What bain-marie means
A bain-marie is a container of hot water used to cook, melt or keep food warm gently and evenly. The name is French and is traditionally said to come from Mary, an early alchemist, after whom a similar device for gentle heating was named. Because water cannot get hotter than boiling point, food surrounded by water or heated by steam is protected from the fierce, direct heat of an oven or hob, which can make delicate foods curdle, split, crack or burn.
There are different forms. In the oven, a bain-marie usually means a deep roasting tin partly filled with hot water, in which ramekins, cake tins or terrines are placed to bake, as for crème caramel, crème brûlée, baked custards, cheesecakes and pâtés. On the hob, it means a heatproof bowl set over a pan of barely simmering water, used for melting chocolate, making hollandaise, sabayon and other egg-based sauces, and gently cooking lemon curd.
A purpose-made double boiler, with a smaller pan fitting into a larger one, does the same job. In professional kitchens and buffets, a bain-marie can also be a heated unit or water-filled tray that keeps sauces and food hot during service. You will see bain-marie in baking, dessert and sauce recipes.