A week in images at Manabe Sumihira’s zuku-oshi tatara
Mr.Manabe uses akome-satetsu, a type of complex iron ore that includes various types of oxides (Fe3O4, Fe2O3, FeO), a considerable amount of silica oxide as well as other important impurities. Most iron ores available for industrial purposes are closer to what is called masame-satetsu in Japan, composed mainly of the overly strong Fe3O4, difficult to reduce in a continuous process tatara.
First, the iron ore must be sifted and prepared into pellets to help the reduction process and concentration of the liquified iron into heavier lumps that flow more efficiently to accumulate at the bottom of the furnace. This involves the preparation of very fine charcoal powder, a very time consuming and dirty job performed by apprentices, or me in this case. I processed over 100kg of charcoal in order to sort it through five different mesh sizes and isolate the powder. I also had to produce some by crushing fine charcoal in order to meet our required quantities.
Meanwhile, the furnace is repaired and prepared for the next smelt. This involves patching with refractory cement, de-icing remaining slag and iron on the furnace walls, putting it together.
It is important to dry the furnace by pouring a good load of hot coals let to burn free for an entire night in order to make sure the furnace is free of any moisture… or it could explode!
Pellets are prepared ideally in advance to be allowed to dry, but M.Manabe has taken the habit of getting this done the same day without any negative effect he could notice.
The furnace is fired early in the morning and for about 10-12 hrs. Each load of pellet-shaped ore is added with a load of different sizes of charcoal, allowing to control the speed at which the process advances, as well as the more important reduction-carburization process.
At some point, slag accumulates and can be seen through the windowed tuyères. It must be let out. Bits of liquified cast iron are trapped while flowing through the slag, and escape with it, burning instantly into beautiful fireworks.
In preparation for the harvest, the crucible is pre-heated in a forge (seen in the background on this picture) in order to dry it and minimize thermal shock.
In order to pour the accumulated cast iron out into cake-sized molds, the top section of the furnace must be lifted away, and its base tilted to help the metal flow out into a crucible.
Mr.Manabe receives the liquid metal into the crucible, and rapidly pours it into the molds.
The furnace is taken apart and its bottom section emptied of remaining charcoal, metal and slag.
This time only hachime (« honeycomb pattern ») cast iron was produced, a rather low carbon type (between 2 and 2.3%). In comparison, koorime (« ice pattern ») includes more, sometimes as high as 4%, and is clearly more fluid.
This cast iron must now be decarburized into steel. This is done about a month later.
More about the zuku-oshi tatara: Home-made steel: A week at Manabe Sumihira’s zuku-oshi tatara