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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]

For the contemporary formulary, use boiling water for the linseed, charcoal and mustard poultices, and water at 100°F for the yeast poultice. These are preparation temperatures, not temperatures at which boiling material is placed on skin.

For Cataplasma Lini, hot water hydrates the ground linseed into a cohesive moist mass; the contemporary account also notes the addition of olive oil when oil has been removed from the meal. For Cataplasma Carbonis, the hot bread-and-linseed basis is made with boiling water, then the charcoal is incorporated and part used at the surface. At ordinary atmospheric pressure the boiling-water preparation temperature is approximately $\boxed{212{}^{\circ }\mathrm{F}=100{}^{\circ }\mathrm{C}}$. ^[ref]

For Cataplasma Fermenti, the stated $\boxed{100{}^{\circ }\mathrm{F}\approx 37.8{}^{\circ }\mathrm{C}}$ permits the yeast to ferment the flour mixture. Boiling would injure its fermenting activity. The flour, yeast and warm water are mixed and kept warm while the mass rises. This is a historical compounding process, not evidence that a fermented dressing treats every wound. ^[ref]

The mustard entry requires a historical qualification. The contemporary British recipe reproduced by the text directs powdered mustard and linseed to be mixed, then stirred into boiling water; the same text observes that excessive heat impairs development of the pungent volatile oil. Thus “boiling” is the contemporary formula's instruction, while lukewarm preparation better preserves development of mustard's pungent volatile oil. These are different claims. The later practical preference for lukewarm water should not silently replace the 1864 recipe, nor should that recipe be mistaken for optimal modern biochemical practice. ^[ref]