WHAT IS A FIRE ASSAY?

Fire assaying is the oldest and most reliable method of determining gold and silver in rock or concentrate samples. This method is still the industry standard. It is called ''fire'' assay because it involves smelting the sample which has been mixed with lead oxide. Until electric furnaces were available, samples were smelted literally in a fire.

Six steps

There are six steps in the fire assay procedure: splitting - weighing - mixing firing - cupelling - parting.

First, the crushed and ground sample of ore or concentrate is carefully split down to a smaller and, one hopes, representative sample. This is usually done using a riffle splitter. Finally a small sample of only around 30 grams is weighed out and added to a crucible. To this is added a mixture of lead oxide, a reducing agent and fluxes. The fluxes usually consist of silica sand, borax and sometimes additional additives like fluorite. The fluxes, reductant, lead oxide and sample are then mixed and fired in a, muffle furnace.

In the furnace the complete contents of the crucible are melted. In the presence of the reducing agent, typically carbon in any form, e.g. flour, the lead oxide is smelted to lead metal which "collects" any silver and gold that may have been in the sample. The molten mass is taken from the furnace and swirled to mix before being poured into a cone-shaped mold and allowed to cool. The molten lead sinks to the bottom of the mold, carrying any gold and silver with it, while the rest of the components of the ore along with the flux turn into a glassy slag that floats on top of the mold.

After cooling, the metallic lead "button" at the bottom of the mold is separated from the glassy slag which is discarded.

The metallic lead button is placed into a cupel, which is a small dish made from bone ash, and placed into a cupelling furnace. In the "cupelling" process lead metal turns back into oxide which volatilizes away from the precious metals and soaks into the bone ash cupel, leaving the minute amount of precious metals as a metallic speck of metal called a "bead" on the bottom of the cupel.

Next, the bead is weighed on a microbalance to determine the amount of gold and silver that was extractable from the original ore sample. The bead is next heated in hot nitric acid which dissolves away the silver, leaving any gold that may have been present. This step is called "parting" because the nitric acid "parts" the gold from the silver-gold mixture in the bead.

The parted bead is then carefully weighed and this amount of gold is related back to the weight of ore or concentrate sample in the first crucible that was fired.

In more modern laboratories, the bead of precious metals that is recovered in the cupel after the lead has been removed is dissolved in aqua regia. The resulting solution is then analyzed by atomic absorption spectrometry, allowing the grade of gold and silver in the original sample to be back calculated.

Is the fire assay sample big enough?

The problem with fire assaying is not with the method itself, but rather with the sample size that is used. Fire assaying generally uses about one "assay ton" of pulverized sample, i.e. 29.84 grams of material.

If much of the gold occurs in a deposit as small "nuggets", then the chance of the assay ton sample's being representative is remote. If even one of the smallest nuggets or particles of visible gold gets into the small sample that is fire assayed, then the result will be incredibly high. On the other hand, if none of the minute gold particles in the ore get into the assay ton that is fire assayed, then the assay result is likely to be lower than reality.

Fire assaying is a science, and also to some extent still an art. Certain types of ore contain elements that may interfere with the result. A good fire assayer knows how to modify the composition of the flux to avoid these problems.

The fire assayer knows how to determine the gold and silver content of the assay ton of sample that has been presented to him or her. The big question is whether that assay ton is truly representative of the sample of core that it came from, and whether the core sample itself is truly representative of the ore deposit from which it came.

B.C. & YUKON
CHAMBER OF MINES

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