FLOORING-PLASTERS AND HARD-FINISH PLASTERS. 77 



lumps of gypsum. The following analyses show the composition of 

 gypsum from Virginia and Kansas, both of which have been used in 

 the preparation of a domestic Keene's cement: 



Kansas. Virginia. 



Lime sulphate 77 . 46 \ 



Water 20 . 46 J 



Iron and aluminum oxides . 10 .036 



Silica and insoluble 0.19 0.116 



Magnesium carbonate . 34 . 221 



Lime carbonate 1 . 43 



It will be seen that both materials are very pure gypsums, and that 

 there is no apparent reason why the Virginia material should not be 

 as satisfactory as that from Kansas. 



The calcination of the product is usually carried on in small ver- 

 tical kilns closely resembling those which are in common use for lime- 

 burning. These kilns are charged with alternating layers of fuel (usu- 

 ally coal) and lump gypsum. Small rotary kilns have been used experi- 

 mentally, but have not proven successful, as the calcined product from 

 a rotary kiln is discharged in small fragments which cannot be treated 

 satisfactorily in the alum-bath. After burning to a red heat the gyp- 

 sum is submitted to the action of a 10 per cent alum solution. It is 

 then recalcined and finally ground in emery-mills. 



The product is a very finely grained white powder. On the addi- 

 tion of water this cement hardens, but the. hardening is slow relative 

 to that of other plasters. Another peculiarity of the material is that 

 even after the hardening has commenced, the partly set cement may 

 be reworked with water and will take its set just as satisfactorily as 

 if the process of hardening had not been interrupted. 



An analysis of a Keene's cement manufactured in Kansas is given, 

 in "Tests of Metals, etc., at Watertown Arsenal, 1897", p. 403: 



Silica (SiO 2 ) tr. 



Alumina (A1 2 O 3 ) tr. 



Iron oxide (Fe 2 O 3 ) 



Lime (CaO) 42.04 



Magnesia (MgO) tr. 



Sulphur trioxide (SO 3 ) 56.54 



Carbon dioxide (CO 2 ) 1 .37 



According to a circular issued by the sales-agent for one brand the- 

 following tests of two brands of Keene's cement were made by Lath- 

 bury and Spackman. The table below shows the tensile strength 

 obtained, in pounds per square inch, at the end of seven days. 



