A bain-marie (also known as a water bath) is a French term for a piece of equipment used in science, industry, and cooking to heat materials gently and gradually to fixed temperatures, or to keep materials warm over a period of time.
The bain-marie comes in a wide variety of shapes, sizes, and types, but traditionally is a wide, cylindrical, usually metal container made of three or four basic parts: a handle, an outer (or lower) container that holds the working-liquid, an inner (or upper), smaller container that fits inside the outer one and which holds the material to be heated or cooked, and sometimes a base underneath. Under the outer container of the bain-marie (or built into its base) is a heat source.
Typically the inner container is immersed about halfway into the working-liquid
The smaller container, filled with the substance to be heated, fits inside the outer container, filled with the working-liquid (usually water), and the whole is heated at, or below, the base, causing the temperature of the materials in both containers to rise as needed. The insulating action of the water helps to keep contents of the inner pot from boiling or scorching.
When the working-liquid is water and the bain-marie is used at sea level, the maximum temperature of the material in the lower container will not exceed 100 degrees Celsius (the boiling point of water at sea level). Using different working-liquids (oils, salt solutions, etc.) in the lower container will result in different maximum temperatures.
Electric bains-marie can also be wet, using either hot water or vapor, or steam, in the heating process. The open, bath-type bain-marie heats via a small, hot-water tub (or "bath"), and the vapour-type bain-marie heats with scalding-hot steam.
Bains-marie were originally developed for use in the practice of alchemy, when alchemists needed a way to slowly and gently heat materials. In that early form of chemical science, it was believed by many that the best way to heat certain materials was to mimic the supposed natural processes, occurring in the earth's core, by which precious metals were germinated.
The device's invention is popularly attributed to Mary the Jewess, an ancient alchemist traditionally supposed to have been Miriam, a sister of Moses. The name comes from the medieval-Latin term balneum (or balineum) Mariae — literally, Mary's bath — from which the French bain de Marie, or bain-marie, is derived.