Incorporating the technology used at the Kai Gold Mine and the Iwami Silver Mine brought further developments to the Sado Gold and Silver Mines.
This section introduces Sado’s mining technology, from extracting ore to minting koban coins.
Edo period (1603–1868) mining technology
1Excavation of ore (mining)
In the early stages of mining, “surface mining,” in which veins appearing on the mountain’s surface were dug along with the sediment, was mainly conducted. The Doyu-no-warito Opencut Site, a symbol of the Sado Gold and Silver Mine, is a representative example of surface mining.
When it became impossible to dig deeper from the surface of the ground, technology called “tunnel mining” came to be conducted to dig tunnels from the side of the mountain. The ore at the Aikawa Gold and Silver Mine was said to be so hard that only about 10 cm per day could be dug.
Surveying methods were developed because of the need for advanced surveying techniques to dig the tunnels horizontally. As the tunnels became deeper, underground water posed a problem, and a tool called suisho-rin was introduced for efficiently draining the underground water. Yamadome (earth-retaining) technology was developed to reinforce weak areas inside the tunnels by using timber.
When mining the ore, the miners clamped the mining chisel with pincers called uedabashi and hit the chisel with a hammer. Since uedabashi were used for clamping, even shortened chisels could be used. Because the ore was hard, each miner was said to have gone through about one chisel in two days. A winnowing fan used in rice sorting was also used to ventilate the tunnels.
Mining in the tunnels was a constant battle against water, and water drainage was the most important part of the work inside the tunnel. In the olden days, suppon-toi (water pumps based on the principle of a syringe) were used, but in 1653, suisho-rin (Archimedean screw pumps) were introduced for efficient drainage of water.
Suisho-rin is a water drainage device that draws water up by rotating a spiral plate attached to the inside of a cylinder, and has long been used in the West as the so-called Archimedean screw pump. Suisho-rin were introduced to Japan via China during the Edo period.
The mined ore was brought to the ore-dressing plant, crushed with an iron hammer, and ground into finer granules with a stone mill. The ore was then placed in a water tub and sifted using a pan to separate the lighter sand from the heavier gold and silver. The remaining sand, which still contained a small amount of gold and silver, was subjected to a process called neko-nagashi (blanket sluice). A cotton cloth was laid on a wooden frame shaped like a slide, and the sand was poured over the cloth so that the gold and silver contained in the sand would adhere to the cloth. This activity was repeated many times to recover the gold and silver.
The stone mills used in Sado for crushing ore were ingeniously designed for efficiently grinding ore by changing the stone quality of the upper and lower parts of the stone mill. The upper part of the stone mill was made of spherulitic rhyolite from the Fukiage quarry, and the lower part was made of conglomerate from the Katabe-Kanoura quarry and other quarries on the island.
Gold collecting procedure
01.Ore is crushed with an iron hammer
02.The ore crushed with a hammer is further ground with stone mills.
03.The ground ore is poured along with water onto a cloth drawn below, and the gold adhering to the cloth is recovered (neko-nagashi).
Fukiage Quarry Site
This was the stone-cutting plant for the upper part of the stone mill for mining, where quarrying was carried out for an extended period from the 17th century to the early 20th century. Numerous ya-ana holes (arrow holes) and tagane (mining chisel) marks remain on the rocky shoreline. (National Historic Site/National Important Cultural Landscape)
Smelting was done in a place called a tokoya (smelting plant). First, gold and silver recovered at the ore-dressing plant are melted along with lead over a charcoal fire to form an alloy of gold, silver and lead. Next, the alloy is heated in an iron pot covered with ash, and the lead soaks into the ash, leaving only gold and silver. This process is called cupellation, which was introduced from the Iwami Silver Mine. Two methods were used to separate the gold-silver alloy into gold and silver, the “separating silver with sulfur” method, in which sulfur was added, and the “cementation with salt” method, in which common salt was used. These methods were combined and repeated four to five times to increase the gold’s purity to match the koban coins’ grade (66-87%).
The remains of a furnace and earthenware excavated from the Sado Magistrate’s Office Site, as well as surviving historical documents such as technical manuals and picture scrolls describing the work procedures of Sado’s cementation method have revealed the operations at that time. Sado is the only place in Japan and one of only few throughout the world where one can find remnants of the cementation method. Ore-dressing plants and smelting plants were scattered throughout Aikawa; however, to address reduced production and manage operations, Ishigaya Kiyomasa, the magistrate of Sado at the time, consolidated the ore-dressing and smelting plants under the magistrate’s office in 1759, to efficiently manage and operate them as a yoseseriba (ore-dressing plant) and a yosedokoya (smelting and refining plant).
01.Lead is added to the ore powder containing gold obtained in the ore-dressing process to make work lead and remove impurities.
02.Ash is spread in an iron pot; the work lead is placed on top of the ash and heated. The lead melts first and soaks into the ash, leaving only gold.
The gold obtained by cupellation is crushed.
04.Mixing with salt:
The finely crushed cupellation gold is mixed with sea salt and hardened into a cylindrical shape.
05.The cylindrical lumps are placed between clay pots and arranged in a furnace, and charcoal is filled around them.
06.After heating for a long time, the silver reacts with the salt to form silver chloride and separates from the gold.
※Photograph on the rightPhotograph on the under：Site of the furnace for cementation at the Sado Magistrate’s Office
07.Crushing in water:
When the burnt lumps are broken down by placing them in a water tub, the silver chloride dissolves in water and only the gold sinks to the bottom of the tub.
Producing Koban Coins
Aikawa not only produced gold and silver from ore but also minted koban coins. This was made possible by the isolated geography of the island and the magistrate’s office’s centralized management system. It is extremely rare throughout the world that everything from ore mining to minting koban coins was done at the same mining site. Koban coins were minted at the koban coin minting plant, located next to the magistrate’s office. First, gold is melted and hammered into plates, then cut with scissors into pieces as heavy as one koban coin. The pieces are then hammered into the shape of the koban coin, and a pattern called gozame (a pattern on a rush mat) is embossed on the coins. The coins are then subjected to the iroage (re-dyeing) process, in which they are coated with chemicals and heated to extract a better golden color before they are finished.
Sado’s koban coins, gold and silver were transported to Edo (present-day Tokyo) every year under strict guard. They were transported from Aikawa to Ogi and from Ogi to Izumozaki on the coast of present-day Niigata Prefecture by boat and further to Edo via the Hokkoku Kaido and Nakasendo routes.
Mining technology since the Meiji period (1868–1912)
After the Meiji period, the mining technology was mechanized. Rock drills and gunpowder replaced hand mining, and pumps came to be used for water drainage. Ore was transported by trolleys or ropeways, sorted, and crushed into small pieces by a stamp mill※.
In the smelting processes for extracting gold and silver from ores, the amalgamation method, using mercury combined with gold and silver to recover the precious metals, was introduced at the beginning of the Meiji period, but in the latter half of the period, the gold cyanidation method became mainstream, in which gold was dissolved in cyanide and then salvaged. In the early Showa period (1926-1989), flotation methods were developed. After recovering gold and silver, the dregs were mixed with a flotation reagent to recover even the fine grains of gold and silver that floated up with the bubbles.
※A machine to crush ores with a pestle made of iron
Ainoyama Stamp Mill
Completed in 1891, this facility was where ore was ground and smelted using mercury. This made it possible to extract gold and silver from previously discarded ores. (National Historic Site/National Important Cultural Landscape)