Check out these Basic Books on Gold Refining from AbeBooks.com

Basic Refining Books Gee
Basic Refining Books C.W.Amman
Basic Refining Books George Gajda
Taxay in U.S. Mint and Coinage has several chapters on Refining Processes, especially the Mint using salt precipitation with nitric refining of coin silver in the early years. His other books are full of refining references as well.

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