MANUFACTURE OF THE BRICK. Soft, homogeneous clay may be run through rollers, to crush the lumps, and from the crusher it may go directly to the brick machine; but it is usually desirable to run it first through a pug mill, where it is mixed and worked with water into a plastic mass. Hard clays and shales are usually reduced to a powder in a dry pan, which consists of two heavy rollers or wheels running in a revolving pan having a perforated bottom. It is important to have the clay finely pulverized, be cause it will then fuse at a lower temperature, and also because fine ness is necessary to the production of an even and close-grained texture which conduces to make the brick tough and impervious. The powdered clay is screened and then tempered with water in the pug mill or a wet pan. Fire clays are sometimes both crushed and tempered in a wet pan, which is similar to a dry pan except that the bottom is water tight. The wet pan gives better results than the pug mill, as the clay can be retained in the pan until it is thor oughly tempered; but as it requires a large plant, and takes more labor and power, it is not usually used for paving brick. The more thoroughly the clay is worked or tempered, the more uniform and better will be the brick.
Molding.Paving brick are usually made by the stiff mud process, although a few yards still use the old-fashioned soft mud and re-pressing system. The molding is usually done by an auger machine which forces the tempered clay or stiff mud through a die, thus giving a continuous bar of compressed clay which passes under an automatic machine that cuts the bar into brick of the desired size. Instead of an auger producing a continuous stream of clay, reciprocating plungers are sometimes employed, which give an intermittent bar. The auger machine is the cheap est, and is almost universally used. Formerly the dies were made about 41 X 2 inches in size, producing an end-cut brick; but of late dies 9 X 4i-inches are being used, a process which gives a side-cut brick. An active discussion is now going on as to the relative merits of the resulting brick; but apparently there is no material difference between the two. The weak point of the stiff
mud process is the laminations that must inevitably result from pushing a stream of clay through a fixed die. The friction of the sides of the die will cause differential speeds in the flow of the clay, and these variations must necessarily result in laminations in the clay bar. If the air has been expelled from the clay by the pug mill, these lines can be largely closed up again by a properly shaped die, and a first-class brick will result in which the lamina tions will be inconspicuous and of no importance; but if the air has not been expelled, or if the former and the die are not properly designed, there will be a number of concentric lines that divide the cross section of the brick into a series of shells or concentric cylinders which greatly weakens the brick. These laminations vary with the character and the condition of the clay; and as a rule, the more plastic the clay the more prominent the laminations.