(From the same). Plaster. Plasters are compositions for external use: they are not always applied for any medical virtue; but chiefly used to retain other dressings, or to keep the parts to which they are applied warm and tight. These effects they produce more equally and steadily than a bandage of linen, especially if there be no swelling. They are composed of oily and unctuous substances, united with powders, into such a consistence, that the compound may remain firm in the cold without sticking to the fingers; that it may be soft and pliable in a gentle heat; and that, by the warmth of the human body, it may be so tenacious as readily to adhere. When a plaster is softened to the consistence of warm wax, it is called cerate; though the term is generally confined at present to such plasters as contain wax in their composition: when so soft as to spread easily whilst cold, yet not to run with the heat of the body, an ointment; and if betwixt the consistence of an ointment and oil, a liniment.
Calces of lead boiled with expressed oils unite into a plaster of a good consistence, and are a proper basis for several other plasters. Plasters may also be made of resins, gummy resins, etc. without wax, especially in extemporaneous prescription; but for officinal compositions they are less proper, as they soon grow too soft in keeping, and lose their form in a warm air.
As some difference is observed in the hardness of a plaster for the breast or stomach, and one that is to be applied to the limbs, the following proportions are generally directed. For a soft plaster, take one ounce of expressed oil, one ounce of wax, and half an ounce of any powder; for a harder, add an ounce more of wax, and half an ounce of powder.
M. Deyeux, in the 33d volume of the Annales de Chimie, has added some chemical refinements with respect to this officinal preparation, which merit our attention. He considers plasters to consist merely of the union of oil, or a mixture of oil and wax with metallic oxides. Those in which these substances are united with vegetable juices he styles ointments.
The union of oils with metallic oxides he supposes to be a true chemical combination, which some pharmaceutical authors have styled soaps; he thinks without reason, as they are neither soluble in water nor alcohol. In proof of the chemical union, he adds, that some metallic oxides, particularly those of iron, refuse to unite with oils; for, though they apparently mix, yet, when diluted, the oxide separates, which is by no means the case with similar combinations. The oxides of lead, bismuth, and mercury, unite with oil, though not with equal facility.
There are three methods of uniting oil with metallic oxides: the first is by agitation, without heat. In this way the oxide of lead combines with oil; but the operation is slow and laborious: and, though the union appears to be complete, the plaster never attains a proper consistence. The second is' to boil the oxide and oil with water, and in this case the water acts only as a balneum mariae, to facilitate the union, by bringing the particles of the oil and oxide together in an attenuated state. By this process we usually obtain a plaster of a proper consistence. The third is the common method of uniting the oxide with the heated oil by agitation. Plasters, thus made, are always dark in their colour, and exhale a peculiar odour, which distinguishes them. In every process the oxide should be in the minutest state of division, particularly in the last, since the metal would be otherwise revived, which sometimes happens, particularly in a saturnine ointment, called by the French pharmaceutists, unguentum de la mere; an event in part owing to the large proportion of animal oils which it contains. The only method of avoiding this inconvenience is to hasten the union, which is best effected by a minute division of the oxides. Though all oils unite with metallic oxides, the results are different. With oxides of lead, for instance, particularly litharge, linseed oil unites freely, and softens by the heat of the hand only: while, with olive oil, it is so dry as to admit of being powdered, and must be heated to be properly spread. In general, drying oils afford the softest plasters; and olive oil, digested with the root of althaea, gives a softer ointment than it would have done, previous to the process. Those oils which are not drying are preferable; but the olive oil, generally sold, is seldom uniform in its properties.
The metallic oxides differ perhaps as much as the oils. Litharge affords drier plasters than minium or the white oxides of lead. Other oxides may unite readily with oil; but a sufficient number of experiments has not yet been made. M. Deyeux suspects that the very pure red oxide of mercury, if finely powdered, so as to prevent its being deoxydated, might advantageously supersede the oxide of lead in many plasters.
When plasters, from age, become too dry, they must be moistened with a due proportion of oil; but, in general, the proportion of oil in those liable to become brittle by age is too small. The access of air often changes the colour of plasters, and very probably their qualities; so that they should be carefully guarded from it, especially those subject to such a change.
Emplastrum adhaesivum nigrum. The black
S Court Plaster, and the
Dissolve twelve ounces of gum benjamin in twelve ounces of rectified spirit of wine: in a separate vessel dissolve a pound of the best isinglass in five pints of pure water; strain each solution; mix them, and let them stand in a narrow vessel, that the grosser parts may subside: when the clear liquor is cold, it will form a jelly; and it must be warmed when spread. This quantity suffices for covering ten yards of half-yard-wide silk: the silk must be stretched in a frame, and the mixture spread on it with a brush. As each spreading dries, it must be repeated to the tenth or twelfth time; and a gloss is obtained by a light touch of the brush at the last operation.