Furukawa Ichibei acquired the Ashio mine in 1877 with financial backing from Shibusawa Eiichi. By 1922 the operation had consolidated its three separate ore-processing plants into one. The Tsudō Ore-Dressing Plant, on the Watarase River, was held up at home and abroad as a model facility for metal mines.
METI's 2007 register of modern industrial heritage carries the plant as item 02 of Story 12, the Ashio modernisation and pollution-prevention narrative. The 2008 National Historic Site designation, by contrast, covers the mine shafts and the smelter but leaves the dressing plant unlisted.
Inside, the process ran top-to-bottom by gravity. Raw ore dropped through hoppers, then through hand-riveted ball mills, then into timber froth flotation cells dating from 1918. At one of the hoppers, the ore is still piled around the spade in the position the operator left it.
The Ashio mine closed in 1973 and the dressing plant stopped with it. The buildings sit on Furukawa Machinery & Metals private land, not publicly accessible. Water still runs through the PVC pipes inside under gravity, likely part of the environmental controls keeping contaminated water out of the Watarase.