One of the key processes in the manufacture of Japanese swords is the fold-forging of the steel. The steel block is heated, hammered and stretched out. Then an incision is made, and the block is then folded in half. This process is repeated a number of times. This folding process helps to remove some of the remaining impurities, while evenly distributing the carbon and other elements throughout the steel. The resulting layers (1 fold = 2 layers, 2 folds = 4 layers, 15 folds = 32,768 layers) produce a beautiful wood grain like pattern in the surface of the steel called jihada. The outer steel (kawagane) is then wrapped around a softer core (shingane) and then hammered out into the shape of the blade. The softer core-steel gives the blade a degree of flexibility so as to resist bending or breaking on impact. A typical Japanese sword is made from a traditionally produced steel called tamahagane. They are commonly a single edged curved blade, with a ridgeline (shinogi) running the length of the blade. This type of manufacture has existed since the late Heian period (794-1185). However, the Japanese sword is not merely a highly effective weapon, it is also considered a beautiful art object, a symbol of authority and honor, a treasured offering to the gods of Japan, and was presented as elite gifts among the nobility.