The heat balance of a laboratory electroslag furnace working in quasisteady state is considered in detail. Results are presented for all possible melting mode combinations of direct and alternating (60Hz) current, with live and insulated molds, in air or argon atmospheres. The material studied is AISI 4340 steel, using a slag of CaF2 + 25 wt pct A12O3. The temperature and potential fields of the slag are determined, together with heat fluxes in the furnace. Heat balances are given for each subregion of the process, showing good agreement where results are available which permit cross checks of the balance. Suggested explanations are given for the differing behavior of the various melting modes based on variations in effective slag resistivity due to electrochemical reactions, and on variations in current path. The most important factors in determining the ingot heat balance are shown to be the electrode immersion, the slag volume’s dimensions, and the depth of the cylindrical liquid metal head on the ingot.