Bain-marie

A bain-marie (English: /ˌbænməˈriː/ BAN-mə-REE, French: [bɛ̃ maʁi]), also known as a water bath or double boiler, a type of heated bath, is a piece of equipment used in science, industry, and cooking to heat materials gently or to keep materials warm over a period of time. A bain-marie is also used to melt ingredients for cooking.
The name comes from the French bain de Marie or bain-marie, in turn derived from the medieval Latin balneum Mariae and the Arabic حمام ماري ḥammām Māriyya, all meaning 'Mary's bath'. In his books, the 300 AD alchemist Zosimos of Panopolis credits for the invention of the device Mary the Jewess, an ancient alchemist. However, the water bath was known many centuries earlier (Hippocrates and Theophrastus), and the balneum Mariae attributed to Mary the Jewess was used to heat its contents above 100 °C, while the bain-marie that continues to be used today only heats its contents up to a gentle heat of less than 100 °C.
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 inner container, filled with the substance to be heated, fits inside the outer container filled with the working fluid (often water, but alternatively steam or oil). The outer container is heated at or below the base, causing the temperature of the working fluid to rise and thus transferring heat to the inner container. The maximum obtainable temperature of the working fluid is dictated by its composition and boiling point at the ambient pressure. Since the surface of the inner container is always in contact with the working fluid, the double boiler serves as a constant-temperature heat source for the substance being heated, without hot or cold spots that can affect its properties.
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 °C (212 °F), the boiling point of water at sea level. Using different working fluids such as oil in the outer container, or pressurizing the outer container, will result in different maximum temperatures obtainable in the inner container.
A contemporary alternative to the traditional, liquid-filled bain-marie is the electric "dry-heat" bain-marie, heated by elements 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.
In cooking applications, a bain-marie usually consists of a pan or pot of water in which another container or containers of food to be cooked is/are placed.
In small scale soap-making, a bain-marie's inherent control over maximum temperature makes it optimal for liquefying melt-and-pour soap bases prior to molding them into bars. It offers the advantage of maintaining the base in a liquid state, or reliquefying a solidified base, with minimal deterioration. Similarly, using a water bath, traditional wood glue can be melted and kept in a stable liquid state over many hours without damage to the animal proteins it incorporates.