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When did iron manufacturing (the smelting of iron) begin in Japan?

Was there iron manufacturing during the Yayoi Period?

Given that no definite traces of iron manufacturing dating to that period have been discovered, the current established theory is that iron manufacturing did not take place during Yayoi times.

At present, those remains that are known to indicate iron working date to the first half of the 6th century C.E. (the Kanakuro-tani and Tonomaru-yama ruins in Hiroshima Prefecture, or the Imasaya-yama ruins in Shimane Prefecture, for example). However, considering that a large-scale blacksmithing group was established in the Onaru ruins in the city of Shobara, Shimane Prefecture, and the formations dating to the second half of the 6th century in the Enjo ruins (Tango Peninsula, Kyoto Prefecture)—a complex that included numerous ironworks and metal working furnaces—then it is possible to believe that iron manufacturing had already started in the 5th century.

On location at the excavation of an ancient iron manufacturing site (the later 6th century Enjo ruins)

Was there iron manufacturing during the Yayoi Period?

There are strongly grounded opinions on the one hand that iron manufacturing did take place in the Yayoi Period. Given that no iron manufacturing furnaces have been found, these opinions come from an emphasis on the following archaeological background factors.
(1) Stoneware rapidly disappears from after the mid-Yayoi Period, and ironware appears throughout the entire country.
(2) The use and manufacture of ironware elsewhere in the world, such as areas that are now England and Germany, occur at this time.
(3) The technology for manufacturing glass was present during Yayoi times, a process that involves attaining temperatures up to 1,400°C to 1,500°C.
(4) Large bronze bells were being cast in the late Yayoi Period (C.E. 200 to 300), and the most advanced metallurgical technology at this time was in East Asia.

The fact that the Komaru ruins discovered recently in the city of Mihara, Shimane Prefecture date to the 3rd century sparked excitement in the Japanese media, as they may be relics of iron manufacturing dating to the late Yayoi. Excavations have also been made that may be the remnants of ironworks dating to some point between the Yayoi Period (approximately 300 B.C.E. to C.E. 300) and the Kofun Period (approx. C.E. 300 to 538) in Kyono (in the town of Chiyoda) and the Nishimoto No. 6 (in the city of Highashi-Hiroshima) ruins in Hiroshima Prefecture.

As an explanation for the illogical chronological gap between the diffusion of ironware at the end of the Yayoi Period and the supply source for that ironware, the mainstream theory until now has been that the people of the time were dependent on the Korean Peninsula for all iron resource materials. However, on the basis of the discoveries of these various ruins it could be that a new page is being written on ancient iron manufacturing.

Iron slag found near an ironworks furnace in the Imasaya-yama ruins, Shimane Prefecture (Wako Museum)

Iron slag is an impurity produced when iron is smelted.

An Epochal Shift in Iron Manufacturing Technology around the 6th Century

Whatever the case may be regarding the start of iron manufacturing in Japan, what is definite is that it entered a new era around the 6th century. Even assuming that a previous method for manufacturing iron existed before then a Yayoi Period, it was most likely primitive and on an extremely small scale, perhaps involving the use of small furnaces to obtain limited amounts of reduced iron, and forging wrought iron mainly through metal working. The new era begun in the 6th century was brought about through the technologies and techniques of immigrant craftsmen who came from the Korean Peninsula.

According to the Kojiki, a Korean blacksmith named Takuso was brought to the imperial court from Kudara during the reign of Emperor Ojin. (The 15th emperor of Japan according to the Kojiki and the Nihongi (“Chronicles of Japan;” also called the Nihon shoki). There are no firm dates for his reign, but he is the oldest of the emperors in those chronicles whose existence is accepted as having been real.) The Kojiki also tells of an exceptionally talented blacksmith from Silla who was employed by the court in the 12th year (C.E. 583) of the reign of Emperor Bidatsu, and it was he who provided instruction in the techniques for working hagane. The exact content of those techniques is unknown, but it could be that it involved iron-manufacturing techniques based on a chamber-style furnace, in which iron ore was the primary material used. Perhaps these technologies and techniques included the new bellows technology, and the techniques for large-scale metal working that involve decarburizing and working of pig iron.

These iron-manufacturing methods undertaken by the early state were likely passed on to the Yamato and Kibi districts that were the backbone of support for the Yamato court (Japan's first relatively centralized government and the origin of the imperial family), and that the manufacture of iron using iron ore flourished for a time in ancient days. On the other hand, there is also a tradition of iron sand smelting focusing on Izumo, Japan's other major cultural heartland. It is not known when or from where this tradition arose, but it may date to a period after the technological revolution of the 6th century. It may be that the chamber-style furnace method of manufacturing iron was adopted from among a variety of techniques brought to Japan, thus giving rise to an ancient version of the tatara iron manufacturing method that blended with existing iron sand iron manufacturing.

The mysteries of ancient iron manufacturing are enveloped in the same deep fog that envelopes so much of Japan's ancient history.