Puddling (metallurgy) - Process

Process

The process begins by preparing the puddling furnace; this involves bringing the furnace to a low temperature and then fettling it. Fettling is the process of painting the grate and walls around it with iron oxides, typically hematite; this acts as a protective coating keeping the melted metal from burning through the furnace. Sometimes finely pounded cinders from a charcoal forge, puddling furnace, reheating furnace, or blast furnace is used instead of hematite. In this case the furnace must be heated for 4–5 hours to melt the cinder and then cooled before charging. Either white cast iron or refined iron is then charged into hearth of the furnace. For wet puddling, scrap iron and/or iron oxide is also charged. This mixture is then heated until the top melts, allowing for the oxides to begin mixing; this usually takes 30 minutes. This mixture is subjected to a strong current of air and stirred by long bars with hooks on one end, called puddling bars or rabbles, through working doors. This helps the oxygen from the oxides react with impurities in the pig iron, notably silicon, manganese (to form slag) and to some degree sulfur and phosphorus, which form gases and escape with the exhaust of the furnace.

More fuel is then added and the temperature raised. The iron completely melts and the carbon starts to burn off as well. When wet puddling the mixture will begin to "boil" due to the added iron oxide. 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 from 1,150 °C (2,100 °F) to 1,540 °C (2,800 °F), so the furnace has to be continually fed during this process. Eventually the carbon is mostly burned off and the iron "comes to nature", forming into a spongy material, indicating 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 35–40 kilograms (70–80 pounds) each, and 30–38 centimeters (12–15 inches) in diameter. Sometimes a large pair of tongs are used to remove the puddle balls.

These puddle balls are then transported to the hammer or squeezer by dragging them along iron slopes built between the furnace and the shingling equipment or, more commonly, the puddle balls are loaded into iron wheelbarrows and transported to their destination. Shingling expels slag and welds shut internal cracks, while breaking off chunks of impurities. The iron is then re-heated and rolled out into flat bars or round rods. For this, grooved rollers were used, the grooves being of successively decreasing size so that the bar was progressively reduced to the desired dimensions. Some of the iron oxide is from the scales that form in the later steps of shingling and rolling. The quality of this may be improved by faggoting.

Working as a two man crew, a puddler and helper could produce about 1500 kg of iron in a 12 hour shift. The strenuous labor, heat and fumes caused puddlers to have a very short life expectancy, with most dying in their 30s. Puddling was never able to be automated because the puddler had to sense when the balls had "come to nature".