This section is from the book "Dental Medicine. A Manual Of Dental Materia Medica And Therapeutics", by Ferdinand J. S. Gorgas. Also available from Amazon: Dental Medicine.
Caustic Potash is prepared by boiling a solution of potash until ebullition ceases, and the potassa melts or assumes a solidified consistence, when it is poured into suitable moulds and kept in well-stoppered bottles, as it rapidly deliquesces when exposed to the air. It dissolves in water and alcohol, and attracts moisture. Its officinal form is that of sticks of a white and somewhat transparent color, but upon exposure to the air, or if it is impure, it becomes a dingy gray, greenish or bluish color, and has the odor of slaking lime. When it is digested in alcohol, so as to free it from such impurities as are insoluble in alcohol, it is called alcoholic potassa.
Caustic Potash is the most powerful caustic and escharotic in use, and, when taken internally, is a corrosive poison. It is only employed externally. When applied to a part, it rapidly destroys its vitality to a considerable depth, differing in this respect from nitrate of silver (lunar caustic), as the latter is more limited in its action, and does not liquefy when it comes in contact with the tissues. From the penetrating action of caustic potash, it is necessary to use it with great care. It is very deliquescent, which is a great objection to its use in some cases, but when mixed with an equal quantity of lime, the deliquescent action is in a measure prevented; it is then known as potassa cum cake - potassa with lime, and is in the form of a grayish-white powder, which is sometimes made into a paste, under the name of Vienna Paste, which is milder and less deliquescent.
Caustic potassa is chiefly employed to open abscesses, and in the treatment of chancres, hospital gangrene, eczema, malignant growths, to arrest the sloughing of carbuncles; in tetanus, applied to the spine; bites of rabid animals and venomous reptiles; phlegmons and incipient carbuncles, to arrest their progress ; to form issues, etc. To prevent its coming in contact with neighboring parts, a piece of adhesive plaster is used, with an opening corresponding in size to the surface on which the caustic is to act. When mixed with water, in the proportion of caustic potash, to water,gangrene of the mouth (cancrum oris), malignant growths, fungous growths of gum, ulcers, etc.; for opening abscesses, when it is not prudent to use the lancet.