μπεν μαρί -> bain-marie, double boiler, water bath
A bain-marie (also known as a double boiler) 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 lower container of the bain-marie (or built into its base) is a heat source.
The smaller container, filled with the substance to be heated, fits inside the lower 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. Because the working-liquid in the lower container is closest to the heat source, it heats more quickly and is also higher in temperature than the material in the upper container. Because of this, it is both easier to regulate the temperature of the material in the upper container, and to keep it from boiling or scorching.
Typically, the material in the upper container is heated, to a temperature normally below boiling, by bringing the working-liquid in the lower container to a boil. 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.
A contemporary alternative to the traditional, liquid-filled bain-marie is the electric "dry-heat" bain-marie, heated by element below both pots. The dry-heat form of electric bains-marie often consumes less energy, requires little cleaning, and can be heated more quickly than traditional versions. They can also operate at higher temperatures, and are often much less expensive than their traditional counterparts.
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.