Silver Refining in 17th Century

The silver trade was the blood circulation flow of the 17th century global commerce. In a previous blog post, we discussed the many forms of silver in different countries surrounding the East Asia trading activities. This post will cover the silver refining techniques adopted in the two main sources of silver: Japan and Spanish Americas.

Japanese Silver Refining: The Haifuki

Japan launched itself as one of the main sources of silver in the world in the 16th century with the discovery of the Iwami Ginzan and other mines. With the discovery of the silver sources in the form of raw silver-sulfide, the corresponding refining technology, Haifuki, was imported from Korea. This technique belonged to a general category of pyrometallurgy, meaning metallurgy for extraction of metals by thermal treatment. Lead was the essential additive material for this technique.

Step 1: Smelting with Lead

The process began with smelting. To extract the silver, the refinery process calls for smelting the silver-sulfide ore in a furnace with an intentional addition of lead. At high temperatures, the lead would act as a “collector,” fusing with the silver to create a lead-silver alloy.

Step 2: Purification

The second stage was the purification of this lead-silver alloy. The lead-silver alloy was placed in a shallow hearth or furnace and heated to a high temperature (around 900–1000°C) while a strong blast of air was forced over the surface. The hot air oxidized the lead, turning it into lead oxide (known as litharge). Silver, being a noble metal, did not oxidize. This molten lead oxide was either absorbed into the porous hearth or skimmed off the top, revealing the pure silver at the bottom.

The Iwami Ginzen: https://en.wikipedia.org/wiki/Iwami_Ginzan_Silver_Mine#/media/File:Iwami_Ginzan_Silver_Mine,_Ryugenji_Mabu_Mine_Shaft_001.JPG

Spanish American Silver Refining: The Patio Method

Prior to the Spanish rule, the natives in Americas had used smelting furnaces to refine high grade silver ores. By mid-16th century, the Spanish had run out of high-grade silver ores and was faced with large deposits of lower grades silver ores in the mines. Developed by Bartolomé de Medina, a Spanish merchant, the patio method belongs to a general category of techniques called hydrometallurgy, or the extraction of metals by water. As with the Haifuki method and its additive lead, a key additive material was used in the patio process: mercury. The development of the patio method allows the Spanish to refine silver from the lower grade ores on an industrial scale, thus enabling the East Asian trades in the 17th century.

Step 1: Crushing and Mixing

First, the silver ore was crushed into a fine powder, often using large, mule- or water-powered stamp mills. This powder was then spread in a large, stone-paved courtyard (the patio).

Step 2: Amalgamation

The ore powder was then mixed with water and salt, copper sulfate, and liquid mercury (also called “quicksilver”). For several weeks to months, this toxic “paste” was continuously mixed. This was often done by laborers, who would tread through the mud barefoot to facilitate the chemical reaction. The mercury chemically bonded with the silver particles in the ore, dissolving them to form a thick, pasty silver-mercury alloy.

Step 3: Separation

After the reaction was complete, the entire mixture was washed with water. The lighter waste rock and mud would wash away, while the much heavier silver-mercury amalgam remains.

Step 4: Purification

The final step was to separate the pure silver from the mercury in the amalgam. The amalgam was heated intensely. As mercury has a much lower boiling point than silver, the heat vaporized the mercury as a gas. Silver, in a sponge form, emerged with the mercury vaporized.

Mines in Pachuca, Mexico in the 1940’s. Pachuca was the city where de Medina first applied the patio method in scale. https://picryl.com/media/pachuca-mexico-silver-or-copper-mine

Conclusion

This blog post covers the two different techniques of silver refinery Japan and the Spanish Americas. The two main sources of silver supply in the 17th century adopted different methods that were suitable for their available ores. The successful extraction of silver in these two regions allowed the trade relationships to develop between places around the world.