The Shogun's Golden Touch
On Sado Island, the waste products of a centuries-old gold rush show the remarkable metallurgical skill. From 1601 until 1989, the Sado Kinzan, particularly the Aikawa mine, was a source of wealth for Japan, first for the Tokugawa Shogunate and later for the modernizing nation. In the early 17th century, at its pre-mechanized peak, the mine produced over 400 kg of gold annually, underpinning the shogunate's finances and its national currency. The purity of the refined gold reached an astonishing 99.54%.
This level of purity was achieved through a sophisticated, non-mechanized process known as haifuki-hō, or cupellation. The raw ore from the Aikawa mine contained a natural gold-silver alloy, or electrum. To separate the precious metals, artisans first melted the crushed ore with lead. The lead acted as a collector, bonding with the gold and silver to form a dense alloy. This alloy was then placed into a cupel—a porous crucible made from bone ash—and heated again. As powerful bellows blasted air across the molten surface, the lead oxidized into litharge (lead oxide). This litharge was either absorbed by the porous cupel or skimmed from the surface, leaving behind a purified button of gold and silver.