A technique for casting pieces of glass for mirrors which results in a preformed concave surface, thus considerably reducing the work required to produce the correct figure. Either a kiln containing pieces of glass in a mould is raised to a temperature of 1200°C while being rotated at a few revolutions per minute, or already molten glass is poured into a rotating mould. Centrifugal force causes the surface of the molten glass to take up a parabolic curve. The technique is particularly valuable in preparing large mirrors of small focal ratio.