A bain-marie 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.A bain-marie is a simple technique used for baking delicate dishes like custards, cheesecakes, mousses and terrines. It works to prevent cracking or curdling by surrounding the food being baked with hot water to produce an even, gentle heat.