Sword making, historically, has been the work of specialized smiths or metalworkers called bladesmiths or swordsmiths. Swords have been made of different materials over the centuries, with a variety of tools and techniques. While there are many criteria for evaluating a sword, generally the four key criteria are hardness, strength, flexibility and balance. Early swords were made of copper, which bends easily. Bronze swords were stronger; by varying the amount of tin in the alloy, a smith could make various parts of the sword harder or tougher to suit the demands of combat service. The Roman gladius was an early example of swords forged from blooms of steel. A good sword has to be hard enough to hold an edge along a length which can range from to more than . At the same time, it must be strong enough and flexible enough that it can absorb massive shocks at just about any point along its length and not crack or break. Finally, it should be balanced along its length so that it can be wielded effectively. Bronze swords Bronze items are typically cast, not forged. Secondary operations involve removing material from the roughcasting, polishing, and the application of decorative elements. Some Chinese swords used high-tin bronze for the edges, since it is harder, and keeps a sharp edge longer, but is more brittle than the softer, lower-tin alloy used for the blade's core. Bronze alloys with lower tin content are tougher, or more resistant to fracturing. Japanese katana Forging Japanese smiths discovered, similarly to many others, that iron sand (with little to no sulfur and phosphorus) heated together with coal (carbon) made the steel they called tamahagane. This allows the sword to have the strength and the ability to hold a sharp edge, as well as to cause the sword to tend to bend rather than flex under stress. The process starts in the combining of the iron and carbon, by heating iron sand to 1200-1500 degrees Celsius in a traditional furnace, or tatara, for 72 hours. The tamahagane is then cooled and the smelter selects the best pieces to send to the swordsmith. Swordsmith The swordsmith takes the tamahagane nuggets and forges them into a block. In the process of forming, the block is heated to around 900 degrees Celsius. Taken out of the fire and hammered into a thinner block, the block is scored in the center