This method is applicable for the determination of gold content ranging from 30% to 99.9% in alloyed gold and jewelry. The test results of this method can serve as a reference standard and have an arbitration effect.
Tools and consumables required for fire assay of gold alloys (jewelry gold):
Cupellation Furnace
Analytical balance
Lead beads making machine
Sampling shovel and sample layout shovel
Cupel plate
Hand hammer and shims
Rolling mills are used to flatten gold-silver beads into thin strips before the acid parting stage.
Fire Assay Procedure
1. Wrap silver and gold with lead, then press into shape using a lead ball machine.
0.18g of gold, K white, K yellow, or pure gold—each requires different amounts of silver and lead.
K gold is divided into K yellow and K white. The terms “yellow” and “white” refer to the alloying elements used to adjust color, known as “coloring agents.” These are the main impurities in K gold.
For K white, the primary impurity is nickel; for K yellow, it’s copper.
For 0.18g of K white and K yellow, the amount of silver added is similar, but the amount of lead differs significantly.
K yellow contains copper and is used for higher karat content, requiring 3–4g of lead.
K white contains nickel and is used for lower karat content, requiring 7–8g of lead.
This is because nickel is harder to melt compared to copper.
Silver addition = sample weight × 2.4 × karat purity – sample weight / inherent silver content of the sample
2. Ashing at 700°C for 60 seconds
Holding at 1100°C (for K gold) for 1800 seconds
Melting at 1100°C for 60 seconds
Cupellation at 1100°C for 1500 seconds
Annealing at 700°C ————
1000°C is the temperature for pure gold
3. After cupellation, annealing must be performed inside the furnace; otherwise, small bubbles will appear on the surface of the pressed gold.
Remove the gold-silver alloy granules, hold them with tongs, and hammer them into small button shapes.
First pressing: place the buttons on paper, press them into strips, then use a flame gun to anneal.
Second pressing: re-press the strip-shaped samples using a different press machine, followed by another annealing with the flame gun.
Then roll the alloy strip in one direction and place it into the parting pan.
4. Parting:
Parting pan made of titanium alloy
Soak briefly in distilled water—optional.
Use 3:4 nitric acid for 8–9 minutes; when brown-yellow gas gradually disappears, the reaction is complete.
Use 2:1 nitric acid for 8–9 minutes; no obvious changes occur, though slight brownish-yellow gas may form.
The 8–9 minute duration applies to separating K gold; pure gold should soak in nitric acid for half an hour—specifically in 2:1 nitric acid for 30 minutes. For 3:4 nitric acid, completeness is determined by absence of brown-yellow gas.
Remove the parting pan and rinse quickly with distilled water.
Place the parting pan in an oven at 220°C to dry.
Then anneal in a furnace at 770°C for 5 minutes.
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