Transfer thermal plasma of argon (Ar) was applied onto the surface of 25 mol% Na₂O-SiO₂ molten slag using a hybrid plasma furnace composed of non-transfer and transfer thermal plasma. A tungsten rod was used as an electrode. The flame from the transfer plasma spread over the surface of the slag. The direct current of the transfer plasma was a maximum just after ignition and then decreased. When the transfer plasma was applied with a voltage less than 80 V, the iron dissolved and diffused into the molten slag as Fe²⁺ at the iron plate anode electrode according to Faraday’s law. For voltages over 90 V, oxygen gas bubbles evolved at the anode in addition to iron dissolution. SiO gas vaporized more than sodium (Na) gas from the slag surface, although the opposite phenomena should take place thermodynamically and kinetically. Furthermore, the quantity of SiO gas was 10 to 100 times more than that expected from Faraday’s law.