Gold can be purified to its most highly-refined state through the Wohlwill process, which was invented in 1874 by Emil Wohlwill. In this electrochemical process, impurities are removed from the ore by first melting it with chloroauric acid, which serves as an electrolyte, and then passing an electric current through it. The resulting electrolytic reaction will cause the gold to collect at the cathode end of the system through ion transfer while the impurities will settle to the bottom of the electrolyte as particulates.
The Wohlwill process requires the anode portion of the system to be made from an ingot of 95+ percent pure gold, which is often called a Dore bar. The cathode portion of the electrolytic system is made from either small sheets of 24k pure gold or from stainless steel. When the process is completed, the cathode will be plated with gold obtained from the reaction and is then removed to be converted into the form required for sale. The Wohlwill process produces the highest possible gold purity that can be obtained, 99.999 percent, and which can be used for the most demanding applications.
When the need for purity is not as high, an alternative method, called the Miller process, can be used. This method will produce gold that is only 99.5 percent pure, but its greater ease of use and shorter turnaround time make it an attractive alternative for meeting less demanding gold purity requirements.