50 



CEMENTS, LIMES, AND PLASTERS. 



80 C. Between the forewarmer and the chimney the dust-chamber is 

 located. Here all of the finer particles are allowed to settle and the 

 air passes on to the chimney practically free from dust. To calcine 

 one ton of gypsum by this method experience has demonstrated that 

 on the average only 100 Ibs. of rather inferior bituminous coal are required. 

 An automatic recorder indicates constantly the heat of the rotary 

 cylinder, and this, with the mechanical stoker, insures an even tempera- 

 ture during the entire process of calcining. From the rotary cylinder 

 the gypsum is again elevated to the floor above and passes through a 

 spiral conveyor which is surrounded by a water-jacket. Here the plaster 

 is cooled and passes on to the sieves. That portion of the plaster which 

 does not need further grinding is separated by the sieves and the rest 

 goes to the vertical mills." 



The process shows economy in fuel, labor, and power over the older 

 methods. On the other hand, "a limited amount of soot settles in the 

 plaster, and it is slightly coated with calcium sulphide, due to the reac- 

 tion on the gypsum of the sulphur present in the coal. For ordinary 

 building purposes, however, these do not injure the plaster." 



Addition of retarders and accelerators. It is now the common prac- 

 tice for plaster manufacturers to add retarders to their product, in order 

 to prevent its setting too rapidly for the convenience of the workman. 

 The general discussion of the character and effects of retarders and 

 accelerators can best be taken up in the following chapter. At present 

 it is only necessary to state that the retarder is best added after the 

 plaster is entirely cool, as otherwise most retarders will melt and form 

 lumps in the plaster. From 2 to 15 Ibs. of retarder are usually added 

 to the ton of plaster, and the mixing is generally accomplished in a 

 Broughton mixer or some similar device. 



The following data relative to the Broughton mixer, which is shown in 

 Fig. 16, are taken from the catalogue of the Des Moines Manufacturing 

 and Supply Company, and will serve to give some idea of the capacity, 

 cost, etc., of the machine. 



TABLE 7. 

 SIZES, CAPACITY, ETC., OF BROUGHTON MIXERS. 



