Sidney Gilchrist Thomas (16 April 1850 – 1 February 1885) was an English inventor.
Thomas was born at Canonbury, London and was educated at Dulwich College. . His father, a Welshman, was in the civil service, and his mother was the daughter of the Rev. James Gilchrist. His father's death left his family with a considerably reduced income, he gave up his original idea of becoming a doctor and obtained an appointment as a police court clerk, in which he kept until May 1879.
During these twelve years, besides the work of a busy police court, which brought him into intimate contact with social problems, he found time to study chemistry, and attended lectures at the Birkbeck Institute (which later became Birkbeck College). He set himself to solve the problem of eliminating phosphorus from iron produced by means of the Bessemer converter, and by the end of 1875 was convinced that he had discovered a method. He communicated his theory to his cousin, Percy Gilchrist, who was a chemist at the former Blaenavon Ironworks, Blaenavon in Wales, and experiments were made which proved satisfactory. Edward Martin, manager of the Blaenavon Works, gave facilities for conducting the experiments on a larger scale and undertook to help in taking out a patent. In March 1878, the first public announcement of the discovery was made at the meeting of the Iron and Steel Institute and took out a patent in May, but without attracting much attention. In September a paper was written by Thomas and Gilchrist on the "Elimination of Phosphorus in the Bessemer Converter " for the autumn meeting of this institute, but was not read till May 1879. Thomas, however, made the acquaintance of Edward Windsor Richards, the manager of Bolckow Vaughan & Co.'s works at Cleveland, Yorkshire, whom he interested in the process, and from this time the success of the invention was assured and domestic and foreign patents were taken out.
The 'Basic process' invented by Thomas was especially valuable on the continent of Europe, where the proportion of phosphoric iron is much larger than in England, and both in Belgium and in Germany the name of the inventor became more widely known than in his own country. In America, although non-phosphoric iron largely predominated, an immense interest was taken in the invention. The improved process resulted in much more slag forming in the converter. Thomas discovered this 'Basic slag' could be useful and profitable as a phosphate fertiliser.
Thomas had been overworking for years, and his lungs became affected.
A long sea voyage and a residence in Egypt proved unavailing to restore his health and he died in Paris in 1885 and was buried at Passy.
He had what William Ewart Gladstone, in a review of the Memoirs published in 1891, described as an "enthusiasm of humanity," and he left his fortune to be used for the promotion of philanthropic work. A police court mission was endowed in his memory.
In July 1960 an obelisk, dedicated to his memory, was erected in South Wales, UK, by the Newport and District Metallurgical Society in conjunction with the Iron and Steel Institute. Financial contributions came from many sources and reflected acknowledgment of his work from the United Kingdom and other countries. The former American Society for Metals, in 1985, struck a commemorative plaque in recognition of the historical importance of the ironworks and honouring his work there. The obelisk and plaque may be seen at the Ironworks.