A modern bladesmith rooted in the old traditions

Japanese tatara

This will describe the building and running of my first kodai tatara-like smelter. Many things have changed since then in the way that I do things now but it was a good first step. Operating the tatara alone by myself proved to be exhausting and very gratifying in the end when steel was recovered from the bottom of the smelter.

Charcoal is the fuel, the heat source and the source of carbon. Charcoal has to be chopped to a specific size for the operation to run smoothly. The bore of the furnace is 11 inches so one inch cubic seems to be the right size for the charcoal: 8-10% of the inner diameter of the furnace.

To keep things single I used an ore with a known iron content from a ceramic supply store. The name of this particular compound is Spanish Red. It is a light dust having the bad habit of staining everything it touches permanently red and contains 80% hematite.

The furnace design is based on the shell of a water heater. It has been cut in three sections for easier assembly and dis-assembly. Here is a diagram.

The parts were cast with refractory made mostly of sand with an inner lining of high temperature furnace cement. It held up pretty well to the heat but crumbled at the time of taking it apart.

The tuyeres were made of simple black iron pipe fittings. They stayed cool by the constant air flow through them and since they don’t protrude in the inner chamber they didn’t melt away.

Here is an additional picture of the bottom part of the assembly after pouring the refractory and letting it dry. It is shaped like a funnel as in the original Japanese tatara.

View of the assembled parts half way through the construction.

Here is the setup with the smelter put together and starting the pre-heat.

A home made manometer to gauge the air flow pressure. The manometer was indirectly estimating the amount of air flow.

Charges started at 1/2 kilogram of ore and half the way through I went up to 1.25 kilograms. Using about 2 kilograms of charcoal per charge.

When I heard bubbling at the tuyeres I started taping the arches but keeping the tuyeres open and trying to keep up with the schedule of charges proved to be very challenging for a lone operator.

As I was reaching exhaustion I started to take it apart the different sections. The top third of the smelter lifted off easily but the refractory crumble to pieces on the rims. So much from trying to have a system that was reusable! The parts will need to be repaired each time.

The middle third has now been removed as well as the tuyeres.

After cleaning up the charcoal a little bloom is peaking inside. It looked like Mount Fuji covered in snow.

Here is the bloom. A total of 5 pounds of consolidated steel and lots of smaller pieces. A very poor result by today’s standards but 10 years ago it was a victory.

The spark test looked pretty good.

This is the largest chunk of the bloom divided in two with the band saw. Resembles a Rorschach Ink Blot Test. What do you see?

After polishing the surface and etching a bit you can see all these beautiful dendrites. This is an indication that the core of the bloom had reached liquidus at some point.

 

And here is a little bit of friendly chemistry for us all.

 

Either hematite (Fe2O3) or magnetite (Fe3O4) can be effectively reduced to metallic iron in the right conditions.

At around 600ºF~1500ºF        Fe3O+ CO   →   3FeO + CO2

At around 800ºF~1800ºF        FeO + CO   →   Fe + CO2

At above 1400ºF~1600ºF        3Fe + C   →   Fe3C

 Charcoal is the fuel of choice and the source of carbon. Mixed with oxygen at the right temperature will form carbon monoxide, which is the reducing agent. Depending on the height of the stack, the time span during which the ore is exposed to the reducing atmosphere in the furnace, the temperature and other factors, the end result will be reduction of the iron oxide to metallic iron and absorption of carbon. The usual result of this type of furnace is a mixture of cast iron, steel, and wrought iron combined in a non-homogeneous mass called “bloom.”

The quality of the slag and the atmosphere in the hearth of the furnace will be greatly responsible for the final carbon content in the bloom.

 

And here is a little video of the process.