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Fire (Dry) Assays


No attempt has been made to explain or illustrate the myriad of possible complexities regarding fire assaying methodologies relating to the liberation and capture of precious metals within rocks that differ in intricacy as do the number of people trying to extract these metals. For obtaining or refreshing the knowledge base please read some of the Fire Assaying books available within silverprospector.

In my opinion Fire Assaying is the artful expression of endeavoring to employ time-tested scientific principles (verifiable & repeatable) of extracting precious metals from rocks. Furthermore, like it or not, sooner or later the precious metal prospector must become at least partially familiar with Fire Assaying techniques or remain ignorant and at the mercy of those who claim to know how to properly assay for the precious metals.

The primary purpose of these few fire assays is meant only to provide visual reference relating to the specific procedure(s) utilized, as well as providing limited visual insight as to what occurs when specific metals and fluxing agents are subjected to intense heat. Plus, the amazing similarities of gold and PGM’s when dissolving (Parting) the silver beads.

Flux formula for the following precious metal assays including the blanks.
20 grams of silica 
10 grams of Borax
35 grams of sodium carbonate
80 grams of Red Lead oxide
2.5 grams of activated charcoal
      Silver metal bead of a known amount
      Liquid gold or a PGM of a known amount.

Normally a 29.16 gram pulverized rock sample is assayed, but the following assays only have a 1 milliliter (mL) precious metal chloride inserted (inquarted) into each crucible flux charge. In addition a small silver metal bead/prill is also inserted into the fluxes to act as a collector within the reduced metal portion of the Red Lead Oxide also known as litharge.

The amount of each Fluxing agent is determined by what the actual content of the pulverized mineral (rock) sample matrix is composed of. But, because these fire assays are essentially the same the exact same fluxes are used, thereby providing a little better understanding of the associated pyro-metallurgical process. 

Needless to say, to obtain the best likely assay the pulverized (minimum of -100 mesh and preferably -150 mesh or finer) rock sample should be intimately mixed with the fluxes that are of the same particle size as the pulverized rock.
Although a gas fired furnace is usually employed for fire assaying and an electric oven is used for cupellation I have chosen to use only the electric furnace for both the fusion’s and cupellation’s.

Remember – that adding or subtracting any of the fluxing ingredients can have profound effects upon assay results. Furthermore, normal fire assays are primarily designed to collect silver and gold, but sometimes the typical assay can or will gather one or more of the PGMs, which is why I almost always add a known amount of silver to act as an additional collector of the pgms.

Please keep in mind – despite the fact that there are numerous ways to harvest gold and silver from rocks the world has standardized upon fire assays and deviation of this methodology will be met with a raised eyebrow.

Before proceeding – short cuts seldom have desirable destinations, contamination is a constant companion and although mistakes and books are requirements nothing, including notes and pictures replaces experience.

Blanks      Au      Au & Pt     Pt & Pd      Rh      Ir