Gilchrist–Thomas process
The Gilchrist–Thomas process, also called the Thomas process, is a historical method for refining pig iron into steel. It was developed from the Bessemer converter and patented in 1877 by Percy Carlyle Gilchrist and his cousin Sidney Gilchrist Thomas.
Why it mattered: It let steelmakers use phosphorus-rich iron ore, the most abundant ore globally, which helped the steel industry grow outside the United Kingdom and the United States.
What makes it different: The converter lining is basic, made from dolomite fired with tar, instead of the acidic lining used in the Bessemer process. This basic environment enables phosphorus to move from liquid iron into the molten slag.
How it works: Phosphorus moves into the slag and is removed, producing steel of acceptable quality. The slag also contains phosphate fertilizer, known as Thomas meal.
Advantages and drawbacks: The process makes it possible to use common ore and yields a useful fertilizer byproduct. However, it has higher iron loss and requires more frequent relining of the converter.
Historical impact and decline: It spurred growth in iron and steel regions like Lorraine. Over time, it declined in favor of the Siemens–Martin open-hearth furnace, which also used a basic lining, and later with the shift to oxygen-based production in the mid-20th century.
Note: Modern oxygen converters operate differently and are not the same as the early Gilchrist–Thomas process.
This page was last edited on 3 February 2026, at 02:51 (CET).