The clay is fed into the rolls by means of a vibrating apron, and the flow of clay is evenly regulated by adjusting screws. This machine is built very strong and rigid, and is a favorite among brick manufacturers. The driving pulleys are 34 inches in diameter, 12 inch face ; speed 175 and 250 revolutions per minute. Capacity, 30,00o to 50,000 brick per day, according to nature of the clay; weight, 4,765 pounds.
This machine is fully protected by patents.
From the dry-pan the ground clay is conveyed to the upper floor of the factory, where it is separated, the fine clay falling into a bin and the "tailings" or coarse clay passing by gravity back into the tailings crusher shown in Fig. 66 or into the dry pan, to be again ground until it is reduced to such a degree of fineness as to allow it to pass through the screen into the bin.
There is not much room for economizing in the grinding and tempering of clay to be used in the manufacture of payers, as the best grade of such brick can only be made from the ma terial which has been properly ground and prepared.
Mr. G. H. Brown, of Sioux City, Iowa, says : " The first step in making a paving brick is the thorough preparation or " tem pering" of the clay. It must be ground fine. I do not believe
that a first-class paving brick can be made where there are lumps as big as peas scattered all through them. It may make a hard brick, good enough for sewers, and for similar purposes perhaps, but it won't be a paving brick. Our own clay is hard to manage—what is known as refractory clay. We run it through two dry pans and a pug-mill, but are dissatisfied with the texture even then. The clay is dug with a steam shovel, and consequently goes into the pans direct from the bank. It is damp, very tough, and inclined to be sticky and to pack under the ' ploughs ' in the pans. The finest paving brick I ever saw in my life, bar none, are being made to-day at the Northwestern Sewer Pipe Works at Sioux City, where the clay, after going through a dry pan, is put through two wet pans, and that, to my mind, is a solution of the question—provided that sufficient capacity can be obtained by means of wet pans. That is a point which I am not competent to determine, but one that the makers of wet pans would do well to study, and perhaps improve upon."