A bain-marie is a pan full of water into which a vessel containing a sauce is placed; the water in the outer pan is then brought to a near boil, which heats the sauce without danger of burning it—a double boiler is therefore a kind of bain-marie. The name of this device, which appeared in English in the early nineteenth century, is a French phrase meaning Mary’s bath; the French name, however, is itself a direct translation of the Latin balneum Mariae, the name of a similar vessel originally used by medieval alchemists in their quest to transmute base metals into gold. The Mary these alchemists named their utensil after was the sister of Moses, known both as Mary and Miriam: because she is called a prophetess in the Bible, alchemists came to see her as a patron of their own mysterious art. Eventually, however, as the practice of alchemy faded away, cooks began to assume that their kitchen bain-maries were the namesake of a more famous Mary, the mother of Jesus: it was the gentle warming of the bain-marie, they supposed, that led to its being named after the warm and gentle mother of God. A bain-marie may also be called a balneum, derived directly from the Medieval Latin name of the vessel.
The apparatus known as the French double boiler, or the water bath, is a culinary device that is composed of a capacious container that retains the water, and several minor containers that harbor the victuals to be cooked. This tool is typically implemented for the purpose of protractedly cooking the comestibles, for steaming them, and for sustaining their warmth sans desiccating them.
A contrivance for maintaining the high temperature of edibles without bringing them to a boil is a specialized instrument that requires a bit of finesse to use. One can accomplish this by placing one or more saucepans or bowls into a large pan filled with boiling water that is then placed on a low heat. A smaller variation of this, known as a double boiler, achieves the same objective. Employing this method is particularly advantageous when making hollandaise sauce, zabaglione, or any dish or condiment containing eggs that tend to coagulate when exposed to intense heat.