Bain-marie
An open saucepan or kettle of nearly boiling water, in which a smaller vessel can be set for cooking and warming.
This is very useful for keeping articles hot, without altering their quantity or quality.
If you keep sauce, broth or soup by the fireside, the soup reduces and becomes too strong, and the sauce thickens as well as reduces; but this is prevented by using the bain-marie, in which the water should be very hot, but not boiling.