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 oxygen 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.