text: 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 cinder was 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 placed in hearth of the furnace, a process known as charging. 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 doors in the furnace. This helps the iron-III (the  species acting as an oxidiser) from the oxides to react with impurities in the pig iron, notably silicon, manganese (to form slag) and to some degree sulfur and phosphorus, which form gases that escape with the exhaust of the furnace.
  More fuel is then added and the temperature is raised. The iron completely melts and the carbon starts to burn off. When wet puddling, the formation of carbon monoxide (CO) and carbon dioxide () due to reactions with the added iron oxide will cause bubbles to form that cause the mass to appear to boil. 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 , so the furnace has to be continually fed during this process. The melting point increases since the carbon atoms within the mixture act as a solute in solution which lowers the melting point of the iron mixture (like road salt on ice).
  Working as a two-man crew, a puddler and helper could produce about 1500 kg of iron in a 12-hour shift.  The strenuous labour, 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".