Schematic of an improvised double boiler, as used in outdoor cooking
The double boiler 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 fluid, 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 fluid.
The smaller container, filled with the substance to be heated, fits inside the outer container, filled with the working fluid (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 fluid 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 (212 °F), the boiling point of water at sea level. Using different working fluids (oils, salt solutions, etc.) in the lower container will result in different maximum temperatures.
Stand Style Bain Maries