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Sidney Gilchrist Thomas

English inventor and metallurgist, Sidney Gilchrist Thomas, discovered a method of eliminating phosphorus (an impurity in some iron ores) in the Bessemer converter – the industrial process used for mass producing steel from molten pig iron.

In his honour, the method became known as the Thomas-Gilchrist process, the basic process, or the Thomas process, as it changed the face of steel production. After experimenting and perfecting his product, Thomas applied for a patent in 1877.


The problem of removing phosphorus from the Bessemer process had perplexed chemists for many years. English inventor Sir Henry Bessemer had patented the Bessemer process in 1856. It was the first affordable industrial process that enabled the mass production of steel, prior to the introduction of open hearth furnaces.

The key principle is the use of oxidation to remove impurities by blowing air through the molten iron. The oxidation process also increases the temperature of the iron mass to keep it molten. As an amateur scientist, Thomas was inspired by the challenge of removing the phosphorus and he researched how to solve the problem of the Bessemer converter.


Early life

Born in Canonbury, London, in April 1850, Thomas attended Dulwich College and intended training to become a doctor. However, after his father’s death left the family with a reduced income, he had to take work as a police court clerk instead.

He carried on working in his day job as a Metropolitan Police Court clerk while studying chemistry in his spare time, attending the Royal School of Mines, where he studied applied chemistry and metallurgy.

He was fascinated by the process of steel production, as it had been discovered that most of the world’s iron ore had high levels of phosphorus and the resulting steel was brittle. This meant steelworks were spending extra money importing expensive non-phosphoric ores, putting the price of steel up.

In 1870, Thomas attended a class held by tutor Dr George Chaloner at Birkbeck College in London. Dr Chaloner set a challenge, telling his students that the man who eliminated phosphorus in the Bessemer converter would make his fortune.


Thomas-Gilchrist process

Thomas began researching the subject in earnest and by 1875 he had determined how to solve the problem. He discovered that to remove phosphorus in the Bessemer converter, it was necessary to use a lining made of a strong basic substance, like burned limestone, so the phosphorus would combine with it. Then, it would be eliminated in the slag.

Thomas perfected his product, helped by his cousin, Percy Carlyle Gilchrist, who was an analytical chemist at Blaenavon steelworks in Wales. In 1877, Thomas and Gilchrist worked together, at their own cost, on experiments to perfect the design.

Edward Pritchard Martin, general manager of the Blaenavon Company, learned of the cousins’ research and bought a share in the patent. The Blaenavon Company provided a Bessemer converter so that larger scale tests could take place.

Thomas patented his invention in 1877. The discovery was announced publicly in March 1878 at a meeting of the Iron and Steel Institute. In September of that year, Thomas and Gilchrist’s paper, The Elimination of Phosphorus in the Bessemer Converter, was published.

Thomas made contact with Edward Windsor Richards, manager of Bolckow Vaughan & Co Steelworks, in Cleveland, Yorkshire. Richards was interested in the process and subsequently, the success of Thomas’s invention was assured.


Later life

As a result, Thomas was finally able to give up his job as a police court clerk, so he could pursue his ambition of devoting more time to other challenges that interested him.

The famous Scots-born American industrialist, Andrew Carnegie, went on to invest $250,000 for the right to use the process in America. He commented that Thomas and Gilchrist had done more for “Britain’s greatness” than “all the kings and queens put together”. He described their invention as a “miracle”, as they had transformed “useless” phosphoric ore into steel.

Sadly, Thomas had little time to enjoy his new-found fame and wealth. His discovery had cost him his health. He was frail due to years working in the dark, dank courts, which had affected his lungs. His experiments further damaged his lungs and he fell ill.

He took a long sea voyage and stayed in the warmer climate of Egypt to try and improve his health, but to no avail. Tragically, he died in Paris on 1st February 1885, at the age of just 34. A renowned philanthropist, he left his fortune to be used for charitable work.

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