In the summer of 2010, Achim Wirtz (check some of his work here)  & Pavel Rihacek did build and operate a low-furnace tatara (kera-oshi tatara) with the help of their wifes Katerina & Alexandra and the folks from Lohmann Stahl.

They started to build the tatara on Saturday the 12th and finished it by Wednesday the 16th (so you thought you could build that in a single afternoon ).

The original tatara, as you can find them in Japan are 3m long and 1m wide but since they only had 1 week time to build and operate it, they scaled it down to 1.30m long, 1m wide and 1.3m high.

Here you can see the charcoal they gathered to keep the furnace going :

This is how 3700Kg charcoal looks like

16 Big bags with a weight of 200-250kg each. So if we take our calculator, that’s around 3700kg total. At the end , they have been using ‘only’ 3000kg of it. This charcoal was made of pinewood in a traditional way. These are not the same as the charcoal we use to have a barbecue party.

That kind of charcoal is too heavy, can’t reach the high temperatures they need and is glowing too long.

The iron ore that has been used, Sishen (coming from South Africa), is exact the same as the Japanese satetsu when looking at the chemical components within it . The difference here is that the japanese satetsu is black and the Sishen obviously not. They had round 2000kg of ore but ‘only’ 1300 kgs has been used during this process.

Sishen iron ore

And off we go, they digged a pit, 25cm deep and light a fire to make it completely dry.

Making a first fire to get the pit dry

The pit is filled with small pieces of charcoal

pit filled with charcoal

Basic construction : to build the furnace, they used 250 fireproof stones, 500 Kg clay and 700 Kg sand (quarz).

top view

Bottom part of the furnace

Pavel is finishing the bottom part with clay

Done

The finished furnace, ready to go...

Thursday 17th, 9.00h…Katja Lohmaan-Hütte, owner of Lohmaan-Stahl, is activating the furnace.

Let it burn..

Pavel checking the vents

Time for a first check

The First slag is coming out after 17 hours

Slag coming out

During the second night (Friday to Saturday) there was “a sea of magma”

Magma

Saturday 12h, letting the slag out for a last time

Almost there

Saturday 13h, filing the furnace for a last time with ore

Filling the furnace with ore for the last time

Saturday 15.30h, starting to take the tatara apart. At first sight it didn’t look like a lot of steel was produced

Taking the tatara down

Hey…they did find a little piece of steel

The first piece tamahagane

With 1300Kg ore being used, the little piece was only a fraction of what was coming…The next piece they found was a ‘bit’ bigger and too heavy to get it out by hand so they had to use other equipment…

Time to get some help to lift the big chunks

There were 2 big blooms produced, one of 180Kg and another one weighting around 210Kg. Besides that they did found another 25-30kg in the furnace and after cooling down there was 20-25kg cast iron found at the bottom of the tatara.

The biggest blooms were taken to the forge, where a 1500kg (air) hammer was used to make smaller pieces out of the big chunks.

The 1500 kg air hammer

The tamahagane that has been produced was of an excellent quality. No slag, very little wholes and big grains.

After building the furnace (12hours/day) and working 55hours straight to operate the furnace (every 10-15 minutes adding 10Kg charcoal and 7Kg ore) they finally had what they were looking for…real tamahagane..

Steel is iron that contains carbon, so the more carbon a steel has , the harder it is. Most of the Japanese sword smiths are working with the traditional steel that is better known as tamahagane. In Japan there’s only 1 official smelter left. This Japanese style smelter is called a tatara. The tatara relies on the propensity of very hot iron to combine with carbon in its vicinity to produce steel.

In the tatara, it is the burning charcoal that supplies the carbon. These typical Japanese smelter is made of clay, about 5 feet wide, 48 inches tall and 15 feet long, where it’s walls are 10 inches thick.

One operating cycle of the tatara takes 5 days. One day to build the walls (clay and sand), 3 day’s to smelt and 1 day to remove the iron out of the tatara. In this 5 days process , they need about 13 tons of charcoal and 8 tons of satetsu (black sand) to produce 2 tons of iron and steel, which is called Kera. [Read more...]

When the clay coating process is done, the blade is heated again until it reach it’s critical temperature, which depends on the kind of steel that is used but will have an average of 750 – 800°C.

Once the blade reached the 750-800°C, the structure of steel changes to austenite. Now, when the blade is quickly cooled by quenching (in water), austenite changes to martensite, the hardest type of steel. However, because of the clay application, the blade will cool more slowly where the clay is thick (on the back of the blade), turning not into martensite but instead forming ferrite and pearlite, which are softer and more flexible. [Read more...]

Lately I was involved in a discussion about the hardening process of a Japanese sword and I did found it very inspiring to write a small piece about that just to give you a better understanding of that part of the forging process.

The hardening process of the blade is in one of the most important and perhaps the most difficult part of the  sword making process. It is the correct hardening of the Japanese sword that gives the blade its ability to take and retain an amazing sharpness.

After the blade is forged and finished to it’s final (raw) shape, the differential hardening process can begin. [Read more...]