MANUFACTURE OF PLASTERS. 33 



commercially as plaster of Paris is usually more finely ground than 

 stucco and is as white as possible. 



Wall-plasters are made by adding not only retarder but also hair 

 (or some other fiber) to calcined plaster. 



Keene's "cement", Parian " cement", etc., are plasters used as 

 hard finishes in buildings. Their properties are due to certain pecu- 

 liarities of their manufacture, for which reference should be made to 

 Chapter IV. 



In the present chapter the manufacture of plaster of Paris, cement 

 plaster, and wall-plaster will be taken up, and followed by a chapter on 

 the properties of the resulting products. The manufacture and properties 

 of the flooring and hard-finish plasters will be discussed together in 

 Chapter IV. 



Manufacture of Plaster of Paris, " Cement Plaster ", and Wall-plaster. 



Though plaster of Paris and "cement plasters" are very distinct 

 so far as properties and fields of use are concerned, their processes of 

 manufacture are so similar .that they will be treated together in this 

 chapter. It will be recalled that in manufacturing plaster of Paris a 

 pure gypsum is used, so that the product sets very rapidly, while in 

 making cement plasters slowness of set is obtained either by using a 

 naturally impure gypsum or by adding a retarder to the material during 

 or after its manufacture. Aside from this difference, and a slight 

 difference in the calcining temperature, which is usually somewhat 

 lower for plaster of Paris than for cement plaster, the methods employed 

 in making the two products are closely similar. 



Two operations are necessary in manufacturing both kinds of plaster: 

 the raw material must be properly calcined and finely ground. The 

 grinding may either precede or follow the burning, for the order of 

 the two operations depends largely upon what calcining process is used. 

 If the burning is carried on in kettles, the grinding is usually done first; 

 but if the burning is carried on in ovens or rotating cylinders, the raw 

 material is necessarily or advisably fed in lumps, and the fine grinding, 

 therefore, follows the burning. In the present chapter the subject 

 will be discussed under the following headings: 



(1) Grinding gypsum and plaster. 



(2) Calcining by the oven process. 



(3) Calcining by the kettle process. 



(4) Calcining by the rotary cylinder process. 



(5) Addition of retarders and acceleration. 



(6) Costs of plaster-manufacture. 



