Plaster of Paris, diluted with water into the consistence of a soft or thin paste, quickly sets or grows firm, and, at the instant of its setting, has its bulk increased ; for ill r. Boyle has found, that a glass vessel, filled with the fluid mixture, and closely stopped, bursts while the mixture sets, and some times a quantity of water issues through the cracks.
This expansion of the plaster, in passing from a soft to a firm state, is one of its valuable properties, rendering it an excellent matter for filling cavities in sundry works, where other earthy mixtures would shrink and leave vacuities, or entirely separate from the adjoining parts.
It is probable, also, that this expansion of the plaster might be made to contribute to the elegance of the impres sions it receives from medals, &c., by properly confining the soft'matter, that its expansion may force it into the minutest traces of the figure ; the expansion of the matter doing the same office as the pressure by which the wax is forced into the cavities of a seal.
Plaster of Paris promotes the fusion of forged iron.
This substance is commonly used for taking casts and impressions from figures, busts, medals, &c., as it is adapted to the double use of making both casts and moulds for forming them.
There is also a plaster of a coarser sort than the plaster of Paris, which is sometimes used in this country for floors in gentlemen's houses, and for corn-granaries: it is made of a blueish stone, taken out of quarries, which are generally at the side of a hill, much like the stone of' which Dutch terms is made ; the stone is burnt like lime, becomes white by burning, and, when mixed with water, does not ferment like lime : when cold, it is beat into a fine powder ; and, when used, the quantity of about a bushel is put into a tub, and water applied to it till it becomes liquid : in this state it is well stirred with a stick, and used hninediately ; for, in less than a quarter of an hour, it becomes hard and good tor nothing, as it will not bear being mixed a second time, like lint e.