Ferrosilicon is produced by reduction of silica or sand with coke in the presence of iron. Typical sources of iron are scrap iron or millscale.  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. The usual formulations on the market are ferrosilicons with 15%, 45%, 75%, and 90% silicon. The remainder is iron, with about 2% consisting of other elements like aluminium and calcium. An overabundance of silica is used to prevent formation of silicon carbide. Microsilica is a useful byproduct.
A mineral perryite is similar to ferrosilicon, with its composition FeSi. In contact with water, ferrosilicon may slowly produce hydrogen.  The reaction, which is accelerated in the presence of base, is used for hydrogen production. The melting point and density of ferrosilicon depends on its silicon content, with two nearly-eutectic areas, one near FeSi and second spanning FeSi-FeSi composition range.