Salmon brick, molded by the dry-clay process, are largely employed for dwelling houses and other light work, and are not objectionable when they are kept free from damp positions, and when so employed answer very well for the purposes for which they are used ; but in warehouse construction and simi lar heavy,work brick of this character are practically worthless.
In selecting a clay suitable for the manufacture of brick by the dry-clay process, it will become apparent from what has already been said in the foregoing part of this chapter that a clay that is weak in its texture and that contains a fair proportion of silica or of iron, lime or other fluxing elements, will make a better class of dry-clay brick than a clay of a stronger nature, which is hard to flux and which will come out of the kiln very much in the same condition that it went in.
The preparation of the clay is a very important part of the process of making brick by the dry-clay method, and in this connection there is always an opportunity to acquire knowledge by closely noting the peculiarities of the clays used.
After the clay has been dug the first step is to properly dry the clay, and usually a few hours' exposure to the sun and wind is sufficient for this purpose.
Richardson says, in his address delivered at the Second An nual Meeting of the National Brick Manufacturers' Association : " If the clay is near the surface, it is generally plowed to the depth of a few inches and left on the beds to dry. In the hot summer months the clay is usually allowed to remain exposed only about two hours, but in the spring and fall it is well to dry a day and have but one plowing. If too far below the sur face, or on ground too uneven to admit of plowing, the clay, after having been mined with pick and shovel in the usual manner, is conveyed • in carts to large drying grounds, over which it is dumped to the depth of about a foot. I know of no cheaper way of getting the clay upon the drying grounds than by carts—tramways cannot be used advantageously, as the clay must be dumped over so large a space. While the clay is upon the drying ground, it is gone over several times with a disk harrow, having about sixteen knives and working on a beam, and so arranged that it can be turned on a handle, and at the same time that the knives turn they cut and tear and break up the lumps and expose fresh portions to the sun and air. Some of the manufacturers of dry-clay brick, located near St. Louis, Mo., use an apparatus which travels back and forth over the plowed clay and which throws it up into ridges, and then men throw it into carts with shovels and it is then hauled to the sheds. When the clay, or a portion of it, has become suf ficiently dry, it is taken by wheel-scrapers into large sheds to be stored for future use. The proper dryness of the clay de pends upon the time it is to remain in the sheds, and the depth or height to which it is to be piled. The object in storing
large quantities of clay is not only to insure no stoppage during wet weather, but to allow the clay to sweat and become uni form in moisture throughout, as thereby alone can good results be obtained, and the longer the clay remains in the shed the better brick it will make. The average cost of getting the clay into the sheds in proper condition ranges from fifty-five cents to seventy cents per thousand when the clay sheds are not more than 500 feet from the banks. The drying grounds are between the clay beds and the sheds. The entrance to the shed, if only one, is in the centre of the side toward the drying grounds, so that while one end is being emptied for manufacture the other can be filled. From the shed the clay is taken to the elevator opposite the entrance, either by scraper or tram cars, and raised to a height sufficient to allow of its shooting into the pulverizer screens. and presses without being again elevated. The manner of pulverizing varies much with different clays, generally two pulverizers being necessary and two cylindrical revolving sieves. No single pulverizer will prepare for the press all the clay it re ceives, though sometimes one is made to do the work by putting through it a second time that which has not been pulverized fine enough. A sieve is placed over the press to prevent any thing but fine clay from being admitted. Such, briefly, is the method of preparing the clay for dry-press brick-making, simple enough in description, but in practice more complex, and in volving more difficulties than one who has not tried it can have any adequate idea. The great problem is to get the clay fine enough and at the same time of the proper degree of moisture. The clay should go into the moulds uniform in fineness and dryness, moist enough to be pressed hard, and dry enough to allow of the brick being set immediately in the kiln thirty to forty high. We usually say that the clay is of the proper dry ness when if squeezed in the hand it will just hold together and retain form after the pressure has been removed. To get it into this condition is not an easy matter. If the clay is too moist it cannot be pulverized fine enough ; if too hard it cannot be pressed hard enough. Brick presses will generally work damper clay than pulverizers. Strong, plastic clays work best, as they can be pressed in a drier state than weak, sandy clays. In considering this question, however, regard must always be had to difference in clays and machinery. It is of the greatest importance that machinery be selected suitable to the clay. No machine has ever been constructed that will work success fully all clays, and some machines are failures under ordinary circumstances, and have ruined many a man's business.