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.