1. The Raw Materials: Iron Sand Gathered with Magnetism, and Charcoal
The first step involves gathering black sand using magnetism. This sand, known as "masago" iron sand, is the raw material for the "kera-oshi" method of making steel. At the Nihonto Ho Tatara, it is collected by hand using magnets in an area adjacent to the nearby Hanaidani Mine [4]. The charcoal is also produced on-site. It is made from miscellaneous trees in the surrounding mountains, burned in charcoal kilns within the facility grounds [4].
For one single operation, the amount used is about 10 tons of iron sand and about 12 tons of charcoal [2]. These numbers will appear again later in the article.
2. The 3-Day Process: Adding Iron Sand and Charcoal Alternately
The general order of the operation is as follows: build the furnace, light the fire and blow air, add iron sand and charcoal alternately, remove the melted waste (called "noro"), reduce the amount added toward the end, and finally break the furnace to remove the kera (iron lump) [9][1]. At the Nihonto Ho Tatara, this 3-day, 3-night operation is counted as one "yo" (cycle). It is said to be performed three times between mid-January and early February, when humidity is low [4].
It is important to note that the specific times and amounts vary by region and era. One document describing the procedure refers to tatara in the Hoki province, which must be read separately from the Nihonto Ho Tatara details [9].
In the past, as furnaces grew larger, a building called "takadono" (高殿) was constructed indoors. Air was blown into a furnace approximately 250–300 cm long, 70–90 cm wide, and 110 cm high using a large bellows device called "tenbin-fuso." The person who stepped on this device to pump air was called a banshi (番子) [8].
3. The Murage: The Leader Who Reads the Fire, Wind, and Sound
The leader of the tatara operation is called the "murage" (村下). The Japan Heritage Portal describes this person as the chief engineer [3].
Regarding the murage, an article introducing the Nihonto Ho Tatara states that they "read the fire, read the wind, and read the sound of the boiling sand" [4]. Since it is difficult to look directly inside the furnace, this appears to mean that they infer the conditions inside from the fire and sounds coming out. Another crucial task is adjusting the amount of "noro" (melted waste other than iron) so it is neither too much nor too little [3].
4. Breaking the Furnace to Remove the Iron Lump and How Much Becomes Tamahagane Steel
The most intense moment comes after 3 days and 3 nights. Breaking the furnace to pull out the iron lump is called "kera-dashi." The Japan Association for the Preservation of Japanese Art Swords introduces this as the most tense and powerful scene in the entire operation [1]. At the Nihonto Ho Tatara, about 2.5 tons of this lump, called "kera," is produced [1].
The kera is not directly "tamahagane" (the high-quality steel used for swords). According to the Nagoya Sword Museum, from one operation, about 2.5 tons of kera is produced, of which about 1 ton becomes tamahagane [2]. This is about 40% (1 divided by 2.5). Tamahagane is graded from 1st to 3rd class based on carbon content, impurity content, and the uniformity of the broken surface [2]. The tamahagane produced in one cycle is provided to about 200 swordsmiths nationwide [4].
In historical descriptions of places like Sugatani Tatara, it is said that about 10 tons of sand and 12 tons of charcoal produce about 3 tons of kera and about 1 ton of tamahagane [3]. This differs slightly from the Nihonto Ho Tatara's 2.5 tons, so these figures should not be mixed.
5. Why Taking Time Slowly Makes the Iron Purer
An explanation by Nakagawa Masashichi Shoten states that tatara is a method of slowly extracting iron from sand using sand and charcoal over time. During this process, impurities dissolve out, resulting in higher purity [6]. The carbon content of tamahagane is introduced as around 1.2%, and around 0.7% after forging, but these figures vary by source, so they should be read as rough estimates [6].
The clay walls of the furnace also play a role. They are explained as working like a catalyst to expel impurities [7]. According to Nakagawa Masashichi Shoten, about two-thirds of the iron sand becomes kera, while about one-third falls to the bottom as sand and reacts with the furnace soil. This ratio is from one specific source, so it should be read within the scope of "it is said that" [6].
6. Making Charcoal: The Successors Training to Become Murage
The people burning the charcoal are successors called "murage-yosai-in" [4]. The Association's article reports that 9 people are refining their skills as murage-yosai-in [5]. However, this is the number at the time of publication, and current numbers have not been verified.
Before the person who reads the fire stands by the furnace, there is the work of making charcoal. There are people learning these skills there too. It is not just the leader of the operation, but also those involved in making the materials, who keep the tatara tradition alive.
7. Try It Yourself: Finding Black Grains in Sand with a Magnet
If you have a magnet at home, try bringing it close to sand placed on paper (such as soil from a flower bed or sand from a potted plant). If you try this in a park sandbox, please be careful of the people around you and the park rules. Black grains may stick to the magnet. In some places, nothing sticks at all.
Comparing places where it sticks and places where it doesn't can help you imagine where iron sand tends to accumulate. Do not put sand in your mouth, and wash your hands after touching it.
If you want to know more, you can read about the operations on the website of the Association for the Preservation of Japanese Art Swords [1][5].