130 NEW YORK STATE MUSEUM 



The plaster is now bolted or screened, and the coarse particles 

 are reground in burstone mills. 



The Cummer process. The first stage in this process is the 

 same as the first stage of the kettle process: i e, crushing in a 

 Blake crusher to about the size of a hickory nut. Usually a 

 nipper is not used; but the rock is screened after being crushed, 

 and pieces that are larger than the desired size are returned to 

 the crusher and broken again. In some mills gyratory crushers 

 are used, and, where a large quantity is to be crushed, these aa^ 

 probably more economical than the jaw crushers. 



The crushed rock is elevated by a bucket elevator to the storage 

 bin, from which it is drawn by gravity to the calciner, which con- 

 sists of a cylinder of boiler steel about 27 feet long and 4 feeit in 

 diameter, which 'i^evolves over the fire. The fire is usually fed by 

 self-stoking machinery, and the smoke passes through the cylinder, 

 so that as much heat may be saved as possible. The cylinder is 

 inclined slightly, so that the material rolls from the inlet to the 

 other end, where it is taken by a bucket elevator to the cooling 

 bin. When the material in the bin becomes cool enough, it is 

 withdrawn through gates at the bottom and conveyed to the mills 

 in which it is ground. In the plants using the Cummer process, 

 emery mills are used in place of burstones, because they do not 

 require redressing so often. After passing through the emery 

 mills, the plaster is screened and the larger particles are re- 

 turned to the mill to be reground. 



TECHNOLOGY OF GYPSUM 



Though, as heretofore stated, the use of gypsum aiS a cement- 

 ing material was known at an early date, and its application to 

 the soil as a fertilizer was also known, yet the most of the 

 properties of this material, together with its chemical composi- 

 tion, were unknown and had never received any investigation. 

 The discoverer of the composition of gypsum was Lavoisier, who 

 in 1765 published the results of his experiments in the Proceed- 

 ings of the Acad^mie des Sciences. After decomposing the 

 gypsum rock and discovering the chemical composition, Lavois- 

 ier produced gypsum by synthesis. 



