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.
"Infusions are aqueous solutions of the soluble principies of vegetable or animal drugs, obtained by maceration or digestion in hot or cold water, and differ from decoction only in the lower degree of heat employed in their preparation. Substances containing volatile or other principles which would be dissipated or injured by boiling, are particularly adapted for the purpose. The infusions are prepared by pouring boiling or cold water on the material, and allowing them to remain in contact for a definite length of time, at the expiration of which the solution is poured upon a strainer, and the solid material expressed to recover the liquid absorbed by it. A convenient apparatus, well adapted for making these preparations, is Squire's infusion pot: this consists of a jar A, with a projecting ledge under the top, which supports a strainer, B or D, containing the material to be exhausted; the jar is closed by a well-fitting cover, C. The advantages of this contrivance are, that the material is exhausted by circulatory displacement, the liquid, as it becomes charged with the soluble ingredients descending to the bottom, giving place to fresh portions of less saturated menstruum, and that no further straining is required, should care have been taken not to use too fine a powder.
"The drugs are best adapted for exhaustion with water, if cut into thin slices by a suitable knife, so that they may easily be permeated by the liquid; when cutting is inadmissible they should be bruised to a coarse powder. Ligneous drugs, however, should be in a fine or moderately fine powder, which is also best adapted for most of those which may be made by percolation.
"The time directed for the maceration with boiling water is, in the United States Pharmacopoeia, usually 2 hours, and in the British Pharmacopoeia 15 or 30 minutes, which is in most cases sufficient". - N. D.
The strength of the infusion varies. It is usually such that the virtues of 1 part of the drug are represented by 10, 20 or 40 parts of the infusion, or that they are made in the proportion of 1 1/2, 4 or 6 parts to 100 parts by weight, or that 100 or 200 parts of infusion represent 1 part of the drug.
Infusions are not intended to be kept, except for a very limited period. Exposed to air, decomposition will ensue after a day or two, and may be retarded somewhat only by keeping the vessel in a cool place, or, better still, on ice. When desirable to have a larger quantity prepared, the liquid may be preserved by Appert's method: it is put into convenient sized bottles, which are heated by the water-bath gradually to the boiling point, and stopped at that temperature.
Infusions, when reduced in volume by evaporation at a moderate heat, may be kept unaltered for a considerable time by the addition of one-third their volume of alcohol. Dissolving 1/2 grain of salicylic acid in each fluid ounce of the infusion will have a similar preservative effect.