GRIMSLEY. | Technology of Gypsum. 107 
the plant is limited, as the supply of material sooner or later is 
bound to be exhausted. Expensive buildings, under such cir- 
cumstances, are certainly not the result of good business judg- 
ment. Where the plant uses the rock gypsum, the supply of 
which can be determined to a better advantage, the character of 
the buildings can be judged of much better, and can be made to 
correspond with the probable durability of the supply of the 
material. 
GENERAL REQUIREMENTS OF A PLASTER MILL. 
In connection with the discussion of the methods of manu- 
facturing the different grades of plaster of Paris and cement 
plaster, it may be desirable to give a short discussion of the 
most important requirements for a modern plaster mill. 
As has already been stated, the difference in the mode of 
manufacturing plaster from the rock gypsum and from the 
gypsum earth consists essentially in but two points: First, in 
crushing the rock gypsum so as to reduce it to a powder before 
calcining ; and second, in mixing a little more retarder with the 
plaster before it is made into the mortar. As the trade demands | 
a plaster ready for immediate use, the retarder is now univer- 
sally mixed with the plaster at the factory. As the methods of 
crushing the rock gypsum and of grinding the plaster to a fine 
powder have already been described in considerable detail, noth- 
ing further need be added to such descriptions. 
The essential requirements for a gypsum plaster mill may be 
enumerated as follows: First, the most convenient location 
possible, considering the character of the grounds around the 
mine and the proximity of the mine to one or more railroad 
lines; second, the proper arrangement of the buildings with 
reference to each other, having regard for the cost of the build- 
ings, the conveniences afforded by their arrangement, and the 
safeguards against fire, etc.; and third, the internal arrange- 
ment of calciners and machinery. 
Location of Mills, ete. 
The exact location of a plaster mili is often a difficult ques- 
tion to decide. When the mines are some distance from a rail- 
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