A double boiler is a stove top
apparatus used to cook delicate sauces such as beurre blanc,
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-degrees C
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.