GYPSUM 411 



is occasionally throxsTi in waves out of the kcttlo. 'i'lie 

 heated material boils like thick cream, and runs almost like 

 water. The boiling is, of course, due to the escape of the 

 water of crystallization and the hygroscopic water that may be 

 present. The boiling takes about throe hours; a short time 

 before completion, when the temperature reaches 27(3° F., 

 there is a sudden setting down and the steam ceases to rise, 

 markmg the period of formation of the first h}drate. Soon the 

 mass begins to boil again; after a certain time, quite accurately 

 determined by the expert calciner, when a temperature of aj)- 

 proximately 350° F. is reached, the charge is done, the dis- 

 charge gate is opened, and the finished product nnis out into 

 a vault. The refilling of the kettle for the succeeding charge 

 begins at once. 



As plaster of Paris, when mixed with water, sets in from 

 six to ten minutes, material is incorj^orated therewith, either 

 during its manufacture or at the time of its preparation for use, 

 to delay or hold back the set, in order that the cement })laster 

 can be prepared in quantity and applied with unifonn results. 

 These compounds or admixtures are known as retarders or re- 

 strainers. In the early days of cement plaster, glue was added 

 by the workman when he prepared the material for use; but 

 now to secure uniform results, the retarder is, as a nile, incor- 

 porated with the plaster in the process of manufacture. Citric 

 acid was used for a time, but this was expensive, and the re- 

 sults were often uneven. As magnesian limes set more slowly 

 than calcareous limes, it was thought that the addition of mag- 

 nesian limestone would serve as a retarder, but the use of this 

 substance is said not to be a success. In the days of the Uo- 

 mans, blood was used to retard the set of plaster of Paris, and 

 organic matter from slaughterhouse refuse (tankage, bones, 

 hair, etc.) forms the base of many of the retarders now in use. 



Any substance which, when added to the water with which 

 calcined gypsum is mixed, or to the diy plaster, will keep the 

 molecules apart or from too close contact will delay the cr>'s- 

 taUization and retard the setting. Such substances are dirt 

 or organic matter not of a crystalline character. Insoluble 

 material or carbonates of the alkaline earths will dilute the 

 plaster so that it will not set as quickly as the unadulterated 



