Stiff-mud Brick. A brick moulded from dry or semi-dry clay. It is moulded by machinery.
Pressed Brick. A brick moulded with semi-dry or dry clay. Re-pressed Brick. A brick made of soft mud, which, after being partly dried, is subjected to great pressure.
(b) The classification with regard to their position in the kiln applies only to the old method of burning. With the new methods, the quality is nearly uniform throughout the kiln. The three grades taken from the old-style kiln were: Arch Brick. Bricks forming the sides and top of the arches in which the fire is built are called arch bricks. They are hard, brittle, and weak from being over-burnt.
Body, Cherry, or Hard Brick. Bricks from the interior are called body, cherry, or hard brick, and are of the best quality.
Pale, Salmon, or Soft Brick. Bricks forming the exterior of the kiln are under-burnt, and are called soft, salmon, or pale brick. They are used only for filling, being too weak for ordinary use.
(c) The classification of brick in regard to their use or shape has given rise to the following terms: Face Brick. Brick that are uniform in size and color and are suitable for the exposed places of buildings.
Sewer Brick. Common hard brick, smooth and regular in form. Paving Brick. Very hard common vitrified brick, often made of shale.
They are larger than the ordinary brick, and are often called paving blocks. Compass Brick. Brice-having four short edges which run radially to an axis. They are used to build circular chimneys.
Voussoir Brick. Brick having four long edges running radially to an axis. They are used in building arches.
26. Crushing Strength. The results of crushing tests of brick vary greatly, depending on the details of the tests made. Many reports fail to give the details under which these tests are made, and in that case the real value of the results of the test as announced is greatly reduced.
The following results were obtained at the U. S. Arsenal at Watertown, Mass., by F. E. Kidder. The specimens were rubbed
on a revolving bed until the top and bottom faces were perfectly true and parallel.
27. Fire Brick. Furnaces must be lined with a material which is even more refractory than ordinary brick. The oxide and sulphide of iron,which are so common (and comparatively harmless) in ordinary brick, will ruin a fire brick if they are present to a greater extent than a very few per cent. Fire brick should be made from nearly pure sand and clay. There is comparatively little need for mechanical strength, but the chief requirement is their infusibility, and pure clay and silica fulfil this requirement very perfectly.
28. Sand=Lime Brick. Within the last few years, the lime brick industry has been developed to some extent. The terials for making this brick consist of sand and lime; and they were first made by moulding ordinary lime mortar in the shape of a clay brick, and were hardened by the carbon dioxide of the atmosphere. There are two general methods of manufacturing these bricks: (a) Brick made of sand and lime, and hardened in the atmosphere. This hardening may be hastened by placing the brick in an atmosphere rich in carbon dioxide; or still less time will be required if the hardening is done with carbon dioxide under pressure.
(b) Brick made of sand and lime, and hardened by steam under atmospheric pressure. This process may be hastened by having the steam under pressure.
When sand-lime bricks are made by the first process, it requires several weeks for the bricks to harden; and by the second method it requires only a few hours; the latter method is the one generally used in this country. The advantages claimed for these bricks are that they improve with age; are more uniform in size, shape, and color; have a low porosity and no efflorescence; and do not disin tegrate by freezing. The compressive strength of sand-lime brick of a good quality ranges from 2,500 to 4,500 pounds per square inch.