1. J. Clark, “Method of Reducing Metals from their Ores,” U.S. patent 277,884 (22 May 1883).
2. J. Clark, “Method of Reducing Metals from their Ores,” U.S. patent 277,885 (22 May 1883).
3. W. Calver, “Apparatus for Making Gas,” U.S. patent 549,765 (12 November 1895).
4. K.C. Wideen, “Reducing Oxides or Other Metallic Compounds,” U.S. patent 683,089 (24 September 1901).
5. James O. Gibson, “Apparatus for the Carbothermic Reduction of Metal Oxides Using Solar Energy,” U.S. patent 4,627,418 (9 December 1986).