MANUFACTURE OF BRICK.
The manufacture of brick may be classified under the following heads: Excavation of the clay, either by manual or mechanical power.
Preparation of the clay consists in (a) removing stones and me chanical impurities; (b) tempering and moulding, which is now done almost wholly by machinery. There is a great variety of machines for tempering and moulding the clay, which, however, may be grouped into three classes, according to the condition of the clay when moulded: (1) soft-mud machines, for which the clay _ is reduced to a soft mud by adding about one quarter of its volume of water; (2) stiff-mud machines, for which the clay is reduced to a stiff mud; (3) dry-clay machines, with which the dry or nearly dry clay is forced into the moulds by a heavy pressure without having been reduced to a plastic mass. These machines may also be divided into two classes, according to the method of filling the moulds: (1) those in which a continuous stream of clay it forced from the pug mill through a die and is afterwards cut up into bricks; and (2) those in which the clay is forced into moulds moving under the nozzle of the pug-mill.
Drying and Burning. The bricks, having been dried in the open air or in a drying-house, aie burned in kilns; the time of burn ing varies with the character of the clay, the form and size of the kiln, and the kind of fuel, from six to fifteen days.
Color of Bricks depends upon the composition of the clay, the moulding sand, temperature of burning, and volume of air ad mitted to the kiln. Pure clay free of iron will burn white, and mixing of chalk with the clay will produce a like effect. Iron pro duces a tint ranging from red and orange to light yellow, according to die proportion of the iron.
A large proportion of oxide of iron mixed with pure clay will produce a bright red, and where there is from S to 10 per cent, and the brick is exposed to an intense heat, the oxide fuses and produces a dark blue or purple, and with a small volume of manganese and an increased proportion of the oxide the color is darkened even to a black.
A small volume of lime and iron produces a cream color, an crease of iron produces red, and an increase of lime brown. Magnesia in presence of iron produces yellow, and clay containing alkalies and burned at a high temperature produces a bluish green.
The best quality of building brick and probably the majority of paving brick or block, are manufactured from shale. The process of manufacture is similar to that of clay-brick, the shale being first ground very fine. If the shale is nearly free from impurities, the resulting product will be a cream colored brick. To give the brick any desired color, the shale is mixed with clay containing the proper proportions of lime, iron, or magnesia, giving almost any shade from a cream to a dark wine color or even a black.
Classification of Brick. Bricks are classified according to (1) the way in which they are moulded; (2) their position in the kiln while being burned; and (3) their form or use.
The method of moulding gives rise to the following terms: Soft-mud Brick. One moulded from clay which has been re duced to a soft mud by adding water. It may be either hand-moulded or machine-moulded.
Stiff-mud Brick. One moulded from clay in the condition of stiff mud. It is always machine-moulded.
Pressed Brick. One moulded from dry or semi-dry clay.
Re-pressed Brick. A soft-mud brick which, after being par tially dried, has been subjected to an enormous pressure. It is also called, but less appropriately, pressed brick. The object of the re-pressing is to render the form more regular and to increase the strength and density.
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 advantage or disadvantage. In hand-moulding, when sand is used for this purpose, it is certain to become mixed with the clay and occur in streaks in the finished brick, which is very undesirable.