A 600-year industrial complex
In the Fanshan area of Wenzhou, a vast network of tunnels and chambers cuts deep into the mountains. This is not a natural cave system but created by more than 600 years of continuous mining that began in the Ming Dynasty (1368-1644). The objective was to extract alunite, a stone with the chemical compound alum. At its operational peak, the area, known as the "World's Alum Capital," contained 76 distinct mining sites and over 200 mining teams. The total alunite deposits here are estimated to exceed 240 million tons, which accounted for 60 percent of the world's known reserves.
The scale of the underground workings is immense. Generations of miners excavated countless tunnels, creating a subterranean world complete with ventilation shafts and large caverns that supported the industrial activity. This long history of extraction, which only ceased traditional operations in December 2017, shaped the geology and the culture of the region. The site is a national industrial heritage location, with sections like the Jilong Mountain caves being developed into an underground museum.
The chemistry of extraction
The entire mining enterprise existed to produce potassium alum (KAl(SO₄)₂·12H₂O), a salt used for many purposes. The source material is alunite (KAl₃(SO₄)₂(OH)₆), a potassium aluminum sulfate mineral. Transforming the raw stone into purified alum crystals is a multi-step chemical process that explains the massive industrial scale. The low yield of the final product required processing enormous quantities of rock.
The process began with quarrying the alunite ore. Miners would then heat the rock in a process called calcining, or roasting, at temperatures between 520 and 620° C. This thermal decomposition drives off water and begins to break down the mineral structure. After roasting, the material was steeped in water. This leaching step dissolves the valuable aluminum sulfate and potassium sulfate, creating an impure chemical solution. The final stage involved boiling this liquid to concentrate the dissolved salts until, upon cooling, crystals of pure alum would form and precipitate out of the solution.