This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
"When the active principles of vegetable drugs are exhausted by boiling with water, decoctions are obtained. Such preparations are obviously not adapted to drugs, the activity of which depends upon principles of a resinous nature, which are insoluble in water, nor to such as contain volatile oils or other volatile substances which would be dissipated with the vapor of water.
"Formerly, decoctions were usually made by using a large quantity of water and boiling it down to one-half or even to a less amount. No obvious advantage was gained by this method, and in many instances it proved to be decidedly disadvantageous, owing to the alteration and darkening of the extractive matters, and in some cases to changing the nature of the active principles". - N. D.
Take of the substance, coarsely comminuted, 10 parts, water a sufficient quantity to make 100 parts. Put the substance into a suitable vessel provided with a cover, pour upon it 100 parts of cold water, cover it well and boil for 15 minutes; then let it cool to about 45 ° 0. (113° F.), strain the liquid, and pass through the strainer enough cold water to make the product weigh 100 parts.
"The use of cold water to begin with ensures the complete exhaustion from the drug of all its soluble principles, by the gradually-heated water, the albuminous principles being subsequently coagulated as the heat is increased to near the boiling point. If, on the other hand, the drug be at once immersed in boiling water, the albumen contained in cells would be coagulated, and thus seriously interfere with the extraction of the other principles. In preparing compound decoctions all the drugs may be added to the cold water, with the exception of those which are injured by long-continued heat, or which contain aromatic or other volatile principles. Such should be added when the decoction is ready to be removed from the fire or steam-bath, and allowed to digest until it is sufficiently cooled for straining. The material should in all cases be cut or bruised, the degree of fineness depending upon the nature of its tissue.
"Unless the liquid is to be considerably boiled down, decoctions are best prepared in a vessel provided with a cover, which may be loosely put on until the boiling is completed, when the vessel should be well closed, particularly if additions have been made at the close of boiling. Porcelain is undoubtedly the best material for vessels used for preparing decoctions, since it is not acted upon by the various vegetable principles; for similar reasons glass flasks will answer a useful purpose in making small quantities of these preparations.
"As a rule, it is best to avoid metallic vessels, except when made of block tin and used in connection with the steam-bath. As many drugs contain tannin, vessels made of iron are not adapted for preparing their decoctions, and the usually imperfect covering of galvanized (or zinc) or tinned sheet iron, renders the vessels lined with such material but little better suited for this purpose, and still inferior to properly-enamelled iron vessels.
"As a rule, decoctions should be allowed to cool to below 50° C. (122° F.) before they are strained; principles which are soluble only in hot water are then mostly precipitated, and removed without, in most cases, weakening the medicinal effects of the preparations. But even with this precaution the strained liquid may become unsightly in appearance by the further deposition, on cooling, of apotheme or matter soluble only in hot water. Decoctions cannot be depended upon for more than one, or, at the utmost, a few days". - N. D.