Plasters are designed to be applied upon the skin or surface of the body; they are of much thicker consistence than cerates, require a certain degree of heat to soften them sufficiently for spreading, and are very adhesive when applied to any part of the body. They are intended to fulfil four indications, viz.: to give mechanical support or pressure to certain parts; to hold cut surfaces in contact; to protect parts from atmospheric action; and to produce sedative, stimulant, or other therapeutical influences, according to the nature of the medicines associated with them. They are most usually composed of resins combined with wax, fats, and other substances, and frequently in combination with the oleo-stearate and palmitate of lead or lead plaster as a basis. Plasters should be prepared in some metallic vessel, as tin, or iron, which, in consequence of the uprising many articles undergo when heated, thereby augmenting their volume, should be considerably larger than required to hold the components of the plaster when in an unmelted condition. The heat should be continued no longer than is necessary to effect the proper amalgamation of the ingredients; and those of a volatile character should be added at as late a period during the cooling of the plaster as is consistent with their intimate combination with it. After having melted the wax and resinous substances, etc., together they should be strained while hot, to remove impurities, and, as the several articles required to form the plaster are added, the mass should be well stirred. Some plasters require to be stirred constantly till cold, while others are poured into cold water during their melted state, and worked by the hands, kneading them until nearly cold, and then forming them into cylindrical rolls, or sticks of various dimensions and shapes to suit the views of the operator. The cylinders are usually made by rolling portions of the plaster on a hard, smooth surface, kept constantly wet during the operation. If some of the components of a plaster are soluble in water, the plaster should not be worked in this fluid, but be allowed to cool, either in the vessel in which it is prepared, or in pans or cylinders made for the purpose. As the action of the air exerts an influence upon plasters, it is advisable to cover them with paper or tin-foil, in order to protect them as much as possible against this influence; and they should always be kept in dry and cool situations. Plasters should be hard and not adhesive, at atmospheric temperatures, and should not become too soft, but remain flexible and tenacious when exposed to the natural heat of the body. When a plaster softens under ordinary atmospheric warmth, it should be remelted, and more resin, or other of its solid constituents be added; if it is too firm, not being readily spread at a moderate heat, or not sufficiently adhesive when in contact with the body, a sufficient quantity of olive oil should be added upon remelting it.

Plasters are spread upon various materials in accordance with the object for which they are used. If they are designed to act as mechanical supports, to exclude atmospheric air, etc., white sheepskin is the best material; if they are to be applied to ulcers, to surfaces exposed by the removal of the skin, or to wounds for the purpose of holding the divided surfaces in close contact with each other, some softer material may be used, as muslin, etc. Sometimes oiled silk, or india-rubber cloth is employed, and where economy is desired, they are spread on stout paper. After cutting the leather somewhat larger than the size desired, paste strips of paper, about ½ inch wide, along the edges of the leather, and after having spread the plaster within the space which they inclose, remove them. The plaster should be spread thinly and evenly, always leaving an unspread edge or border, ½ inch wide, which serves to protect the linen worn over it from adhering to it. There are various modes of spreading the plaster; some melt the plaster in a suitable vessel over a gentle fire, and spread it by means of a common spatula; others use an iron instrument made expressly for the purpose, which, when properly heated, they apply to the plaster; as this melts, the fused portion is dropped upon various parts of the leather, and the spreading is accomplished by carefully passing the same heated iron over the surface, carrying portions of the melted plaster along with it. Care must be taken not to heat the irons employed in spreading to too great a degree, else certain parts of the plaster may become volatilized or decomposed. When it is desired to obtain large quantities of plasters, they are spread by machines made for the purpose, and these factory-made spread plasters are now such favorites as to have practically displaced those of the pharmacist.

Adhesive material spread upon silk, muslin, or paper, makes adhesive and court plasters, of which the official Emplastrum Capsici and Emplastrum Ichthyocollae are examples. Such preparations are known in Europe under the name sparadrap. Porous plasters are those which have been closely perforated by means of a metal wheel beset with punches. Elegant plasters are now manufactured on a large scale by specialists who use rubber and other plaster bases.

The following plasters containing mercury are official:

EMPLASTRUM AMMONIACI CUM HYDRARGYRO (U. S. P.), Ammoniac plaster with mercury.—"Ammoniac, seven hundred and twenty grammes (720 Gm.) [1 lb. av., 9 ozs., 174 grs.]; mercury, one hundred and eighty grammes (180 Gm.) [6 ozs. av., 153 grs.]; oleate of mercury, eight grammes (8 Gm.) [123 grs.]; diluted acetic acid, one thousand cubic centimeters (1000 Cc.) [33 fl℥, 391♏︎]; lead plaster, a sufficient quantity to make one thousand grammes (1000 Gm.) [2 lb. av., 3 ozs., 120 grs.]. Digest the ammoniac with the diluted acetic acid, in a suitable vessel, avoiding contact with metals, until it is entirely emulsified; then strain, and evaporate the strained liquid by means of a water-bath, stirring constantly, until a small portion, taken from the vessel, hardens on cooling. Triturate the oleate of mercury with the mercury gradually added, until globules of the metal cease to be visible. Next add, gradually, the ammoniac, while yet hot; and finally, having added enough lead plaster, previously melted by means of a water-bath, to make the mixture weigh one thousand grammes (1000 Gm.) [2 lbs. av., 3 ozs., 120 grs.], and mix the whole thoroughly"—(U. S. P.). This plaster frequently excites an eczematous eruption, and may produce ptyalism. It is more active than plaster of mercury, and is used to discuss syphilitic swellings. It is not employed in Eclectic practice.

EMPLASTRUM HYDRARGYRI (U. S. P.), Mercurial plaster.—"Mercury, three hundred grammes (300 Gm.) [10 ozs. av., 255 grs.] oleate of mercury, twelve grammes (12 Gm.) [185 grs.); lead plaster, a sufficient quantity to make one thousand grammes (1000 Gm.) [2 lbs. av., 3 ozs., 120 grs.] Triturate the mercury with the oleate of mercury in a tared capsule until globules of metal are no longer visible. Then place the capsule on a water-bath, add enough lead plaster, previously melted, to make the contents weigh one thousand grammes (1000 Gm.) [2 lbs. av., 3 ozs., 120 grs.], and mix the whole thoroughly"—(U. S. P.). Used as a counter-irritant and stimulating discutient to remove syphilitic nodes, glandular swellings of syphilis, syphilitic engorgements of the liver, and splenic enlargements of malarial origin. May produce sore gums and ptyalism. Not employed in Eclectic medicine.

The U.S. P. of 1880 directed the following plasters, the amounts being those in the metric system as given in the National Formulary:

EMPLASTRUM AMMONIACI (N. F.) (U. S. P., 1880), Ammoniac plaster.—Formulary number, 116: "Ammoniac, one hundred grammes (100 Gm.) [3 ozs. av., 231 grs.]; diluted acetic acid, one hundred and forty cubic centimeters (140 Cc.) [4 fl℥, 352♏︎]. Digest the ammoniae with the diluted acetic acid, in a suitable vessel, avoiding contact with metals, until it is entirely emulsionized; then strain and evaporate the strained liquid, by means of a water-bath, stirring constantly, until a small portion, taken from the vessel, hardens on cooling"—(Nat. Form.).

EMPLASTRUM GALBANI (N. F.) (U. S. P., 1880), Galbanum plaster.—Formulary number, 120: "Galbanum, sixteen grammes (16 Gm.) [247 grs.]; turpentine, two grammes (2 Gm.) [31 grs.]; burgundy pitch, six grammes (6 Gm.) [92 grs.]; lead plaster, seventy-six grammes (76 Gm.) [2 ozs. av., 298 grs.]. To the galbanum and turpentine, previously melted together and strained, add, first, the burgundy pitch, then the lead plaster, melted over a gentle fire, and mix the whole thoroughly"—(Nat. Form.).

Galbanum plaster of the British Pharm. consists of 1 ounce (av.) each of galbanum, ammoniacum, and yellow wax, and 8 ounces (av.) of lead plaster. The galbanum and ammoniacum are melted together and strained, and subsequently the wax and lead plaster, also melted together, are added, and the whole mass thoroughly mixed together.

EMPLASTRUM GALBANI RUBRUM, or EMPLASTRUM OXYCROCEUM.—The German Pharmacopoeia, of 1872, directed the following plaster, much employed by the German people in America: Melt together, with moderate heat, 6 parts each of resin, burgundy pitch, and yellow wax; strain; now add to 3 parts of turpentine, 2 parts each of powdered ammoniac and galbanum. Add this solution to the first part. Next add to the whole mixture 1 part of powdered saffron, and 2 parts each of powdered mastic, olibanum and myrrh, all previously intimately mixed, and mix the whole mass together. An excellent stimulating plaster for local affections.

EMPLASTRUM PICIS CANADENSIS (N. F.) (U. S. P., 1880), Canada pitch plaster.—Formulary number, 121: "Canada pitch, ninety grammes (90 Gm.) [3 ozs. av., 76 grs.]; yellow wax, ten grammes (10 Gm.) [154 grs.]. Melt them together, strain the mixture, and stir constantly until it thickens on cooling"—(Nat. Form.).

The following plasters are sometimes used:

EMPLASTRUM CERUSSAE (Emplastrum album coctum), White lead plaster.—Lead plaster, 60 parts; olive oil, 10 parts. Melt together. Add lead carbonate (in fine powder), 35 parts. This, with the addition of a little water, as needed, is to be boiled until a plaster consistence is obtained. This is a German official plaster.

MAHY'S PLASTER resembles the preceding, excepting that it contains, in addition, powdered orris root and a small amount of wax.

King's American Dispensatory, 1898, was written by Harvey Wickes Felter, M.D., and John Uri Lloyd, Phr. M., Ph. D.