STATE GEOLOGIST. 173 
finally pressing the material into bricks by means of a press similar to 
the dry presses used in clay. brick making. The capacity of the largest 
machine of this kind is about. 20,000 bricks per day. In selecting a press 
for this work care should be taken to see that its plungers are provided 
with carefully adjusted powerful springs in order that, in pressing, the 
plungers will give some whenever an excess of material happens to be fed 
by the charger, thus preventing breaking of the machine. It is hardly 
necessary to say that the press should be of the heaviest possible construc- 
tion. In Europe a press is used in which the pressure is applied by a 
heavy plunger falling freely upon the material in the mold, thus doing 
away with all possible danger of breakage due to an excessive charge. 
However, the bricks thus made are not as dense as they should be, and 
are apt to be crumbly. It is important that the top and bottom dies 
do not fit too tightly in order to allow the enclosed air to escape. The 
slag brick should not be taken into a dry atmosphere at once, but should 
be allowed to remain in a moist room for several days. After two weeks 
they are, as a'rule, ready for the market. It 1s obvious that in making 
these bricks Portland cement may be added in smaller percentages and 
may be used as a substitute for the lime. 
MANUFACTURE OF NATURAL CEMENT. 
This branch of the cement industry is the simplest one and consists 
solely in taking the rock as it comes from the quarry or mine, in lumps, 
burning it in plain upright kilns, and grinding the burnt, friable pieces 
to a powder. No attempt to regulate or control the qualities of the 
product can be made except by varying the speed of burning, or by 
using a higher or lower temperature, and hence the burning becomes the 
most important operation. 
The rock is either quarried or mined. In the quarries of the natural 
cement mills, all modern quarry appliances are used, steam or compressed 
air drills and power cranes. High explosives are, of course, universally 
employed. 
The treatment of the quarried cement rock differs in different plants. 
The more progressive mills crush the rock into two or three .uniform 
sizes by means of Blake crushers, while others make no attempt to crush 
it, but put it into the kiln in the shape in which it comes from the quarry. 
It is evident. that by crushing the rock, greater uniformity of burning 
must be the result as well as better utilization of the kiln space. 
The rock is burnt until most of its carbon dioxide is expelled, and 
the stone has become very porous, and is quite friable. It is practically 
unavoidable that some of the stone be underburnt, and some of it over- 
burnt, that is, burnt to a vitrified clinker. It is endeavored to sort 
out both kinds as much as possible. 
The sorted stone is now conveyed by means of cars or barrows to the 
intermediate crushing machines, usually a machine of the ‘“‘corn-sheller’’ 
