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
A double boiler is a stove top apparatus used to cook delicate sauces such as beurre blanc, to melt chocolate without burning or seizing, or cook any other thick liquid or porridge that would normally burn if not stirred constantly. It consists of an upper vessel containing the substance to be cooked that is situated above a lower pot of water. When brought to a boil, the steam produced in the lower pot transfers heat to the upper pot.
This apparatus utilizes the properties of water to establish a constant temperature. The phase change of water from liquid to vapor occurs at 100°C (212°F). Therefore, as long as the lower pot does not become pressurized or boiled dry, the maximum temperature contacted by the upper vessel will be the boiling point of water, and scalding or uneven heat is avoided. The steam will either condense on the upper vessel or escape, but the temperature of the vapor phase will remain constant.
The lid on the upper vessel must fit tightly, or else steam may enter the upper vessel and affect the cooking substance.