GRIMSLEY. | Technology of Gypsum. 113 
which is tedious and expensive and requires unnecessary time 
and space. 
The horizontal conveyors which seem to give the best satis- 
faction are the belt conveyor and the auger conveyor, appliances 
similar to those now extensively used in the great milling cen- 
ters. 
USE OF RETARDERS. 
During the process of calcination, about one-half hour before 
the operation is completed, some manufacturers add a retarder, 
in order to hold back the time of setting of the plaster. Plaster 
made from gypsum rock will set in six to ten minutes, and re- 
tarders are added in sufficient amount to hold back the set for 
two to six hours, or in extreme cases for twenty-four hours, in 
order that the workmen may have time to spread the plaster on 
the wall and properly trowel it down. Various substances have 
been used for this purpose. In the earlier days of cement 
plaster glue water was added by the workman as he used the 
material; but this was troublesome, and often resulted in poor 
work from neglect to use the right amount or failure to thor- 
oughly mix the parts. The trade demanded a plaster already 
retarded in a way that would give uniform results. 
Citric acid was used for a time, in the proportion of about 
two pounds to the ton of plaster, but this was expensive, and 
the results were often uneven. Magnesian limes are known to 
set much more slowly than the pure calcareous limes, so it was 
thought that the addition of such magnesian limestone would 
retard the plaster of Paris, but this proved a failure. Sours 
and sweets form the worker’s rule for retarders, and sorghum 
formed a cheap and successful retarder. 
At the present day patent retarders are used, known as Chal- 
lenge, Iola, Webster City, etc., and they are used commonly in 
the proportion of six to fifteen pounds to the ton. In the old 
days of the Romans, blood was used to retard the set of plaster 
of Paris, and to-day the organic material of tankage from 
packing-houses is found to bring about the desired results. 
The writer has in his possession copies of patent specifications 
of nearly fifty of these patent retarders and a few are given here 
