If the brick are to be manufactured by the soft-clay process almost any kind of material can be used that will hold its shape in drying. Clays such as the Hudson river clays, which are largely impregnated with quicksand will not retain their shape, in drying if they are too much pugged, hence the brick machines used in the Hudson river district do not have too great a pugging power for the clays.
Both the stiff-clay and dry-press brick machines require for their successful operation a strong plastic clay. Fine front or pressed-brick require clays having all the qualities necessary for good common brick and in addition the color-producing qualities. Iron is generally supposed to be the all-important constituent necessary to produce a good red color, but there are a number of things and conditions, besides the iron, that are necessary for the successful manufacture of the best classes of salable front brick.
The iron should be equally distributed throughout the mass to be burned. The lack of ability to do this successfully has caused many attempts at artificial coloring of clays to prove failures. The clays must be of such a nature as will enable them to stand sufficient heat, without warping and twisting, to bring out the color. They should also be without those ele ments which, when brought to a high temperature, unite with the iron and carry it out of the brick and kiln in the form of vapor. An analysis which shows a good per cent. of iron in clay does not always prove that the brick will burn a good red color.
Clays which shrink a great deal are not the best for front brick making, as they are liable to warp and crack in burning.
Such clays are more liable to show stripes and brown edges than those that shrink less.
In erecting works for the manufacture of building-brick, it is necessary first to determine whether the clay located upon the property is suitable, not only in its nature, but in the quantity present. The digging of one or two pits will not determine the question. Examinations should be made by boring into the earth in a large number of places. This is an expeditious and general method, and it is possible by boring to penetrate all the strata or beds of clay on the property. Large-size boulders cannot be thus penetrated, and layers of gravel are also hard to penetrate, while wet sand, such as quick sand, also offers im pediments ; these obstacles, however, are usually of no great extent, and by making different trials can almost always be avoided. There are a variety of augers and bits used in mak ing explorations of clay by boring.
The specimens of clay brought to the surface by the boring auger are usually fair specimens, disclosing as they do, not only the character, but the thickness of the strata from which they are taken. Of course the number of borings to be made will be governed somewhat by the information obtained from the clays brought to the surfaces. If the strata seems to be of al most uniform depth and thickness, a less number of borings will be required than when less uniformity of the clay forma tions is shown to exist.