A Radio Talk by Charles F. Kettering
But even in the Middle Ages, the steelmaker's trade was a very old one. Thirteen hundred years before that, the Romans defeated the Celts because the swords used by Celts were poorly tempered and had to be straightened after each blow.
In those days, each blacksmith had trade secrets which he had developed himself, or which had been handed down to him by his father. These blacksmiths knew that they had to have good metal to start with; each added his own particular brand of skill along with a little good luck. The latter was very important. Some craftsmen, however, produced consistently fine materials. For instance, the Damascus blade was notable because of its flexibility and strength. Its steel contained a great deal of carbon which normally makes metal hard and brittle. But the sword-maker would heat the blade and hammer it and re-heat and hammer it again and again until he had changed the entire structure of the metal. The more it was hammered the more flexible it became, yet it retained its hardness.