338 CEMENT MATERIALS AND INDUSTRY. [bull. 5 
All the cement rock used is now obtained by mining. Two inclir 
have been run in on the dip of the beds. These inclines are, at a 
near their entrances, entirely in the cement bed, several feet of cenu 
rock being left above and below them to serve as floor and ro< 
Farther in, however, the height of the passages is increased, a 1 
almost the entire thickness of the cement bed is taken out Pillars 
cement rock are left at intervals to support the roof. 
The cement rock is blasted out, sledged when necessary to a s 
convenient for the kilns, and loaded onto cars carrying about 1^ tc 
each. These cars run on narrow-gage tracks, which extend from nc 
the heads of the workings to the kilns. As the grade in the inclii 
is heavy the cars are hauled out of the mine by wire cables, po\N ■ 
being supplied by the crusher engine. Outside of the mine, ho 
ever, the grade to the kilns is sufficiently light to permit the cars to \ 
run by hand to the level from which the rock is fed to the kiln. 
The kilns are of the familiar dome type commonly used in lime a 
natural-cement burning, and are six in number. Four are jacket 
with steel and lined with tire brick, the space between the jacket a 
the lining being filled with clay. The two remaining kilns differ fr< 
these only in the fact that in place of the steel jacket their exter 
surfaces are laid up with rock. These rock-jacketed kilns are said 
be somewhat more satisfactory than those of the steel- jacketed ty]| 
All the kilns are 25 feet in height, and have an output of 60 barm 
of cement each. The kilns are charged to the top with fuel and cenu 
rock, in the proportions of about 300 pounds fuel to 2,500 poum 
rock. The fuel used is coal, the sizes being nut, pea, and slack, 
about equal amounts. Seven or eight days are required, on the av 
age, to "turn a kiln," including charging, burning, and drawing. 
When the kilns are drawn the clinker is picked over and then c: 
ried by a Jeffreys elevator to pot crushers of special design. < 
issuing from the crushers the material encounters revolving screens \ 
about i-inch mesh. All material that does not pass these screens | 
sent back to be recrushed. The liner material which passes throu 
this i-inch screen meets another revolving screen of about 50 me; ■! 
Everything that passes through this tine screen is sent direct b 
conveyor to the packing machines, while the material failing to p; 
the 50-mesh screen is sent to buhrstone mills, where it is groin 
screened, and sent to the packers. 
The Howard cement is marketed as the "Red Keystone" brai 
and is favorably known to the southern cement trade, having be 
extensively used in engineering work. It has certain propert 
which serve to differentiate it from other natural cements. In coi 
it is lighter than any other natural cement known to the writer, i 
set cement being very light gra}^ or }^ellowish gra}^. Its specific grav 
