162 University Geological Survey of Kansas. 
In the process of manufacture, the details of which are given 
more fully in chapter VI, the iron kettles that are used hold 
about eight tons of material, and they are heated directly by 
means of a hot fire of coal beneath. The flame is carried 
around the sides by flues, with the object of heating all parts of 
the kettle. Although many devices have been suggested, such 
as heating by steam, and the use of kettles of peculiar construc- 
tion, nothing has been found to be more practical than the ket- 
tles described (loc. cit.), in which the material is kept in motion 
by means of a mechanical stirrer making fifteen revolutions 
per minute. It is of the greatest importance that the partly 
calcined plaster shall not remain in contact with the heated 
iron, as in case very little water is left to be driven off the heat 
may become too intense, and disastrous results may follow. 
The heated material boils like thick cream, and runs almost 
like water. 
The heat is kept constant, or nearly so, during charging, by 
letting the material drop in gradually, till a sufficient quantity 
is present, and then the boiling will increase in violence, so that 
it is occasionally thrown in ‘‘waves’’ out of the kettle. The 
boiling is, of course, due to the escape of-the water of crystal- 
hzation and the hygroscopic water that may be present. A 
short time before the process is completed (the whole process 
requires about three hours), there is a sudden settling down of 
the contents of the kettle, but soon the mass begins to boil 
again, and after a certain time, that can be quite accurately 
determined by the expert calciner, the charge is pronounced 
done, and, almost instantly, the kettle is opened at the bottom, 
and the finished product slips out into a storage vault. This 
operation also must be carried on very quickly and swiftly, as the 
fire is still burning under the kettle, and if there is no material 
to be heated, the bottom soon gets too hot. The second charge 
follows the first immediately. The calcined cement is then 
elevated to the top of the building and carried to a sifter, and 
the particles (‘‘tailings’’) that are too large to pass through 
this are returned to the kettle and heated again. 
dine boiling material, on account of the steam given off, pro- 
