Clay, as we have seen from the foregoing description, instead of being a mineral formed by the ordinary processes of chem ical synthesis, is the result of decomposition of granite rocks, or by a closer definition, is the result of the decomposition of feldspars or those rocks which yield them, notably granitic and gniessic rock. Orthoclase, the feldspar from which the body of our clays is derived, is a double silicate of potash and alumina ; the other feldspars found accompanying it are albite, or the soda feldspar, and oligoclase, the soda-lime feldspar ; these latter forms are in small proportion compared to orthoclase. Ordinary aqueous and atmospheric agencies are sufficient to decompose feldspar, giving rise to hydrated silicate of alumina or kaolin and a soluble salt of potash, which is carried away by the water which accompanies decomposition. Feldspar beds are fruitful sources of the finest kaolins and china clays, and are rarely found without some kaolin accompanying.
The mineral elements in ganite and gneiss, as is familiar to all, are quartz, feldspar and mica. The first mineral is not affected by air or water, but the feldspars, and to some small extent the micas, are attacked by the atmosphere. When the feldspar decomposes, the bond which holds the other elements together is gone, and the quartz and mica are carried off by water or mixed with the clay in varying proportions, as the conditions of formation vary. The more the water carries off the purer the clay left behind.
The irregularity of composition which is so characteristic of clay, is thus seen to he the result of the differing mechanical conditions which surround the clay as it forms ; it is distinct ively a mineral in which other forces than chemical affinity have left their mark ; and another element of uncertainty is added in the fact that if the mechanical conditions were con stant, clays would vary with the parent rock, which has no fixed structure.
Thus is exposed, in the origin of clay, the reason of that irregularity which has so long baffled or retarded progress in its study, but which once understood, proves the key to all that follows.
The qualifications for a clay for building brick are simple, viz.: Plasticity when wet, and solidity and hardness when burned.
Clays containing a large amount of carbonaceous matter naturally mixed with it are very objectionable, as brick when made from such clays will, when wetted in the wall, pass out soluble compounds, which discolor the walls, whether they are painted or not, and plastering or stucco-work is discolored by them the same as when brick which have once been used in the inside of a chimney flue and become blackened, are re-used in new work.
It would be useless to attempt decorative work of any de scription upon brick walls, the materials of which contain a large amount of carbonaceous matter, or if the brick be made from the alluvial mud of the embouchures of rivers, as no pos sible precaution can prevent the entire destruction of the work.
Carbonate of lime, diffused limestone and lime pebbles, when they are present in brick clays, are a decided hindrance to the production of even a passable quality of building brick, for in the kiln the limestone and lime-pebbles are converted into caustic lime, and when the brick are used below ground, or for exposed walls, the moisture and carbonic acid, which pene trate to every part of a brick, slack the nodules of lime, the swelling causing the brick to burst and break to pieces. Should such brick be used for " filling in," or inside or unex posed walls, the dampness from the mortar used in laying them, and also that contained in the plastering, would, by pro ducing the same bursting and breaking, destroy the finished face of the inside walls.
These are some of the evils which result from the badly-made brick so freely used in Chicago, and arise from the large amount of lime-pebbles in the clay, and the neglect of finely pulverizing or thoroughly sifting the clay, which can easily be done by machinery, at but a small additional cost.
Selecting Clays for Various Kinds of Brick of uniform quality can be made from low grade clays, but good, salable, uniform building-brick require that suitable clay shall be used in their manufacture; the better the grade of brick produced the more remunerative the price which they command in the market. If common mud-brick are to be manufactured by the hand method of moulding and without the use of machinery for crushing the clay preparatory to pugging, a clay should be selected that is very tender and easily soaked and pugged. The pugging qualities of clays can be determined by digging and pugging a small pit full of the material. The plasticity, as well as the moulding and drying qualities of clays, can also be readily tested on a similar small scale. There is no better way for determining the color to which a clay will burn than by making and burning the brick in a test kiln, such as several of the leading brick machinery manufacturers of the country maintain for testing purposes at their factories. Clays manufactured into a few brick and sent to a neighboring kiln to be burned and tested for color often turn out unsatisfactorily, for the reason that different clays burned in the same kiln tend to impart a uniform color to each other.