Plaster 



the water of which it has been deprived by 

 baking, and thus to resume its original 

 condition as stone. It is on this property 

 that the use of plaster depends. When mixed 

 in the bucket the powder quickly absorbs the 

 water restored to it, and hardens into a sub- 

 stance as firm as the gypsum before it was 

 baked. In the case of lime the change is slow, 

 but for plaster it is very quick. 



When baked the plaster is crushed under 

 vertical millstones and then passed through 

 a sieve. The powder must be kept in a very 

 dry place, for it absorbs the damp very 

 readily, and will then no longer harden when 

 mixed with water. It can be easily under- 

 stood that after being more or less saturated, 

 the plaster cannot readily absorb the water 

 which is needed for its conversion into a solid 

 substance. It cannot combine with the water 

 when it is required for use. Damp plaster is 

 quite useless. 



Statues, busts, medallions and other orna- 

 mental objects are prepared from plaster by 

 moulding. This plaster is made with the 

 purest gypsum, with those beautiful trans- 

 parent sheets which have been already men- 

 tioned. It is baked in ovens like those used 

 by bakers, and kept from contact with the 



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