While fired at high temperatures, the tamahagane is never allowed to reach a molten state. This is to ensure that just the right amount of carbon will dissolve into the steel, and that the percentage of carbon will vary throughout the tamahagane (between 0.5 and about 1.5 percent). Katana-makers use two types of tamahagane: high-carbon, which is very hard and allows for a razor-sharp edge, and low-carbon, which is very tough and allows for shock absorption. A sword composed simply of one kind of steel or the other would either dull too quickly or be too brittle. On the third night of smelting, when the tatara masters break open the clay furnace to expose the tamahagane, they use the degree of ease with which the pieces of newly made steel break apart to discern their carbon content.