The high carbon steel (tamahagane) and higher carbon pig iron, called nabe-gane, will then be forged in alternating layers, using very intricate methods to form the very hard steel for the edge, called hagane (edge steel), and the springy metal jacket, called kawagane (skin steel). The most useful process is the folding, where the metals are forge welded, folded, and welded again, as many as 16 times. The folding removes impurities and helps even out the carbon content, while the alternating layers combine hardness with ductility to greatly enhance the toughness.[5][6][7] Currently, tamahagane is only made three or four times a year by Nittoho and Hitachi Metals[8] during winter in a wood building and is only sold to the master swordsmiths to use once it is made.