26 KANSAS UNIVERSITY QUARTERLY. 
The kettle is heated to about 240° F., before filling with the raw 
material, and this temperature is gradually increased to 340° F., as 
evaporation progresses. At the latter temperature the material 
stops the apparent boiling and sett/es down solid, leaving twelve to 
sixteen inches of vacant space at the top and the steam ceases to 
rise. . After about one-half hour the material comes up again and 
then at a certain temperature, which is a trade secret, the whole 
mass is rapidly withdrawn through a gate near the kettle bottom 
and the plaster runs into a fire-proof bin on the ground, ©The ket- 
tle is then refilled, so that three kettles are usually burned in a day, 
and these require about 1400 pounds of coal. 
During the process of calcination a retarder is mixed with the 
gypsum, the object of which is to form a plaster which will not set 
as quickly as the natural plaster of Paris. The latter sets in about 
ten minutes, while the retarder usually is added in sufficient amount 
to make it set in two hours, or in extreme cases in twenty-four 
hours. Various substances are used for this purpsse. Citric acid 
was formerly used with about two pounds to the ton of plaster, but 
the effect was apt to be very uneven, Magnesian limestone has 
been used with poor results. Sours and sweets form the worker’s 
rule for retarders. Sorghum has been’ used with success; glue 
water was long in favor, but now they usually use patent retarders, 
known as Iola and Webster City retarders, and about fifteen pounds 
are used to the ton. 
After the hot plaster passes from the kettle to the eround bin it 
remains about an hour to partially cool, and then it is raised to the 
second story and 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, usually about one 
per cent in amount, are carried back to the buhr mill and reground. 
The fine plaster is then run into 100 pound sacks or 250 pound 
barrels and is ready for shipment. 
The advantages of wall plaster, made from gypsum, are outlined 
in the circulars of the various companies as follows: Being a per- 
fect non-conductor of heat it is valuable for protection of iron joist, 
elevator shafts, and stairs; it sets and the walls dry out much 
quicker than lime work, so that carpenters can follow the plasterers 
almost immediately, as also the painters and paper hangers; any 
color can be mixed with the material in its preparation for mortar 
to produce any tint desired, and it does not affect coloring material 
as lime does; it requires less mortar than other materials; ceilings 
