 Solution

Decoction suits material whose useful constituents are water-soluble and withstand boiling; infusion avoids sustained boiling and can better preserve delicate or volatile constituents. The choice depends on both the plant's structure and the chemistry of the constituent being extracted.

Hard bark, wood and many kinds of root need comminution and more prolonged contact with hot water to penetrate their tissues; a decoction can assist extraction when the desired material is stable. Boiling is unsuitable when it drives off a needed volatile oil, decomposes the active constituent, or extracts an unwanted excess of starch or other material. A hard root is therefore not automatically an instruction to boil. Period examples suitable for decoction include oak bark, logwood and pareira root; appropriate infusions include aromatic cloves and chamomile. Cold infusion of calumba avoids dissolving its starch, illustrating that suitability depends on the desired selectivity of solvent extraction, not merely on whether a drug is a leaf or a root. ^[ref]

For an infusion, the appropriately divided drug is left in a measured solvent for a prescribed time, commonly in a covered vessel. Hot water assists extraction, while a cover reduces loss of volatile matter. Cold infusion can be chosen when heat is undesirable. The result must be strained or filtered; time, temperature, particle size and solvent quantity affect its strength. Neither method guarantees that every constituent is extracted in the same proportion.

A liquid extract is made by extracting the specified drug with a suitable solvent, separating the liquid from the exhausted material, and bringing the extract to its prescribed concentration and volume. Maceration can precede percolation; expression or washing recovers retained liquid. A percolator passes fresh solvent through a packed bed, rather than indefinitely soaking the drug in an increasingly saturated liquid. Where concentration is necessary, controlled evaporation limits damage; an alcoholic fraction can be reserved or recombined as the monograph directs. Different liquid extracts can use aqueous, alcoholic or ethereal processes. For example, the liquid extract of male fern is an ethereal extraction of a resinous and oily fraction, whereas the liquid extract of opium is compounded from a previously made aqueous extract. These are chemically different operations. There is no warrant for assigning all 1864 extracts one modern universal drug-to-extract ratio. ^[ref]