sanded Brick. Ordinarily, in making soft-mud brick, sand is sprinkled into the moulds to prevent the clay from sticking; the brick is then called sanded brick. The sand on the surface is of no serious advantage or disadvantage. In hand-moulding, when sand is used for this purpose, it is certain to become mixed with the clay and occurs in streaks in the finished brick, which is very undesirable; and owing to details of the process, which it is here unnecessary to explain, every third brick is especially bad.
Brick. Brick is frequently described as "machine made"; but this is very indefinite, since all grades and kinds are made by machinery.
Three grades of brick are taken from the old-style kiln: arch brick, body brick, and salmon brick.
Arch or Clinker Bricks. Those which form the tops and sides of the arches in which the fire is built. Being over-burned and partially vitrified, they are hard, brittle, and weak.
Body, Cherry, or Hard Bricks. Those taken from the interior of the pile. The best bricks in the kiln.
Salmon, Pale, or Soft Bricks. Those which form the exterior of the mass. Being underburned, they are too soft for ordinary work, unless it be for filling. The terms salmon and pale refer to the color of the brick, and hence are not applicable to a brick made of a clay that does not burn red. Although nearly all brick clays burn red, yet the localities where the contrary is true are sufficiently numerous to make it desirable to use a different term in designating the quality. There is, necessarily, no relation between color, and strength and density. Brick-makers naturally have a prejudice against the term soft brick, which doubtless explains the nearly universal prevalence of the less appropriate term—salmon.
Compass Brick. One having one edge shorter than the other. Used in lining circular shafts, etc.
Brick. One having one edge thinner than the other. Used in arches; and more properly, but less frequently, called voassoir brick.
Face Brick. Those which, owing to uniformity of size and color, are suitable for the face of the wall of buildings. Sometimes face bricks are simply the best ordinary brick; but generally the term is applied only to re-pressed or pressed brick made specially for this purpose.
Sewer Brick. Ordinary hard brick, smooth, and regular in form.
Paving Brick. Very hard, ordinary brick. A vitrified clay block, very much larger than ordinary brick (see § 83), is sometimes used for paving, and is called a paving brick, but more often, and more properly, a brick Vitrified Brick. The introduction of brick for street pavements about 1890 led to a new grade of building brick, viz., vitrified brick, one burned to the point of vitrifaction and then annealed or toughened by slowly cooling. Vitrified brick and paving blocks, though origi nally made for paving purposes, are now much used in building and engineering structures.