The largest metal mirror is a 183-cm (72-in) reflector, made by the third Earl of Rosse (1800-67), and set up at Birr Castle, Co. Offaly, Republic of Ireland in 1845. The mirror was made of speculum metal (an alloy of copper and tin). With it, Lord Rosse discovered the spiral forms of the galaxies. It was last used in 1909, but was restored with a mirror made of aluminium in 1998.
Comments below may relate to previous holders of this record.