The present invention provides a method of converting copper containing material to blister copper comprising: (a) providing copper containing material comprising copper sulfides and iron sulfides, whereby the copper containing material comprises at least 35 wt % copper of the total weight of the copper containing material; (b) reacting the copper containing material in a furnace with an oxygen containing gas, in the absence of flux, to effect oxidation of iron sulfide and copper sulfide, and controlling injection of the oxygen containing gas and the temperature so that the resulting converter slag is in a molten phase to obtain blister copper and converter slag.