Thomas, Sidney Gilchrist (1850–1885)

English metallurgist and inventor who, with his cousin Percy Gilchrist, developed a process for removing phosphorus impurities from the iron melted during steel manufacture.

Thomas was born in London and for most of his life worked as a police-court clerk. His deep interest was in industrial chemistry; he experimented systematically at home and attended the laboratories of various chemistry teachers.

The steel produced from phosphoric ores (such as most British, French, German, and Belgian iron ore) was brittle and of little use. Towards the end of 1875 Thomas arrived at a theoretical solution to the problem of how to dephosphorize pig iron when it was loaded into a Bessemer converter. He thought of adding lime, or the chemically similar magnesia or magnesian limestone, to the lining of the converter or furnace. Gilchrist, a chemist at a large ironworks, helped him try out this idea, and Thomas's patent was filed in 1878. The slag that formed as a by-product of this process found use in the developing artificial fertilizer industry.