Fire Assay

Fire assaying is the oldest and is considered to be the most reliable method of determining the content of gold and silver in rock or concentrate samples. This method is still the industry standard. The reason why it is called ”fire” assay is simply because it involves smelting the sample which has been mixed with lead oxide.

There are six basic 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 smaller samples only around 30 grams. These samples are weighed out and added to a crucible. A mixture of lead oxide, a reducing agent and fluxes is then added. 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 mixed 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.

After cooling, the metallic lead at the bottom of the mold is separated from the glassy slag which is discarded. The lead is called a “button.”

This metallic lead button is then placed into a cupel, a small dish made from bone ash, and placed into a cupelling furnace. In the “cupelling” process, lead metal turns back into oxide which separates away from the precious metals and soaks into the bone ash cupel, leaving the minute amount of precious metals as a metallic speck called a “bead.”

The next step in the process is called parting where 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 then heated in hot nitric acid which dissolves away the silver, leaving any gold that may have been present.

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.

Fire assaying is a science, and also to some extent is 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.

Source by Robert Allan Young

Fire Assay