History of Japanese Steel
The earliest known iron artifacts were discovered far apart in Iran and Egypt and date from the 5th millennium BC. By the end of the 2nd millennium BC iron was being produced from Sub-Saharan Africa to China. It was not however valued as a cutting tool or weapon due to it being much softer than bronze.
It was the Hittites, an ancient Anatolian people, whose empire spanned modern day Turkey, Syria and Lebanon from 1600 BC, that are credited with the first smelting of iron. Although belonging to the Bronze Age they were the fore runners of the Iron Age. The archeological remains of the earliest known smelting works were discovered in northern Lebanon and are thought to date from 1,200 BC.
Early iron was produced in furnaces where bellows were used to force air through a pile of burning charcoal containing iron ore. Carbon monoxide produced by the charcoal caused the separation of iron oxide from the ore which would collect at the bottom of the furnace as a spongy mass of metallic iron. The pores of the iron were then filled with slag and re-heated. The iron would soften and the melted slag could be forced out by repeated beating and folding.
The Hittite Smiths discovered that this iron could be made much harder by adding carbon, which was done by further re-heating the iron in a bed of charcoal until very hot and then quenching it in water. This process turned the outer layers of the metallic iron into much harder steal.
Around 1,100 BC following a period of wars and invasions by many different tribes and peoples the Hittite kingdom disintegrated and its population was absorbed into the several different city states of the warring factions. Their iron smelting techniques were dispersed amongst different Middle Eastern populations and subsequently carried throughout Africa, India and Asia following the spice routes.
Iron smelting techniques are thought to have first reached Japan, via China and Korea, in the 6th century AD. Over the course of the next 4 centuries these smelting techniques, combined with new forging techniques, would become so refined that by the 11th century Japan was producing the hardest most refined steel and consequently the finest blades in all the world.
The invention of the tatara, a clay smelting furnace designed to reach and maintain intense heat over a sustained period of time, was the most essential and significant development of steel production in Japan. It gave the Japanese sword smiths a steel of remarkable purity and quality.
A clay box construction would be built, about 1.5 meters high, 3 meters long and 1.5 meters wide with the walls being about 30 cm thick. This would be allowed to dry and then fired. Small holes would be made and bellows attached to either side to force air into the lower half of the furnace and the tatara would be ready to use.
Tama-hagane is the name given to the high grade steel made in the tatara. A fire of soft pine charcoal would be started in the bottom of the tatara and once it had reached the correct temperature the smelters would add iron sand followed by another layer of charcoal. The bellows were used to force the heat of the previous layer up through the next layer.
The smelters would continue adding many more layers of iron sand and charcoal until the tatara was full and by adding air from time to time the super high temperature of the furnace (around 1000 degrees centigrade) would be maintained for up to 3 days. It is this process that helps remove impurities and even out the carbon content of the steel resulting in refined multi layered steels.
When the process is finished the clay walls were broken and the steel bloom (tama-hagane) removed. The best steel would be found on the edge of the block where the oxidation process was strongest. The quality of the tama-hagane would be judged by its colour, with the brightest silver pieces being very good for making blades.
It would take about a week to construct the tatara and complete the conversion of iron to steel. During this time the tatara would have consumed about 10 tons of iron sand and 12 tons of charcoal, creating about 2.5 tons of tama-hagane.
Yasugi-hagane is a sophisticated version of tama-hagane and was developed by Hitachi Metals, Ltd who continue to use the tradition of smelting high quality iron sand, with modern production methods, to create the premium grade high carbon steels used in the very best Japanese kitchen knives and modern day swords.
Hitachi metals also make the highest grades of stainless steel, used for Western style knives, of which ZDP-189 is the highest performing steel used in the knife industry today.
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