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GYPSUM. 
Gypsum is a soft, white mineral, composed of hydrated 
calcium sulphate (CaS0 4 -2H 2 0). The names terra alba and 
land plaster are also applied to it. It occurs usually in beds, 
often of great thickness. 
The transparent crystallized or foliated variety is called 
selenite, and the fine fibrous, opalescent variety, satin spar. 
When the massive variety is of very fine texture and translu 
cent it is called alabaster. 
Gypsum is frequently coloured by the presence of impurities, 
especially the massive varieties. 
USES. 
The principal use of gypsum is for the manufacture of 
plaster of Paris, which consists of partially dehydrated gypsum. 
On heating finely powdered gypsum, within certain limits of 
temperature, it gives off part of its water of crystallization but 
retains the power of again taking up a like quantity of water, 
and, at the same time, forming into a solid mass. 1 This prop 
erty of the calcined gypsum or plaster of Paris finds for it 
many uses in the arts and trades. A partial list of the uses is 
as follows:—wall plaster and decorations, moulds and pat 
terns for various purposes, casts of art objects, etc., surgical 
and dental purposes, and as a cement. It is also the base of 
alabastine, used for tinting walls. 
In the manufacturing of portland cement, gypsum is in 
troduced into the cement for the purpose of regulating the 
rapidity of setting when mixed with water. Some cement 
mills purchase the gypsum ground very finely, while others 
purchase it in lump form or crushed to inch. As a rule a 
minimum of 36 per cent of sulphur trioxide (SO3) is demanded. 
Considerable quantities of ground gypsum and plaster of 
Paris are used by asbestos manufacturers in the manufacturing 
of pipe and boiler coverings, mill board, etc. 
1 For technology of Gypsum see “ Gypsum in Canada”, by L. H. Cole, No. 245, Mine* 
Branch.