Due to the quality of metal found in Japan, Japanese bladesmithing became an extremely rigid, precise process, involving folding and forge-welding the steel many times over to create a laminated blade. By the time of the Kamakura period (1185–1333 AD), Japan was under the rule of a military class and repelling Mongol invasions. This became known as the "Golden era" of Japanese bladesmithing under Emperor Toba II, who became a bladesmith himself. After abdicating, Toba II summoned Japan's finest bladesmiths around him in an effort to develop the perfect sword. It was determined that a sword had to be hard in order to maintain a sharp cutting edge, yet hard steel is brittle and can shatter under the stress of a heavy blow. Swordsmiths in Japan found the solution by wrapping a softer low-carbon steel core such as wrought iron, in a jacket of high-carbon steel and then hardening the edge. However, under heavy usage, the edge would be more prone to chipping than its European counterparts, which were typically designed to deal with heavier armor than Japanese blades. This was answered by allowing projections of softer steel known as "ashi" to form in the hardened cutting edge during differential hardening of the blade.