Puddling is a metallurgical process which was used during the Industrial Revolution as a means of making iron and steel. It is used by subjecting pig iron or any other impure metal to intense heat and frequent stirring inside a furnace, and in the presence of oxidizing substances. This process frees the metal from some of its impurities, such as sulfur and carbon.
The process of puddling begins by preparing the furnace. While at a low temperature, the furnace is painted on the inside with iron oxides such as hematite. This step prevents molten iron from burning through. Scrap iron or cast iron is then added and heated until it melts at approximately 2,800°F (1,538°C). The liquid mixture is then hit with a strong current of air, using long bars with hooks at one end, called puddling bars. This helps oxygen react with the impurities in the liquid metal, which then form gases that escape from the furnace as exhaust.
At this stage, more fuel is added to the furnace to raise its temperature. Once the mixture gets hot enough, carbon starts to burn off. Fuel must be added constantly at this step, because the burning off of the carbon raises the melting point of the metal. Once the carbon has mostly burned off, the metal is removed by the hooks on the end of the puddling bars. The metal then goes through the process of shingling, which expels leftover impurities, and closes any internal cracks in the cooling metal.
Puddling was one of several methods for producing iron that were developed in Europe during the 1700s. It has the distinction, however, of being the most successful method in the history of metallurgy for making pig iron into bar iron without using charcoal. Its success made possible the great expansion of iron production in Great Britain in the late 1800s, making the Industrial Revolution much more notable than it would have otherwise been.
The puddling process began to be obsolete as it was replaced by the Bessemer and Aston processes, which greatly reduced the cost and time requirements of making steel when compared to puddling. Puddling was also limited because it could only be done on a small scale, and could only be expanded by building more furnaces. Nevertheless, it represented a significant advance for its time in the field of metallurgy.