BUILDING BRICK CLAYS.
Brick-makers divide clays into three classes ; First. Plastic or strong clays, which are chiefly a silicate of alumina, which are by the workmen called " foul clays ; " a more fitting name, and one by which they are also called, is " pure clay." Second. Loams and mild clays are those in which there is a considerable proportion of sand intermixed.
Third. Marls or calcareous clays are, as their name indi cates, clays containing a notable quantity of carbonate of lime.
" Malm " is the name applied by English brick-makers to an artificial marl, made by adding to and intermixing with the clay a proper proportion of carbonate of lime.
As a general rule, a clay fit for the manufacture of a first class quality of building-brick is not met with in nature. There is almost always a deficiency of sand and lime. A good brick clay is one that contains sufficient fusible elements to bind the mass together, but not so much as to make the brick adhere to each other or become vitrified. Such clays contain from 20 to 30 per cent. alumina, and 5o to 6o per cent. silica, the re mainder consisting principally of carbonates of lime and mag nesia, and oxide of iron.
Pure or "foul clays" are sometimes used for brick without any admixture of substances to improve the material. Brick thus made are generally deficient in weathering qualities. The color of brick depends upon the composition of the clay, the character of the added ingredients, the temperature at which they are burnt, and the amount of air admitted to the kiln. A clay free from iron will burn white, but as a general rule car bonate of lime (in the form of chalk) is added to produce white brick. The presence of iron oxides produces a tint which varies from light yellow to dark red, the intensity of color in creasing with the greater quantity of the oxide. If 8 or to per cent. of iron oxide is present, and the brick becomes intensely heated, the red oxide of iron combines with the silica and fuses, producing a dark-blue or purple color. The presence also of a
small quantity of oxide of manganese, in addition to the oxide of iron, will cause a material darkening of the red tint. By the presence of small quantities of lime the red color of iron oxide is modified to a cream tint, while larger quantities make a brown color. Magnesia also changes the red tint to yellow.
In the clays from which the famous Milwaukee cream-colored brick are made, the proportion of lime and magnesia runs up to twenty-three per cent. carbonate of lime, and seventeen per cent. carbonate of magnesia, with nearly five per cent. of iron. The average brick-clays of the drift show from three to ten per cent, of lime, and in these uses it is a valuable agent, but it would be quite fatal to any of the higher uses of clay. The alkalies, i. e., potash, soda and lithia, are found in all clays to greater or less extent, though not all together by any means. Potash is most common and most detrimental, lithia is most in frequent and in the smallest amounts. Its presence has not heretofore been noticed as an element in Ohio clays, but once detected, it was found in a number of samples. Mention has been made of mica and feldspar as the probable sources of the alkalies in clays, and this theory is strengthened from the fact that the largest source of lithia at present is one of the minerals of the mica group, viz., lepidolite.
The knowledge of the composition and properties of clay now current among the clay manufacturers, is almost wholly practical, and there may seem to be ground for surprise that such excellent results should have been obtained with so little aid from science, but it is to be remembered that much less has been done for this subject than for parallel industries. The scientific research directed to it is much more scanty in pro portion to the interest involved than in almost any similar field.
What work has already been done has proved very valuable, and further study cannot but be productive of good.