PURPOSE: To prevent the oxidation of the surface of liquid metal in a furnace by protecting the surface of the liquid metal by a liquid seal which is stable to the holding temp. in the furnace without correspondence to the liquid metal and is completely separated therefrom by a difference in sp. gr.

CONSTITUTION: The liquid seal 20 is enclosed onto the surface of the molten metal (liquid metal) in the holding furnace 16. This liquid seal 20 does not react any more to the molten metal in the holding furnace 16, is extremely stable to the holding temp. of the holding furnace 16 and is completely separated therefrom by the difference in sp. gr. from the molten metal. The liquid seal 20 has a high b.p. and flash point and is, therefore, in a liquid state at the holding temp. of about 150°C at which the molten metal is held in a molten state, thus shutting off the surface of the molten metal from the air. For example, Bi-Sn alloy is used for the molten metal in the holding furnace 16 and polyethylene glycol for the liquid seal.