The morphological phase composition and chemical composition were studied of ancient iron smelting slags from the 4th century B.C. to the 18th century A. D. The action of the reducing furnace atmosphere contacting the slag was evaluated from the contents of metallic Fe, Fe(II), and Fe(III). The ancient iron smelting process was carried out by the solidification of the Fe melt under an intermittently decantable molten slag at approximately 1200°. -- AATA