96 University Geological Survey of Kansas. 
crystallization. Where the plaster is slightly overburned, the 
crystals are extremely fine and crystallization goes on very 
slowly and imperfectly. 
This theory of the set of plaster seems to accord with the facts 
noted and appears simpler and more plausible than the views 
already advanced. The influence of retarders on the set of 
plaster will be discussed in connection with that subject. 
Process of Manufacture. 
In Kansas there are eleven mills engaged in the manufacture 
of plaster from gypsum, and two equipped mills not now in op- 
eration. Six of the eleven use the gypsum rock and five use the 
gypsum earth. The process of manufacture is practically the 
same in all the mills of the northern and central areas except 
the mills using the gypsum earth do not require crushing ma- 
chinery. } 
The machinery used in the Kansas mills is manufactured by 
Butterworth & Lowe, of Grand Rapids, Mich., and also by two 
Kansas companies: the Hhrsam Machine Company, of Enter- 
terprise, and the Great Western Manufacturing Company, of 
Leavenworth. 
On the ground floor of the mill are placed the crusher and 
nipper. The crusher, shown in Figure 15, has face plates or 
jaws of chilled iron which have a backward and forward crush- 
ing motion, operated by steam or water power. Blocks averaging 
fifty pounds weight are thrown into this machine and crushed 
into pieces about the size of aman’s hand. ‘These small masses 
drop from the crusher into the cracker or nipper, which is set 
in the floor. just under the crusher. This machine, with its in- 
terior revolving shaft, works somewhat like a coffee-mill, and 
further crushes the gypsum into fragments of the size of small 
gravel, which fall into the buckets of a chain elevator, whereby 
they are raised to a bin on the second floor. From this bin the 
gypsum particles pass through a spout into an ordinary buhr 
mill, where it is ground into flour. 
The buhr stones are ground down smooth in course of time 
by the friction of the gypsum particles, and then they must be 
