PLASTER, or Emplaster, in Pharmacy, an external application, of a harder consistence than an ointment, and which is, according to the different circumstances of the wound, place, or patient, spread either upon linen or upon leather.

Plaster, or Plaister, in building, a composition of lime, sometimes with sand, or other substance, to parget or cover the nudities of a building.

PLASTER of Paris, a preparation of several species of gypsum dug near Montmartre, a village in the neighbourhood of Paris, and hence the name.

The best sort is hard, white, shining, and marbly, being known by the name of plaster-stone or parget of Montmartre. It will neither give fire with steel, nor ferment with aquafortis; but it calcines very freely and readily into a fine plaster, the use of which in building and casting statues is well known.

The method of representing a face truly in plaster of Paris is this. The person whose figure is designed 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 of respiration. These tubes being anointed with oil, are supported by the hand of an assistant; then the face is lightly oiled over, and the eyes being kept shut, alabaster fresh calcined, and tempered to a thinnisħ 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, and in about a quarter of an hour hardens into a kind of stony concretion; which being gently taken off, represents, on its concave surface, the minutest part 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 basin over a fire, it will, when heated, assume the appearance of a fluid, by rolling in waves, yielding to the touch, and steaming, all which properties it again loses on the departure of the heat; and being thrown upon paper, it will not at all wet it, but immediately discover itself to be as motionless as before it was set over the fire; wherefore 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 by a sufficiently strong and various agitation of the particles which compose it, and may lose its fluidity when the agitation is discontinued.

Two or three spoonfuls of burned alabaster, mixed up thin with water, in a short time coagulate into a hard lump at the bottom of a vessel full of water. Artificers observe, that the coagulating property of burned alabaster will be very much impaired or lost if the powder be kept too long, especially in the open air, before it is made use of; and when it has been once tempered with water, and suffered to grow hard, they cannot, by any new burning or powdering of it, make it serviceable for their purpose as before.

This matter, when wrought into vessels, or other forms, is still of so loose and spongy a texture, that the air finds an easy passage through it. Mr Boyle, in relating his experiments[7:17:797] with the air-pump, gives an account 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 internal air was exhausted, although the sides of the tube were of considerable thickness. A tube of iron was then put on the engine, 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; the tube might be perfectly exhausted, and would remain in that state for several hours. But on pouring some hot oil upon the turpentine, the case was much altered; for the turpentine melting, became a thinner fluid, and in this state was capable of passing like water into the pores of the plaster. On taking away the tube, however, 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 mundi. In this manner, the weight of air, under proper management, becomes capable of making several sorts of glues penetrate plaster of Paris; and not only so, but also baked earth, wood, and all other bodies porous enough to admit water.