 Steels can be further heat treated to optimise their toughness, which is important for impact blades, or their hardness, which allows them to retain an edge well with use (although harder metals require more effort to sharpen).It is possible to combine different materials, or different heat treatments, to produce desirable qualities in a blade.For example, the finest Japanese swords were routinely made of up to seven sections of metals and even poorer quality swords were often made of two.