A bain-marie (or water bath) is 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.

Description 
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 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 liquid.

The smaller container, filled with the substance to be heated, fits inside the outer 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. The insulating action of the water helps to keep contents of the inner pot from boiling or scorching.

When a bain-marie is filled with water 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.

Alternatives 
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.

Culinary applications 

 Chocolate can be melted in a bain-marie to avoid splitting and caking onto the pot. Special dessert bain-maries have a thermally insulated container and are used as a chocolate fondue. 
 Cheesecake is often baked in a bain-marie to prevent the top from cracking in the centre.
 Custard may be cooked in a bain-marie to keep a crust from forming on the outside of the custard before the interior is fully cooked.
 Classic warm sauces, such as Hollandaise and beurre blanc, requiring heat to emulsify the mixture but not enough to curdle or "split" the sauce, are often cooked using a bain-marie.
 Some charcuterie such as terrines and pâtés are cooked in an "oven-type" bain-marie.
 Thickening of condensed milk, such as in confection-making, is done easily in a bain-marie.
 Bains-marie can be used in place of chafing dishes for keeping foods warm for long periods of time, where stovetops or hot plates are inconvenient or too powerful.

Origin 
Bains-marie were originally developed for use in the practice of alchemy, when alchemists needed a way to heat materials slowly and gently. In that early form of chemical science, it was believed by many that the best way to heat certain materials was to mimic the supposed natural processes, occurring in the Earth's core, by which precious metals were believed to be germinated.

The name comes from the medieval-Latin term balneum (or balineum) Mariae—literally, Mary's bath—from which the French bain de Marie, or bain-marie, is derived. There are many theories as to how the name Marie came to be associated with this equipment:

 According to culinary writer Giuliano Bugialli, the term comes from the Italian bagno maria, named after Maria de'Cleofa, who developed the technique in Florence in the sixteenth century.

 Alternatively, the device's invention has been popularly attributed to Mary the Jewess, an ancient alchemist traditionally supposed to have been Miriam, a sister of Moses. However, according to The Jewish Alchemists, Maria the Jewess was an ancient alchemist who lived in Alexandria, which would seem to contradict the tradition that she was Moses' sister: Alexandria was founded by Alexander the Great in 334 BC, while Moses is thought to have lived around 1450-1200 BC.

 Finally, some consider the name a reference to the Virgin Mary, whose proverbial gentleness can be likened to the gentleness of this cooking technique.

References

 José María de Jaime Lorén. 2003. Epónimos científicos. Baño María. María La Judía. Universidad Cardenal Herrera-CEU. (Moncada, Valencia). 
 Prof. Dr. Hassan S. El Khadem. 1996. A Translation of a Zosimos' Text in an Arabic Alchemy Book. Journal of the Washington Academy of Sciences. Volume 84. Number 3, Pages 168-178. September 1996 
EN 60335-2-50 Household and similar electrical appliances – Safety – Part 2.50: Particular requirements for commercial electric bains-marie (73/23/EEC Low Voltage Directive)

Cookware and bakeware