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How to Fire Assay Gold Dust

Gold ore exists in different forms, and gold dust is one of them. As the dust can be contaminated with other metals, as well as other impurities such as silica, it's important to assess how much gold there actually is in a sample of gold dust. Several methods exist for doing this -- both modern techniques, which require expensive equipment, and ancient methods, which have been used for hundreds of years. The most ancient method -- and one that is still frequently used today -- is the fire assay.

Things You'll Need

Wash pan or centrifuge
Rock grinder
Flux
Crucible
Mold for extracting metal
Cupel, made of bone ash
Nitric acid, diluted
Scales with carat, gram, or fractional-ounce weights

Instructions

Preparing the Sample
- 1
  Extract as many impurities as possible from the gold dust manually, using a wash pan or centrifuge.
- 2
  Crush the gold dust to a fine powder with a rock grinder.
- 3
  Weigh the ore sample and separate a small quantity. Usually, 29.166 grams, also called one assay ton, is used.
Fire Assay Method
- 4
  Mix the flux with a known quantity of the pulverized ore sample.
- 5
  Heat the mix of ore sample and flux in a crucible until it turns bright red.
- 6
  Remove the crucible from the heat and pour off the molten glass, which forms from the flux and the impurities in the sample.
- 7
  Pour the mix of lead and other metals that collected at the bottom of the crucible into a mold.
- 8
  Allow the metal to cool. It will solidify, and the lead will settle to the bottom. The lead has dissolved all the gold and silver that was present in the sample.
- 9
  Remove the lead and discard the rest of the sample in the mold.
- 10
  Place the lead in a cupel (a small porous cup) made of bone ash and heat it while blowing air into the furnace. This will produce lead oxide, which will be absorbed by the bone ash. What remains will be the gold and silver from the original sample.
- 11
  Weigh the remaining bead. The weight is the total weight of both the gold and the silver.
- 12
  Extract the gold by putting the metal bead into a solution of diluted nitric acid, which will dissolve silver but not gold. Pour off the nitric acid.
- 13
  Weigh the remaining purified gold. The weight of the silver can be calculated by subtracting the weight of the gold from the weight you determined in Step 8. The concentration of gold and silver in the original ore can also be calculated from the weight of the original sample.

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