Emplatres, Fr.; Pflaster, G.; Empiastri, It.; Emplastos, Sp.
Plasters are solid compounds intended for external application, adhesive at the temperature of the human body, and of such a consistence as to render the aid of heat necessary in spreading them. Spread plasters having rubber in their composition are most largely used; they are usually perforated. Some plasters have as their basis a compound of olive oil and litharge, constituting the Emplastrum Plumbi of the Pharmacopoeias. Others owe their consistence and adhesiveness to resinous substances, or to a mixture of these with wax and fats.
In the preparation of the plasters, care is requisite that the heat employed be not sufficiently elevated to produce decomposition, nor so long continued as to drive off any volatile ingredient upon which the virtues of the preparation may in any degree depend. After having been prepared, they are usually shaped into cylindrical rolls, and wrapped in paper to exclude the air. Plasters should be firm at ordinary temperatures, should spread easily when heated, and, after being spread, should remain soft, pliable, and adhesive, without melting, at the heat of the human body. When long kept, they are apt to change color and to become hard and brittle, and, as this alteration is most observable upon their surface, it must depend chiefly upon the action of the air, which should therefore be excluded as much as possible. The defect may usually be remedied by melting the plaster with a moderate heat and adding a sufficient quantity of oil to give it the due consistence.
Plasters are prepared for use by spreading them upon leather, linen, or muslin, according to the particular purposes they are intended to answer. Leather is most convenient when the application is made to the sound skin, linen or muslin when the plaster is used as a dressing to ulcerated or abraded surfaces, or with the view of bringing and retaining together the sides of wounds. The leather usually preferred is white sheepskin, or the kind known commercially as "hemlock splits." A margin about one-fourth or half an inch broad should usually be left uncovered, in order to facilitate the removal of the plaster, and to prevent the clothing in contact with its edges from being soiled. An accurate outline may be obtained by pasting upon the leather a piece of paper so cut as to leave in the center a vacant space of the required dimensions, and removing the paper when no longer needed. The spreading of the plaster is most conveniently accomplished by the use of a spatula or plaster iron. This may be heated by means of a spirit lamp or gas jet. Care must be taken that the instrument be not so hot as to discolor or decompose the plaster, and special care is requisite in the case of those plasters which contain a volatile ingredient. A sufficient portion of the plaster should first be melted by the heated instrument, and, having been received on a piece of coarse stiff paper, or in a shallow tin tray open on one side, should, when nearly cool, be transferred to the leather, and applied quickly and evenly over its surface. By this plan the melted plaster is prevented from penetrating the leather, as it is apt to do when applied too hot. Before removing the paper from the edge of the plaster, if this has become so hard as to crack, the iron should be drawn over the line of junction.
The spreading of plasters has become to a great extent a lost art to the pharmacists of this country, owing to the introduction of machine-spread plasters, which contain India-rubber in the adhesive composition. These are reasonably permanent, and do not require the application of heat. When made by reliable manufacturers they are in many cases to be preferred, but the introduction of immense quantities of worthless ones by unscrupulous makers has caused many practitioners to direct plasters to be spread by the pharmacist from official plaster. The perforation, or "porousing," of plasters is usually accomplished on a large scale by expensive apparatus. J. J. Edmondson read a paper before the Pennsylvania Pharmaceutical Association in 1887 giving the process for manufacturing rubber-mass plasters as carried on by Johnson & Johnson, of New York. It is as follows: The ingredients employed are—rubber, two parts; Burgundy pitch, one part, gum olibanum, one part. This may vary with some special plasters, but they constitute the component parts of the mass used for the majority.
The crude rubber is at first steeped in hot water, to cleanse and soften it, then the rubber is passed through the washer and crusher, where it is subjected to severe pressure between two corrugated rolls, eight inches in diameter and one foot wide, while a stream of water falling continually washes it thoroughly, and it comes out in sheets somewhat softened. After these sheets are dried, which requires a number of days, they are passed through the grinder, where they are crushed between two smooth rollers, fourteen inches in diameter and thirty-six inches across. This thoroughly softens the rubber and makes it plastic, so that it can be readily worked up with the other ingredients. The principal operation, the mixing, then takes place. The medication must be carefully combined, and the whole manipulation so managed that the plaster will be uniform, so that age or varying temperature shall not affect it. This operation is performed by means of rollers, sixteen inches in diameter, arranged so that one revolves at twice the speed of the other. Between these large rollers the mass is passed with whatever medication is required: e.g., when a belladonna mass is to be made, a certain amount of the stock mass is taken and extract of belladonna in the proportions corresponding to the formula in the Pharmacopoeia. These are repeatedly passed through the rollers until the extract of belladonna is thoroughly mixed with the mass, care being taken to keep the temperature from rising high enough to decompose or affect the alkaloids. The spreading is also done by rollers, into which the thoroughly mixed mass is fed at the same time that the cloth is fed, the thickness of the plasters being governed by the adjusting of screws on the side of the machine.
The rubber base is pliable, adhesive, it will not become too hard or too soft, but will yield up the medication to the absorbents of the skin, and its properties will be retained indefinitely. For formulas for plasters of former Pharmacopoeias, see U. S. D., 19th edition, p. 436.