BRICK CLAYS AND COLORS Bricks have been used as building material for many centuries, and no known substitute seems likely to make them less popular. They are durable, of good appearance, and comparatively cheap. The fact that brick masonry is but little affected by the chemical agents that often destroy stone, is well known. As heat is used in the manufacture of brick, such material is well able to withstand the attack of fires that would destroy granite. While stone is no doubt preferred by many for its appearance, very pleasing effects may be obtained from the use of brick, and at considerably less expense.
Brick may be defined as a variety of artificial stone produced by baking clay. While made in a great many shapes and sizes, the blocks are generally of rectangular cross-section approximately 81/4 by 4 by inches on the three sides. These small and regular dimensions are factors that make handling, transportation, and laying comparatively easy. No slow and expensive dressing is required ; no heavy hoisting apparatus is necessary for loading; and the separate loads can be made greater or smaller as occasion requires.
Composition of Brick. Brick, as already noted, are made from clay; and the great variety in physical condition and chemical composition of clays enabler man to manufacture brick suitable for his every requirement.
( lay, in its purest form, is known as kaolin, and consists almost exclusively of alumina, silica, and water. Kaolin is the clay from which the highest grade of white porcelain ware is made. This purest clay is supposed to have been formed by the decomposition of feldspar rock. Its composition is generally stated as 16 per cent silica, 40 per cent alumina, and 13 per cent water.
Similar decompositions of other rocks containing silica, alumina, lime, iron, magnesia, etc., and the blending and mixing of all these elements and compounds under varying influences of glacial and water action and changes of temperature, have given us a range of clays yielding to the manufacturer products as widely variant as the white porcelain cup or vase and the "dinas" firebrick, which is mostly silica bonded together only with a small quantity of milk of lime or of clay.
Common clay, chiefly used for making the different grades of building brick, from "backing" or inside brick to front brick, is composed of silica, alumina., mixed with oxide of iron, lime, magnesia, and other impurities.
Silica, if not in too great proportion, makes the brick burn with less warping than would otherwise be the case. Too much uncombined silica (more than 25 per cent) tends to make the product brittle. Lime in excess has a had effect by causing the brick to change shape or swell in the burning and to show a tendency to disintegrate after they are burned and exposed to the atmosphere.
Iron produces strength, and its presence is indicated by the red color. Usually the red brick are—like the red varieties of stone—the stronger.
To produce firebrick a more nearly pure clay is necessary. A fireclay may be regarded as a variety of impure kaolin, a rough analysis averaging 60 per cent silica, 20 per cent alumina, and small but important proportions of lime, potash, and soda. Iron is in this case not desirable; and the amount of lime, soda, potash, and magnesia combined should not exceed 6 per cent. Silica may be in excess if the property of resisting heat be the only consideration.
Color of Brick. The color depends upon the composition of the clay and the amount of heat applied in burning.
In general, oxide of iron and lime are the controlling factors in determining the color of brick. Magnesia is found in smaller quantities in common clays, and exerts a coloring influence similar to that of lime.
Broadly stated, the red color of clay products is due to the presence of iron oxide; and the white, cream, or yellow, to the presence of lime or magnesia, or both.
Seger's classification is that clays (a) Rich in alumina, poor in iron compounds—burn white or cream color.
(b) Rich in alumina (20 to 30 per cent), moderate amount of iron (1 to 5 per cent)—burn yellow to brown, as most plastic clays and fireclays.
(c) Poor in ahunina, rich in iron compounds—burn red, violet, or black, according to temperature.
(d) Poor in alumina, but rich in iron and lime compounds—burn yellow owing to the formation of the double silicate of calcium and iron.
The conditions of firing also affect the color of goods, as is markedly shown where clays containing iron compounds, and binning red in an oxidizing atmosphere, will, in a reducing atmosphere with low air-supply, burn black or blue.
Clay containing constituents tending to produce a red color will become darker as more intense heat is added. Heat alone will not produce this effect, as is shown in the light-colored firebricks.
In some processes of manufacture, sand is applied to the forms or moulds, to prevent the wet paste from sticking to them. Tf, as often happens, this sand is red, the brick that might otherwise be of light tint takes the color of the sand rather than that of the clay, and the outside is red.
The ordinary clay in this country suitable for brickmaking, is mostly glacial material that has been washed down by the streams and redeposited, while its character has undergone little or no change. This is called alluvial clay, and the larger pebbles and grains of gravel have been left behind by the stream while the fine clay was in suspension.