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.

A temperature measuring system includes a ladle and temperature measuring members. The ladle includes a steel shell, a refractory layer and a permanent layer arranged between the steel shell and the refractory layer. An outer surface of the steel shell is provided with exhaust holes which extend to an outer surface of the permanent layer. The cross-section of the temperature measuring members is less than that of the exhaust holes such that the temperature measuring members can extend into the exhaust holes for temperature measurement without damage to the ladle structure. Outer sides of the temperature measuring members are provided with protruding portions which abut against walls of the exhaust holes to fix the temperature measuring members in the exhaust holes. Gaps are formed between non-protruding portions of the temperature measuring members and the walls of the exhaust holes for discharging gas.