A bain-marie, otherwise known as a double boiler or water bath, is a piece of kitchen equipment used to heat foods gently or to keep them warm. Typically it is used to keep sauces warm or to melt chocolate. The idea is that the food itself does not come into direct contact with the heat. An example is simply a glass or stainless steel bowl containing solid chocolate, standing within a pan of water, which is either already boiled, or sitting over a gentle heat on the stove. The heat from below gradually melts the chocolate without burning or clumping it.
Recently, I came across this beautiful example of a country house steam bain-marie at Petworth House, West Sussex (made by Jeakes & Co c1870). This illustrates the idea of it being a water bath. It resembles a big metal sink with taps running into it (for filling with hot water), and out (for emptying). The various saucepans would have held a variety of pre-made sauces which would have been kept warm for the appropriate dinner course.
Sadly, I don’t I could fit one of those in my kitchen 🙂