REPORT OF THE DIRECTOR AND STATE GEOLOGIST 1903 129 



to about the size of a liickon' nut. The lower part breaks them 

 up to about the size of peas, after which the fragments are ele- 

 vated to a bin on the floor above. From this bin material is 

 drawn through spouts to the burstones, which are usually on the 

 same floor as the crusher. Other processes of grinding the 

 gypsum have been introduced in other states, but so far the im- 

 provements have not been introduced into the mills of this State. 



After the gypsum has been ground, it is elevated to the storage 

 bin, which is located on the top floor of the mill, so that the 

 kettles may be filled from it by merely opening a gate and letting 

 the plaster run by gravity from the bin. The kettles oonsist of 

 cylinders of boiler steel, usually 8 feet in diameter and 6 o'r 8 

 feet deep. Such a kettle is estimated to hold 10 or 12 ton®, 

 though usually the charge is not more than 8 tons. 



For greater strength the bottom of the cylinder is convex 

 upward and is thicker than the sides. The kettle is placed on a 

 brick foundation and is enclosed in a brick jacket with an air 

 space betw^een, which serves as a flue. Two flues pass through the 

 kettle, so that more heat is saved on account of the greater surface 

 exposed to the heat. The gypsum is stirred from the time it is 

 first put into the kettle till the charge is withdrawn by the two 

 arms near the bottom of the kettle, which are fastened to a ver- 

 tical shaft, which is set in motion by a pinion wheel. The plaster 

 is fed gradually into the kettle, because, if it runs too rapidly, 

 the gearing may be broken and the plaster burned on the bottom. 

 It takes about an hour to fill the kettle, and in a short time the 

 temperature of the plaster reaches the boiling point of water, and 

 part of the water of crystallization begins to pass off. When the 

 temperature reaches about 235° F., the mass boils furiously, and 

 the temperature gradually increases to about 260° F., wh^ai' the 

 vapor ceases to pass off and the plaster settles into a solid mass. 

 When the temperature rises to about 290° F., the boiling begins 

 again, and the plaster continues to give off water till the tem- 

 perature of 350° F. is reached, when it settles again. At this 

 point a gate is raised, the plaster rapidly discharged into a bin 

 on the ground and the kettle again refilled. 



