An original piece from Morioka, Iwate Prefecture, Japan
Nanbu tekki (南部鉄器), or Nanbu ironware, were first made in the 17th century, when master craftsmen from Kyoto were invited to Morioka in Iwate Prefecture. Ever since, Nanbu tekki have been considered Japan's finest ironware. The masters' knowledge and techniques are heavily guarded and only passed on from generation to generation.
Cast iron (鋳鉄 chūtetsu), which is normally susceptible to rusting, is baked at 800°C using the special Kamayaki technique. In the process, the iron is specifically oxidised and activated and thus receives a natural rust protection.
The elaborate manual production requires more than 3 weeks. The mould, including pattern, is formed by hand and can only be used for casting about five kettles. Japanese tetsubins are made much thinner and more delicately than Chinese copies. Usually, Chinese cast iron has considerably more manganese and therefore has to be cast thicker. The training for producing high-end Japanese tetsubins takes 15 years and has only been achieved by a handful of masters in Japan.
Coated only with urushi (漆), a purely natural lacquer obtained from the sap of the Japanese lacquer tree, and cha jiru (茶汁, "tea juice") for the noble golden-brown colouring. After heating the tetsubin to 250℃, the natural mixture is applied to the surface with a special brush (kugo-hake くご刷毛).
No coating, the water comes into direct contact with the activated cast iron and can thus react chemically or be improved. If tea is steeped in the Tetsubin, the water may turn dark due to the reaction with the catechins. This is described by Iwachu as normal and harmless. The Tetsubin should be emptied completely after each use and dried relatively quickly (dry as hot as possible), otherwise rust can form despite the activated cast iron.