Alloying can be effected via the addition of the element, e.g. lump silicon for Al-Si alloys, or an alloy, e.g. ferrosilicon for Al-Fe-Si alloys, or via the flux, e.g. titanofluorides, or borofluorides for titanium and boron alloys, which are particularly important in grain refining applications. ‘Irony scrap’, that is aluminium scrap which contains ferrous parts sucg as nuts and bolts, is melted in reverberatory furnaces with slopping hearths. The aluminium melts out (selective melting) at lower temperatures than the iron-bearing materials and separates to the lower end of the furnace. The aluminium is usually highly alloyed with zinc and other low melting point metals, while the residue is mainly ferrous metal which can be sold to the steelmakers. It is not currently possible to extract pure aluminium commercially although it has been achieved on the laboratory scale. The only successful method of removing Fe and Si from molten aluminium is by extremely expensive three layer cell electrolysis techniques (see Section 7.4.2).