One of the major impurities often found in iron ore is phosphorus. During the 1850s, Henry Bessemer developed a conversion process whereby carbonic impurities could be burned away, but his method worked only in phosphorus-free ores. In 1875, Sidney Gilchrist Thomas devised a process of removing the phosphorus using the Bessemer converter. His method was based on the introduction of a basic compound such as burned limestone to the converter. The phosphorous adhered to the compound and could then be removed from the iron ore.

Thomas's livelihood hardly reflected his stature in the history of technology. For twelve years (1867-1879) he held a menial job as police court clerk, continuing even after making his discovery. He had learned of the need for eliminating phosphorous from iron ore during lectures that he attended at Birkbeck College in England. He later studied chemistry and metallurgy at the Royal School of Mines.

Thomas's cousin, Percy Carlyle Gilchrist (1851-1935), organized the initial trials of the new process at the steel works at Blaenavon, where he was employed as a chemist. The process was hence named the Thomas-Gilchrist Process. This and the precipitate fertilizer known as Thomas Slag were patented and protected.

Thomas was forced to leave his court position to keep pace with demands for his new process. His health was failing, however, and an attempt to travel to a healthier climate came too late. He died at the age of thirty-four.