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 on a stovetop
An improvised bain-marie being used to melt chocolate
An alchemical balneum Mariae from Coelum philosophorum, Philip Ulstad, 1528, Science History Institute
Mary or Maria the Jewess, also known as Mary the Prophetess or Maria the Copt, was an early alchemist known from the works of Zosimos of Panopolis and other authors in the Greek alchemical tradition. On the basis of Zosimos's comments, she lived between the first and third centuries A.D. in Alexandria. French, Taylor and Lippmann list her as one of the first alchemical writers, dating her works at no later than the first century.
Engraving depicting Maria Prophetissima from Michael Maier's book Symbola Aurea Mensae Duodecim Nationum (1617)
An alchemical balneum Mariae, or Maria’s bath, from Coelum philosophorum, Philip Ulstad, 1528, Science History Institute