PLASTER OF PARIS, a fissile stone, serving many purposes in building ; and used likewise in sculpture, to mould and make statues, basso- relievos, and other decorations in architecture.
It is dug out of quarries, in several parts of the neighbour hood of Paris; whence its name. The finest is that of Montmartre. See GYPSUM.
Plaster of Paris, amongst our workmen, is of two kinds, viz., crude, or in the stone, and burnt, or beaten.
The crude is the native plaster, as it comes out of the quarry ; in which state it is used as shards in the founda tions of buildings.
The burnt plaster is a preparation of the former, by cal cining it like lime in a kiln or furnace. and then beating it into powder, and diluting and working it. In this state it is used as mortar. or cement, in building.
This, when well sifted, and reduced into an impalpable powder, is used also to make figures, and other works of sculpture; and is, besides, of some use in taking out spots of grease, &c. in stuffs and silks.
The method of representing a face truly in plaster of Paris is this : the person, whose figure is designed to be taken, is laid on his back, with any convenient thing to keep off the hair. Into each nostril is conveyed a conical piece of stiff paper, open at both ends, to allow respiration. These tubes, being anointed with oil, are supportod by the band of an assistant ; then the face is lightly oiled over, and the eves being kept shut, alabaster fresh calcined, and tempered to a thinnish consistence with water, is, by spoonfuls, nimbly thrown all over the face, till it lies near the thickness of an inch. This matter grows sensibly hot, an I about u, in a.mu_ a quarter of an hour, hardens into a kind of stony concretion ; which, being gently taken off, represents, on its concave surface, the minutest parts of the original face. In this a head of good clay may be moulded, and therein the eyes are to be opened, and other necessary amendments made. This second face being anointed with oil, a second mould of calcined alabaster is made, consisting of two parts joined lengthwise along the ridge of the nose; and herein may be cast, with the same matter, a face extremely like the original.
If finely powdered alabaster, or plaster of Paris, be put into a bason over a fire, it will, when hot, assume the appear ance of a fluid, by rolling in waves, yielding to the touch, steaming, &e., all which properties it again loses on the departure of the heat ; and being thrown upon paper, will not at all wet it, but immediately discover itself to be as motionless as before it was set over the fire ; it appears, that a heap of such little bodies as are neither spherical, nor otherwise regularly shaped, nor small enough to be below the discernment of the eye, may, without fusion, be made fluid, barely by a sufficiently strong and various agitation of the particles which compose it ; and, moreover, lose its fluidity immediately upon the cessation thereof.
Two or three spoonfuls of burnt alabaster mixed up thin with water, in a short time coagulate, at the bottom of a vessel full of water, into a hard lump, notwithstanding the water that surrounded it. Artificers observe, that the coagulating property of burnt alabaster will be very much unpaired, or lost, if the powder be kept too long, especially if in the open air, before it is made use of; and when it hath been once tempered with water, and suffered to grow hard, they cannot, by any powdering of it again, make it service able for their purpose as before.
This matter, when wrought into vessels, &c., is still of so loose and spongy a texture, that the air has easy passage through it. Mr. Boyle gives an account, among his experi ments with the air-pump. of his preparing a tube of this plaster, closed at one end and open at the other, and, on applying the open end to the cement, as is usually done with the receivers, it was found utterly impossible to exhaust all the air out of it ; for fresh air, from without, pressed in as fast as the other, or internal air, was exhausted, though the sides of the tube were of considerable thickness. A tube of iron was then put on the machine; so that being filled with water, the tube of plaster of Paris was covered with it ; and, on using the pump, it was immediately seen, that the water passed through into it as easily as the air had done, when that was the ambient fluid. After this, trying it with Venice turpentine instead of water, the thing succeeded very well ; and the tube might be perfectly exhausted, and would remain in that state several hours. After this, on pouring some hot oil upon the turpentine, the case was much altered ; for, the turpentine melting, it became a thinner fluid, and, in this state, capable of passing like water into the pores of the plaster. On taking away the tube after this, it was remarked that the turpentine, which had pervaded and filled its pores, rendered it transparent, in the manner that water gives transparency to that singular stone called oculus fnundi. In this manner the weight of air, under proper management, will be capable of making several sorts of glues penetrate plaster of Paris; and not only this, but baked earth, wood, and all other bodies porous enough to admit water on this occasion.