CRaNE. | Theory of Ore Concentration. 333 
hardness of particles treated, character of roll surface, 
method of feeding, etc., which influence the character of 
crushing and amount crushed. 
It has been found that the best results are obtainable for 
chert (flint) with hard shells; for limestone, with soft shells ; 
although hard shells are commonly used for all kinds of 
work. 
To prevent breaking, it is best to chamfer the edge of 
shells =, inch on the diameter and 1 inch on face for fine, and 
+ by 1 inch for coarse material. 
Slow speeds are probably best for rolls. The range is 
from ten to twenty up to thirty revolutions per minute. 
Rolls should not reduce the material to be crushed more 
than one-third of the size at one crushing. The following 
are standard sizes: Coarse rolls, 24 to # inch; medium, 
#to4inch; fine, ¢to 4 inch. One-sixth inch is probably 
the economic limit for crushing with rolls in this district. 
The sledge is generally used to reduce pieces of rock too 
large for the crusher. 
A crude form of bucking board is used in connection with 
hand jigging, to separate the barren rock from the ore and to 
reduce to a size sufficiently small for jigging. Occasionally 
a hammer is used to remove small pieces of ore from the 
barren rock, which is ‘‘cobbing’’ on a small scale. 
Sizing. 
Sizing is the process of separating material according to 
size by screening. The sizes made should be within the 
equal falling ratio of two grains. To illustrate: A grain of 
chert (quartz) of 1 mm. diameter falls slower in water than 
a grain of 4 mm. diameter; slower also than a grain of ga- 
lena of the same diameter; but the large grain of chert and 
the small grain of galena have very nearly equal falling ve- 
locities. The ratio of chert to galena is about 1:4; pyrite 
to galena, about 1:2. 
When chert and galena alone are to be separated, they 
might be sized between the limits 1 and 4; if mixed with 
pyrite, however, the limit might have to be changed. The 
ratio of 1 to 2 would probably be the most practicable. It 
