I watched a wonderful Nova [PBS] documentary on the making of high quality Samurai Swords. It took the story from the smelting of iron sand with charcoal in an open top clay furnace which took four days of adding more iron sand and charcoal and continuous monitoring. The clay furnace is demolished to get the smelted steel which is cooled and broken into pieces. They contain pieces of both low and high carbon steel. The master sword maker chooses the right combination of both types to make the sword blank. After much heating, hammering and folding the final step is to heat the sword to the corect temperature and then quench it in cold water. Due to the swords construction of hard and flexible steels the sword takes on it's curved shape due to difference in the response to cooling of the different steels. The polishing and sharpening takes many days. The ultimate sword was known as a 'five body sword' since it was supposed to be able to slice through five bodies at one stroke.