 Solution

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]