What is fire assay?

This family of procedures is the most reliable method for accurately measuring precious metal in materials ranging from high purity bullion to parts-per-billion "pathfinders".    Samples are melted (fired); reactions in the molten solution gather precious metals into a collector, such as lead. Usually, additional steps separate and purify the precious metals from this base collector into a purer form for quantitative measurement. (The procedure summarized below follows traditional lead-based fire assay; alternative assay designs can use  bismuth, nickel sulfide, tin, and other metals as the fusion collector.)  Fire assay is over 2000 years old; it remains as the most accurate method for analyzing for gold, silver, and platinum group metals.

-- Process summary --

• Before heating in an oven, a weighed ore sample is mixed with flux reagents in fireclay crucibles.
• Fire assay melts (fuses) this powdered rock / flux mixture to generate a reaction that separates precious metals from gangue (waste) minerals. (These reactions are usually designed to finish at 1950 F, but less common variations use alternative temperatures.)

Fusion combines high temperature acid-base and reduction reactions in a design that balances:

• reaction temperature patterns through full (fluid) melt (usually at 1950 F),
• chemical release reactions (fusion chemistry),
• composition of reaction products (slag and button properties),
• and other reaction factors for optimum precious metal recovery.

Optimum recovery requires that 100% of precious metals collect when a rain of lead metal descends to the bottom of the melt during fusion. (Procedures that depend on secondary recovery are not cost efficient.)

(The previous page in the working fire assayer series shows an assayer pouring a fire assay fusion set.)

• After fusions are poured and cooled, a glassy slag (top layer) and lead button (bottom layer) forms . (An additional layer of alkali slag may form above the glass layer in "alkali" type slag systems.)

 

With correct assay design, all of the precious metals alloy in the lead button; waste (everything that isn't a precious metal) becomes part of the slag. Ideal slags shatter upon cooling to separate cleanly from the lead. A picture of this ideal glass-and-lead melt product looks like the picture at right: Image notes: This picture shows an example of a reaction product formed during a fire assay fusion. The melt product was poured into a cast iron mold and cooled. A lead button, showing clean slag separation, formed at the bottom of the mold. The slag layer above the button is a clear amber. (Amber coloring is supplied primarily by iron in the original sample.) A very light alkali layer, a by-product of consumed sulfide mineral, coats the top of the glassy slag. A partial view of the crucible used for this fusion shows above the slag.

• The slag is broken and discarded; the lead reaction product (button) is "cupeled" to remove lead and any remaining base metal impurities.
• Cupelation uses oxidation to remove impurities: 2 Pb + O2  ---> 2 PbO
• Oxidized products (lead and limited amounts of other base metals) absorb into a cupel; non-oxidized metal does not absorb.
• Cupelation has only a limited capability to accurately cope with impurities; the bulk of impurities must be removed by correct fusion design.
• Recovered precious metal is measured by either:

Parting and weighing --

Selectively dissolving precious metals

then weighing the metal products (gravimetric finish)

to calculate concentrations of gold, silver and other precious metals.

or by digesting the product and measuring metal concentration with modern instruments (usually atomic absorption).

For more information on fire assaying: