Assaying methods have not changed much since the days of the Old West.
There are several different methods of assaying mineral samples for gold or silver content, but the oldest is the fire method. Back in the days of the Gold Rush of 1849 (and long before then), testers would mix the sample with lead oxide and then smelt it in a fire. In modern times, electric furnaces have become the appliance of choice. Fire assay methods involve six steps, whether still over an open flame or in a modern furnace.
Splitting and Weighing
Assayers take the sample for testing and use a riffle splitter to divide the crushed sample down and find a representative blend. Then, 30 grams of the representative sample are put into a crucible. It's not necessary to assay the entire testing sample because the splitter should be able to find a representative sampling from the larger collection.
Mixing and Firing
At this point, the tester adds lead oxide to the representative sample as well as a reductant and some fluxes. Commonly used fluxes include borax and silica. All of these substances are blended together and placed into a muffle furnace, which heats the contents of the crucible to melt them completely. Then, the tester smelts the lead oxide into lead. This will attract the gold and silver. This conglomerate goes into a swirling machine for blending and pouring into a mold. The lead will drop to the bottom along with the gold and silver.
Cupelling and Parting
The lead conglomerate now goes into a cupel (a small container consisting of bone ash), which sits in a furnace. The lead returns to its oxide form and actually blends into the cupel. The silver and gold are left as a precipitate. The tester weighs this precipitate and then places it in nitric acid, which eliminates the silver. Whatever is left is pure gold. The tester weighs this and develops a ratio between the gold and the entire sample weight (30 grams).
Reliability
Because gold often appears in the form of nuggets, if one or more show in a representative sample, they can skew the assumed gold content too high. Experienced gold assayists can analyze a larger ore sample before performing this test, so they can look at the end result and adjust it to a more accurate calculation of a sample's gold content.
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