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 2J 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 4^ inches by 10 
1 " Leanness " and " fatness " refer respectively to the lesser or greater amount of 
silicate present in the material. 
