炎中の火花「沸き花」発生を鍛接開始の指標とする和鉄鍛接機構

“Tsumiwakashi Tanren” is the forge-and-welding process to form a steel block from steel pieces that are produced by Tatara. “Orikaeshi Tanren” is the forge-and-welding process to hold up a steel block to produce a steel plate. Steel blocks were covered with straw ash and mud and heated in blacksmith's furnace. When the temperature of steel blocks increased over 1100°C, the fire flame of charcoal colored yellow followed by orange. The yellow flame was D-line spectrum of sodium with 589 nm wavelength and was a sign of production of molten fayalite slag, so called “Noro”. When temperature increased over 1190°C, small and bright sparks so called “Wakibana” started to appear in fire flame from charcoal. When temperature attained at 1290°C and many “Wakibana” sparks appeared, blacksmith took out steel blocks on a anvil and forged them to weld by hammering. At this time, temperature at the interface of steel blocks increased to about 1470°C by oxidation of iron and decarburization and the surface of steel block melted and wetted. Then, by hammering, “Noro” film was broken and steel blocks easily welded. When small bubbles of CO gas were produced at steel surface by strong force, fine steel particles were caught in CO babbles and oxidized in air to make bright sparks of “Wakibana”. “Wakibana” is a start signal of forge-and-welding process.