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Pan Amalgamation and Variations

After Bartolomé de Medina developed the patio process, silver mining was revitalized in 16th century America. With amalgamations now in their repertoire, precious metal refiners continued to evolve and improve their techniques by using the patio process as a basis.

One of the major weaknesses of the patio process was how long it took. In order to speed up the process, Alvaro Alonso Barba invented the pa process, or pan amalgamation in the early 17th century. Instead of mixing salt, water, mercury, copper sulfate and ground up silver ore in a large outdoor patio, the pan process mixed these ingredients in several shallow copper pans heated with fire (instead of the sun). This reduced the amalgamation process from taking more than a week, to only about 15 hours. 

In the mid-19th century, Almarin Paul further industrialized pan amalgamation by developing the Washoe process in the Washoe Valley (present day Nevada). For this refining method, the copper pans were replaced by iron tanks with mechanical agitators that were heated by steam pipes. A circular iron plate called a muller was lowered into the tank and rotated to provide agitation and additional grinding of the ore.  The iron filings worn from the muller and tank were discovered to be an essential ingredient in the process, improving efficiency and speed even further.

However, the Washoe process didn’t work well with ores that were rich in arsenic, antimony sulfides, galena, or sphalerite. Carl A. Stetefeldt discovered that such ores could be roasted with salt to convert the silver sulfides to silver chlorides, which could then be recovered in amalgamation pans. Since this method first gained popularity in the Reese River district, it became known as the Reese River process.

Eventually, better methods of silver mining and refining were developed and amalgamation-based processes fell by the wayside. But without them, silver mining would not be as developed as it is today.

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