Re-pressed Brick weighs about 150 lb. per cubic foot, common brick 125, inferior soft 100. Common bricks will average about 41 lb. each.
Hollow Brick, used for interior walls and furring, are usually of the following dimensions: Single, S in. long, 3i in. wide, 21 in. thick.
Double, 8 " " 7 " " 41 " " Treble, 8 " " 71 " " 7f " " Roman Brick, 12 in. long, 4 to 41 in. wide, 11 in. thick.
are used wherever high temperatures are to be resisted. They are made from fire-clay by processes very similar to those adopted in making ordinary brick. Fire-clay is also used in the manufacture of paving-blocks or pavers, especially in West ern Indiana; and many of the streets of our Western cities .are laid with fire-clay block, forming a smooth and durable roadway.
Fire-clay may be defined as native combinations of hydrated silicates of alumina, mechanically associated with silica and alumina in various states of subdivision, and sufficiently free from silicates of the alkalies and from iron and lime to.resist vitrification at high tem peratures. The presence of oxide of iron is very injurious; and, as a rule, the presence of 6 per cent justifies the rejection of the brick. The presence of 3 per cent of combined lime, soda, potash, and mag nesia should be a cause for rejection. The sulphide of iron—pyrites —is even worse than the substances first named.
A good fire-clay should contain from 52 to 80 per cent of silica and 18 to 35 per cent of alumina and have a uniform texture, a some what greasy feel, and be free from any of the alkaline earths.
Good fire brick should be uniform in size, regular in shape, homogeneous in texture and composition, strong, and infusible and break with a uniform and regular fracture.
A properly burnt fire-brick is of a uniform color throughout its mass. A dark central patch and concentric rings of various shades of color are due mainly to the different states of oxidation of the iron and partly to the presence of unconsumed carbonaceous mat ter, and indicates that the brick was burned too Fire-brick are made in various forms to suit the required work. A straight brick measures 9 X 41 X 21 inches and weighs about 7 lb., or 120 lb. per cubic foot; specific gravity 1.93. One cubic foot of wall requires 17 9-inch bricks; one cubic yard requires 460. One ton of fire-clay should be sufficient to lay 3000 ordinary bricks. English fire-bricks measure 9 X 41 X 2+ inches.
To secure the best results fire-brick should be laid in the same clay from which they are manufactured. It should be used as a thin paste, and not as mortar: the thinner the joint the better the furnace wall. The brick should be dipped in water as they are used, so that when laid they will not absorb the water from the clay paste. They should then receive a thin coating of the prepared fire-clay, and be carefully placed in position with as little of the fire clay as possible.