A method and device for treating a metal or a molten metal alloy using an addition agent, wherein the addition agent is deposited in a local cavity arranged at the bottom of a treatment ladle and surrounded by a protruding wall, and a closing member connected to movement means is able to form, with the bottom of the treatment ladle, in a low insulating position, a chamber including said local cavity and comprising an intermediate annular space around the small wall. Application to the treatment of a molten cast iron using pure magnesium or magnesium alloy.