Puddling furnace

The puddling furnace is a metalmaking technology to create wrought iron from the pig iron produced in a blast furnace. Pig iron contains high amounts of carbon and other impurities, making it brittle. The puddling furnace burns off these impurities to produce a malleable low-carbon steel. It is the process of manually handling and stirring the iron during and after this process that gives us the name "wrought". Invented by Peter Onions and patented by Henry Cort in 1783-4, the puddling process, along with the blast furnace, led to a massive increase in the amount of iron being produced every year.

The furnace was constructed to pull the hot air over the iron without it coming into direct contact with the fuel, a system generally known as a reverberatory furnace or open-hearth process. After lighting and being brought to a low temperature, the furnace is prepared for use by "fettling", painting the grate and walls around it with iron oxides, typically hematite. Iron is then placed on the grate, normally about 600 lbs, and allowed to melt on top, mixing with the oxides. The mixture is then stirred vigourously with a "rabbling-bar", a long iron rod with a hook formed into one end. This causes the oxygen from the oxides to the mix to react with various impurities in the pig iron, notably silicon, manganese and to some degree sulfur and phosphorus, which form gases and are removed out the chimney.

More fuel is then added and the temperature raised. The iron completely melts and the carbon starts to burn off as well. The carbon dioxide formed in this process causes the slag to "puff up" on top, giving the rabbler a visual indication of the progress of the combustion. As the carbon burns off the melting temperature of the mixture rises, so the furnace has to be continually fed during this process. Eventually the carbon is mostly burned off and the iron forms into a spongy plastic material, indication that the process is complete and the material can be removed.

The hook on the end of the bar is then used to pull out large "puddle-balls" of the material, about 40 kg each. These are then hammered using a steam hammer to weld shut internal cracks, while breaking of chunks off impurities. The iron is then re-heated and rolled out into flat bars or round rods.

The puddling furnace was widely used as the first step in making crucible steel as well. The low-carbon wrought iron was packed in large stone boxes with a layer of charcoal powder between each bar, heated to very high temperatures for six days (plus two days for the furnace to heat up and another two to cool down before the metal could be removed) so that some of the carbon from the charcoal was transferred cleanly into the iron. The bars were then broken into small pieces, and melted in crucibles to produce steel.

Both processes, and the puddling furnace, were displaced with the introduction of the Bessemer Process, which produced similar quality steels for a fraction of the cost and time. For comparison, an average size charge for a puddling furnace was 600 lbs, for a Bessemer converter it is 15 tons. Wrought iron is now made from the Bessemer Process as well. Still-liquid steel from the converter is mixed with oxides and stirred as in the former process, lowering the carbon content. The mixture is then poured into a cooler container, where is solidifies almost instantly, shattering as a result, and thereby releasing the combustion gases trapped inside. The resulting low-carbon spongy metal is then hammered and rolled as before.