GRIMSLEY. | Technology of Gypsum. 115 
In some mills ordinary flour-bolting machines are used, while 
in others the plaster descends into a horizontal cylindrical reel, 
forty inches in diameter and ten feet long, slanting downward 
three-eighths of an inch to the foot. ‘This is made of brass wire 
cloth about forty by forty meshes to the inch. The tailings 
or screenings, usually about one per cent. in amount, are carried | 
back to the buhr mill and reground. The fine plaster is run 
into 100-pound sacks or 250-pound barrels and is ready for ship- 
ment. The plan of a modern equipped mill in a Kansas field is 
shown in Plate XXX. 
The calcined plaster which sets in a few minutes, forming a 
clear white finish, is known as plaster of Paris. The plaster 
which sets slowly, whether retarded or not, is called cement 
plaster. When water is added to these plasters they more or less 
rapidly unite with it, setting into a firm, solid mass. 
Hardening of Cement Plasters. 
It has been known for some time that if gypsum stone is 
heated and thrown into a ten per cent. soiution of alum for a 
few minutes, and then heated again to redness, the plaster on 
setting is very much harder than the ordinary plasters. 
Payen, as stated before, in his sixth principle, thought the 
hardening was due to the formation of a double sulphate of 
potash and lime. lLandrin analyzed the alum plasters with the 
following results : 
Landrin’s Analysis of Alum Plasters. 
English cements. 
FE i 
ELEMENTS. : pt . are : Stucco 
Suliphateroteliniveyrem Geren ane cis 96.75 98.19 98 .02 98 .55 
Carbonaveot lime eee ay Nees CANE 1.05 0.41 ORS 0.36 
Sill Carper eens eee So eemianes Srna mean IE ft CB) i Mian naa 0.42 0) {5 
NVR I 21 ae RAR er gs ahr AUST oe ae A Dn Ua 1.48 1 45) 1.19 1.08 
An examination of these figures shows that the alum cements 
are of great purity, and there is no trace of alumina or potash. 
Landrin explains the hardening of alum plasters as due to the 
reaction of sulphate of alumina and potash on the plaster stone, 
converting nearly all the carbonate of lime into the sulphate, or 
