102 University Geological Survey of Kansas. 
In an hour after the kettle is filled the temperature reaches 
230° F. and the mass is seen to be boiling vigorously as the 
water is driven off and out through the vapor stacks above. 
When the temperature increases to 270° F. (132° C.) the gypsum 
settles down solid, leaving twelve to sixteen inches of vacant 
space at the top, and the steam ceases to rise. At 280° or 290° F. 
(138° to 143° C.) the mass boils again, often throwing part of 
the material over the edge of the kettle. When a temperature 
of near 350° F. (177° C.) is reached, the plaster is rapidly with- 
drawn through a gate near the kettle bottom controlled by a 
lever above, as shown in Figures 19 and 20, into a fire-proof bin 
on the ground, and the kettle is refilled. 
It is a difficult matter to tell the exact temperatures in a gyp- 
sum kettle. ‘The usual method is to attach a thermometer to a 
long stick, and insert this through a door at the top of the ket- 
tle. After leaving the thermometer in the plaster a few minutes 
it is drawn up and read, but it is difficult to get the reading be- 
fore the mercury column has dropped somewhat. Gypsum 
plaster is a good non-conductor of heat, and it clings to the 
cooler thermometer placed in the mass, and so causes the instru- 
ment to record a lower temperature. 
In some mills along tube thermometer is kept in the plaster, 
with the register projecting out of the kettle. This obviates the 
difficulty of thermometer cooling while it is being read, but the 
plaster adheres to the tube below, and so gives a lower reading. 
In the other mills electrical wires run from the kettle to an 
automatic registering thermometer which is said to give good 
results. These records are filed away, and any objection to the 
plaster may be explained by the temperature record, and so may 
be corrected in other kettles. The expert calciners, however, 
depend more upon the appearance of the plaster in the kettle 
than upon thermometer readings. 
The whole process takes about two and one-half hours, and 
there is a loss of weight in the gypsum of twenty per cent., due 
to the loss of water. Three kettles are usually burned in a day, 
and these require 1400 pounds of the best coal. Poor coals do 
not give sufficient heating power even when used in large quan- 
