Plaster-masses are intended to be spread upon leather, calico, or other suitable material, the plaster so produced being designed either to maintain a medicinal substance in close contact with the skin, to act as a protective, or to assist in the approximation of the edges of wounds. In spreading plasters, the mass should be melted over a water-bath and precautions taken to avoid destroying alkaloidal principles or dissipating volatile matters by excess of heat. The plaster-mass should be spread as thinly as possible upon the supporting material, a margin of the material at least half-an-inch wide being left bare, to avoid extension of the mass over the edge of the plaster.
Plasters having a basis of lead, resin, or soap plaster require to be warmed before application to the body; those having an india-rubber basis adhere closely by the warmth of the body. Plasters intended to raise a blister are made soft in order that they may be easily removed from the vesicated surface; blistering plaster-masses are usually spread upon adhesive plaster, the edges of the latter being warmed before application to the skin. For use in minor surgery and as protective agents, plasters are prepared spread with a solution of isinglass, the usual backings being felt, of various thicknesses, linen, muslin, and silk; such plasters require to be moistened with warm water before application.