The Puddling Process

In 1784 Henry Cort devised a method of producing wrought iron from cast iron using a Coal fired Reverbatory Furnace. Solid Cast Iron was heated within an enclosed furnace.

A Reverberatory Furnace is a long low structure built out of fire bricks. The coal fire was at one end with the hearth between the fire and the chimney. The hearth was slightly dished with a roof that directed the smoke and flame from the fire well above the iron. By keeping the smoke and flame above the iron, no carbon from the fire came in contact with the iron.

Solid Pig (Cast) Iron was heated vigorously in the hearth until it was all molten. The fire was then damped down and the iron stirred so as to bring as much as possible in contact with the air. As wrought Iron has a higher melting point than Cast Iron, if the temperature in the furnace was correct the iron began to solidify as the carbon was removed. Eventually the Wrought Iron could be worked into a single lump of iron in the centre of the Hearth. Although in theory this was Wrought Iron it was not usable in this form because of the slag within the lump.

For the Wrought Iron to be made usable, it was lifted from the furnace and forged using a 'Shindling Hammer'. Finally it was rolled into bars or sheet. As most of the slag was squeezed out of the iron under the Shindling Hammer this could be a dangerous job, with each drop of the hammer white hot slag would be strayed out across the forge. As the workmen had to hold and move the iron during the forging, there was no option other than for them to dress in heavy protective clothing.

An improvement to Cort's puddling process came from Joseph Hall in 1816. Hall added mill scale (iron oxide formed and broken off during the forging and rolling) to the Cast Iron at the start of the Puddling process. Once the iron had melted, the carbon monoxide formed by the mill scale bubbled up through the iron giving the impression of boiling, thus the common name for this refinement 'Pig Boiling'.