BRICK AND BLOCK MASONRY 56. Clay and Shale Bricks.—The cheapness, case of construction, and durable qualities of good brick masonry make it one of the most desirable materials for general structural work. It is not as largely used in engineering work as stone or concrete, but in building con struction it is very extensively employed. The qualities of clay bricks vary widely according to the character of the clay and methods of manufacture, and care must be taken in selection of material in order to secure good results.
Composition of Clay consists primarily of silicate of alumina.. Common clays also usually contain certain percentages of iron oxide, magnesia, lime, and alkalies. These are known as fluxes, having the effect, when in considerable quantities, of making the clay fusible. Fire clays contain a low percentage of fluxes, and withstand a high degree of heat without fusing.
Sandy clays contain high proportions of silica in an uncounbined state, a factor which, if not in excess, is of value, tending to give stability to the form of the brick. Sand is commonly added to plastic clays for this purpose.
The color of the brick is mainly dependent upon the amount of iron oxide present in the clay. The color varies from white, through huff to red as the percentage of iron oxide increases. The presence of iron oxide is also of value in adding strength and hardness to the brick.
Linie, when present in appreciable quantities, must be finely divided and uniformly distributed through the clay. If in lumps, the slaking of the lime, subsequent to burning, may cause the brick to become distorted and cracked. When in excess, lime neutralizes the color effect of the iron oxide, making the bricks lighter in color, huff or yellow colors being sometimes due to this cause.
Excess of alumina usually makes the clay very plastic and causes it to shrink and crack in drying.
Physical Properties.—The physical properties of clay are of more importance than the chemical composition.
Plasticity is one of the important properties of clay for brick making, as it permits the play to be worked into a plastic mass, and to be molded into the desired form. Clay shrinks in drying and also in burning, very plastic clay shrinking more than that less plastic. Sand is frequently mixed with clay to reduce excessive shrinkage. The degree of plasticity is sometimes controlled by mixing clays which differ in this respect.
When subjected to high heat, clay gradually becomes soft and fuses together, and as the heat is increased the softening and shrink age progresses until the material finally melts sufficiently to lose its shape. The temperature required for burning varies widely with
different clays, and the degree of burning given to brick depends upon the kind of product desired and the fusibility of the clay.
Manufacture.—There are three methods in use for forming the brick. They are known as the the stiff-mud, and the dry press methods.
The soft-mud process consists in pulverizing the clay or shale and tempering it with water to the consistency of soft mud. This paste is then pressed into wooden molds, which are usually sanded on the surface to prevent the clay sticking, thus giving the brick five sanded surfaces.
The stiff-mud process consists in mixing the pulverized clay or shale with sufficient water to form a stiff paste, capable of retaining its form, which is then forced through a clie, resulting in a bar of the section of the brick. The bar is then cut into bricks by wires. These bricks may be either side cut or end cut.
Dry-press bricks are made by pressing pulverized clay containing a small amount of moisture into steel molds, a method used to secure bricks with smooth faces and sharp edges for face brick.
Repressed bricks are made by putting bricks made by the soft mud or stiff-mud methods into presses and subjecting them to high pressure. The purpose is to give the brick more perfect form and sometimes to imprint a design upon the surface.
Bricks made by the wet method must be dried before being placed in the kiln. In some yards this is accomplished by exposing the molded bricks to the air on floors or racks, while in the larger plants the drying is clone more rapidly in dryers using artificial heat.
The burning is accomplished either in temporary kilns, built of the brick to be burned, or in permanent kilns arranged usually with fire boxes on the outside and a- downdraft and intended to give uniform heat throughout the kiln. This cannot he fully accomplished and all of the prick will not he perfectly burned. The degree of burning received by brick in temporary kilns depends upon the position in the kiln. They must be sorted after burning into various shades, varying from the light underburned to the dark arch brick.
Good bricks may he made by any of the methods of manufacture, provided the material is carefully handled and the burning properly regulated. The differences due to method used are mainly those of the form and appearance Of the brick, Dry-press brick are usually somewhat softer and weaker than stiff-mud brick of equally good material.