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Sunday, August 16, 2015

Jewel Steel

Tamahagane, in Japanese a term for the result of a smelt yielding a very high carbon steel bloom, translates in English to Jewel Steel. Height and diameter of the reduction furnace, air pressure and by extension temperature, fuel quality and type, and the type, refinement, and size of the ore all play an intricate part in the production of bloom, each of which can dramatically change its resulting properties.

I had the opportunity to join Mark Green for an experimental magnetite smelt which resulted in a beautiful, carbon rich bloom of surprising yield. Magnetically separated, crushed magnetite was used as the sole ore source, which averaged between 50~60% iron. Typically, ore with less than around 80% Fe is not worth the effort of smelting due to the low yield. Additionally, this magnetite was high in Aluminum Oxide at around 20% despite cleaning and separation. To accommodate the magnetite, the stack was extended by an extra 12 inches for a total of 48 inches from the floor to the top. Because of the extra volume of charcoal, the pressure, and subsequently volumetric flow rate of the air was also increased, which caused the smelt to run a bit hotter and faster than the lower stack.

Throughout the smelt, 19 total charges went in, totalling 51.8lbs of magnetite and 83.75lbs of charcoal plus the initial fill of about 30lbs. The result, 11.25lbs of dense, high carbon bloom ranging between 1~1.1% C.

Due to the nature of the magnetite and likely the high aluminum content of the slag, tapping the furnace was initially difficult, with a thick, sticky slag that did not flow easily.

After a few taps, the slag began to flow more readily. Inside the furnace, the slag bath was very large and active, bubbling and spluttering wildly as it crept higher and higher.

Through the tuyere, the bloom (dark spot) is visible the charcoal faintly visible on top. This is the first clear shot of a bloom I have managed to get through the tuyere. Throughout the smelt, droplets of slag were visible leaping around inside the furnace, sometimes splashing onto the tuyere orifice and needing to be knocked off. During extraction, we found a large plate of slag and bloom impurities along the side with the tuyere about as large as a dinner plate.

When the time came to pull the bloom, another great volume poured out of the bottom, forming a fiery lake of boiling slag.

Left behind, a tentacley beard sat in the opening that would later birth the bloom.

In extracting the bloom from the furnace, a large portion of the wall fell away. Not unexpected, the wall had become fragile over the course of several prior smelts, and all throughout the day grew cracks spouting gouts of flame that crept steadily higher and higher. 

Already a hot day, the radiant heat put off by the opened furnace and the bloom is immense. Weighing in at 11.25lbs, it was like a meteor that could burn you from simply looking at it.

Cut in half with an axe, the two pieces were cooled and weighed, and then tested for carbon content. Consistently, the bloom sparked well into the hyper-eutectic range, around 1~1.1% carbon.

In the beginning it was unclear whether or not the bloom would yield good steel, or if it would turn to the cast iron range with the amount of air to compensate the higher stack. A few changes may be implemented next time, but the test of pure magnetite was overall very successful. 

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