Coking: from wood to coal

Coking fuel is an ancient practice. In early times wood was coked to make charcoal. Roman writers mention charcoal fires burning in the temples of the ancients.

The first written record of coking coal is in 1620 when a group lead by Sir William St John invented and patented the first beehive coke oven. Over the following 200 years a further thirty-five patents for beehive coke ovens were submitted, each offering fresh improvements.

At first, however, iron production did not use coke, because the impurities in coal affected the quality of the iron. In the 17th century Dudd Dudley experimented with using coke for iron production, with some limited success, but it was not until 1709 that the coke smelting process was perfected by a Quaker called Abraham Darby, in Coalbrookdale, Shropshire.

A primary fuel

By the beginning of the 19th century coke had become the primary fuel for producing iron, dramatically changing this industry in Scotland. Whereas blast furnaces were once based in the Highlands - where oak woodlands provided charcoal fuel - coke-fired furnaces were now built in the central belt, where there was plenty of coal to be mined.

The coking process

On reaching the remains of the ovens at Woodmuir Farm you will see that the fronts of the oven have been lost. This, however, gives a view of the inside of each oven to see how they operated.

The principal process of producing coke using coal did not really change from the way people made charcoal from wood.

Civil Engineer Paul Mallmann described the process at a presentation on the history of the coke oven to the Cleveland Institution of Engineers in 1904:

"The coal to be coked forms a heap in the Oven, and is ignited by the heat reflected from the dome, which retains its heat, whereas the side walls are cooled by quenching the Coke in the Ovens itself"

The process heats the coal but does not allow it to burn. While the principles are similar, coal coking is far more complicated than charcoal making. You can see this by comparing the archaeological remains of the charcoaling process at Bonawe to this site.

The long history of patent records outlines the continuous inventions of better methods and designs for the coking ovens. The attempts to develop and improve the coking process continued into the 20th century.