Ferrosilicon is produced by reduction of silica or sand with coke in presence of scrap iron, millscale, or other source of iron. Ferrosilicons with silicon content up to about 15% are made in blast furnaces lined with acid fire bricks. Ferrosilicons with higher silicon content are made in electric arc furnaces. An overabundance of silica is used to prevent formation of silicon carbide. Microsilica is a useful byproduct. The usual formulations on the market are ferrosilicons with 15, 45, 75, and 90% of silicon. The remainder is iron, with about 2% of other elements like aluminium and calcium.