People of Steel: Sidney Gilchrist Thomas and Percy Gilchrist

infographic about Sidney Gilchrist Thomas and Percy Gilchrist, inventors of a method to remove phosphorus from steel made in the Bessemer method.

Sidney Gilchrist Thomas and
Percy Gilchrist
(significant working years 1875-1883)

The Gilchrist-Thomas (or basic) method for processing iron, innovated by cousins Sidney Gilchrist Thomas and Percy Gilchrist, produced low phosphorus steel from lesser quality ores used in the Bessemer process. Bessemer was able to leverage his access to lower phosphorus ores to produce low-phosphorus, less brittle steel, however, most ironmasters did not have that advantage of ore quality. A teacher told Sidney Gilchrist Thomas that the person who found a way to remove phosphorus from the Bessemer process would be a very wealthy man, motivating him to focus his efforts in that direction.

Both Percy and Sidney were chemists, schooled at the Royal School of Mines, with Percy going right after boarding school and Sidney attending at night while working in the police court of London. Percy went to work at a Welsh ironworks, Blaenavon Ironworks. Sidney used to travel to Wales on weekends and conduct experiments on lowering the phosphorous level. When the general manager of the ironworks Percy worked at learned of their experimentation, he offered them the use of a Bessemer converter. Sidney calculated that lining a Bessemer converter with lime or magnesium limestone would cause the phosphorus to oxidize, which allowed it to be removed from the Bessemer converter. This high-phosphorus slag was found to work as agricultural fertilizer, known as Thomas meal.

Percy and Sidney submitted a paper to the Iron and Steel Institute in early 1878, but it was not read until May 1879. By that time, they had worked with an ironmaker and showed it to be successful on a larger scale. Patents were issued and royalties were enjoyed. American Andrew Carnegie paid $250,000 to use this process in the United States.

Sidney Gilchrist Thomas died at age 34 in 1885. His experiences seeing the condition of those who came into the municipal court caused him to leave his royalties fortune in trust for his sister, instructing her to use the money to help the disadvantaged. She used the money to support efforts to improve conditions for workers, especially women. Percy Gilchrist lived to age 83, but at one point was admitted to an asylum for his “eccentricities.”

The Blaenavon Ironworks, where Gilchrist and Thomas did their experiments is now a World Heritage Site.