VITRIFIED BRICK PAVEMENTS FOR COUNTRY ROADS. 3 



the different strata are almost always slightly dissimilar in both 

 physical and chemical composition. By carefully mixing the mate- 

 rials from different strata or from different parts of the bank, there- 

 fore, a resulting material of the desired character may usually be 

 obtained. But it not infrequently happens that in order to secure 

 the best results sand or surface clay must be added in an amount 

 depending on the relative " leanness " or " fatness " x of the material 

 used. In this connection it may be noted, also, that a chemical 

 analysis of a given fire clay or shale does not necessarily indicate its 

 fitness or unfitness for paving brick. The reason for this is that the 

 quality of the brick after " firing " is no less dependent on the physical 

 arrangement of the minerals than on the chemical composition of the 

 material. 



THE MANUFACTURE. 



The general processes of manufacture are the same for both fire 

 clays and shale. The raw material in either case is crushed to com- 

 paratively small fragments and conveyed by some convenient means 

 to a grinding machine, known in the industry as a dry pan. Briefly, 

 this machine consists of a solid iron plate, approximately 5 feet in 

 diameter, surrounded by a perforated iron surface about 2 feet wide. 

 Outside the perforated surface is a rim some 15 inches in height which 

 serves to prevent the material from escaping otherwise than through 

 the perforations. Upon the solid plate rest two massive crushers or 

 mullers, each weighing from 2^ to 3 tons. The pan is revolved 

 rapidly, causing the mullers to rotate by friction. The material is 

 ground between the mullers and the plate and thrown out by cen- 

 trifugal force toward the rim, where it escapes through the per- 

 forated surface into an elevator, by means of which it is conveyed to 

 the screens. 



The particles too large to pass the screens, which should not exceed 

 three-sixteenths inch in mesh, are returned to the dry pan, while 

 the screened material is passed to the mixing machine or pug mill 

 by means of conveyors. In the pug mill, water is admixed with the 

 clay to form a stiff mud, which is fed continuously into the brick 

 machine proper. 



The brick machine is an extremely heavy mechanism. It con- 

 sists essentially of an auger or propeller conveyer, a tapering barrel, 

 and the die or former. The material is forced by means of the auger 

 conveyer into the tapering barrel, which terminates in the die, and 

 issues from the die in a solid column under heavy pressure. For 

 " side-cut " brick this column is approximately 4J inches by 10 



1 " Leanness " and " fatness " refer respectively to the lesser or greater amount of 

 silicate present in the material. 



