212 Solidification of Gypsum. 



gypsum, and thus tend to promote that solidification which is so very 

 characteristic of it in the burnt state. 



The experiment which first exhibited the solidifying property of 

 raw gypsum, was well calculated to give the impression that chem- 

 ical decomposition was necessary for the result. I wished to deter- 

 mine how far fresh precipitated carbonate of lime was capable of 

 improving gypsum, (intending subsequently to burn the mixture ;) 

 with this view, pulverized, raw gypsum was placed on a filter, and 

 a cold solution of carbonate of potassa poured over it. The result 

 was the rapid solidification of the crude mineral and an evident di- 

 minution of the alkali. Upon repeatedly returning the same solu- 

 tion through the filter, turmeric paper ceased to indicate the pres- 

 ence of potassa, and reagents showed that sulphate of potassa had 

 taken its place. In this manner, a saturated solution of the latter 

 salt may soon be obtained. Yet, as has been already stated, a fur- 

 ther examination proved that the sulphate of potassa is not capable 

 of contracting a permanent union with the gypsum. 



Further enquiry will, no doubt, lead to the detection of salts better 

 adapted to the developement of this property than those here noti- 

 ced, but the cheapness of carbonate of potassa seems more likely to 

 recommend its use for practical purposes, provided it shall be found 

 that the solidification of raw or effete plaster, by the process here 

 indicated, equals, in durability, that which has been recently burnt. 

 Gypsum, it is well known, requires judicious treatment, in order to 

 fit it for taking casts, and unless carefully defended from moisture, 

 will soon lose its valuable property. The process of burning may, 

 moreover, not always be convenient, and in this case, a solution of 

 carbonate of potassa, or, for common purposes, the ley from wood 

 ashes, will always enable the operator to effect rapid solidification, 

 and, as far as I have observed, it is perfect. 



