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Precious Metal Refining Science: Karat Metallurgy & Chemical Parting
The conversion of scrap jewelry, broken chains, dental crowns, and casting sprues into pure fine investment bullion is governed by strict stoichiometry and pyrometallurgical principles. Commercial gold jewelry is intentionally alloyed with base metals—primarily copper to impart red tones, silver to brighten yellow tones, and nickel or palladium to produce white gold—both to increase mechanical tensile hardness and to reduce retail metal cost.
1. Karat System Purity Mathematics & Troy Conversions
The karat scale divides alloy mass into 24 proportional units. To determine the exact fine gold content of any lot, each karat category is multiplied by its pure decimal equivalent:
\(W_{\text{fine}} = \sum \left( W_i \cdot \frac{K_i}{24} \right) - W_{\text{stones}}\)
Troy Ounce Weight Conversion:
\(1\text{ oz t} = 31.1034768\text{ grams} = 480\text{ grains} = 20\text{ pennyweights (dwt)}\)
\(W_{\text{oz t}} = \frac{W_{\text{grams}}}{31.1034768}\)
Net Settlement Payout:
\(\text{Payout} = W_{\text{fine, oz t}} \cdot \text{Spot Price} \cdot \left(\frac{\text{Retention \%}}{100}\right) - \text{Assay Fee}\)
2. Chemical Parting Methods: Aqua Regia vs. Inquartation
| Refining Method | Reagents Required | Purity Yield | Primary Industrial Use | Byproduct Waste |
|---|---|---|---|---|
| Aqua Regia | 1 part \(\text{HNO}_3\) + 3 parts \(\text{HCl}\), SMB or hydrazine | 99.99% (24K Four Nines) | Jewelry scrap, high-purity coin bars | Toxic \(\text{NO}_2\) gas, \(\text{AgCl}\) sludge |
| Inquartation & \(\text{HNO}_3\) | Granulated silver or copper + concentrated \(\text{HNO}_3\) | 99.5% (Sponge Gold) | Bench jewelers, historical assayers | Silver nitrate solution (recovered as AgCl) |
| Miller Process | Pressurized \(\text{Cl}_2\) chlorine gas at 1,150°C | 99.5% (Good Delivery) | Large-scale mining smelters (Rand, Perth Mint) | Volatile base metal chloride fumes |
| Wohlwill Electrolytic | \(\text{HAuCl}_4\) electrolyte bath, direct electrical current | 99.999% (Five Nines) | Central bank reserve bars, semiconductor wire | Platinum group metal anode slimes |
3. Why 1 oz Bullion Coins Vary in Gross Weight
A frequent source of confusion among collectors is why a 1 oz American Gold Eagle weighs 33.93 grams, while a 1 oz American Buffalo weighs exactly 31.10 grams. Both coins contain exactly 31.1035 grams (1.000 troy ounce) of pure 24K gold. However:
- American Gold Buffalo & Canadian Maple Leaf: Struck from pure 24-karat gold (.9999 fine). Because there are no alloy additions, the coin's gross scale weight is identically 31.10 grams.
- American Gold Eagle & South African Krugerrand: Struck to durable 22-karat crown gold standards (.9167 fine, alloyed with 5.33% copper and 3.0% silver). To provide a full troy ounce of pure gold, the gross weight of the coin must be \(31.1035 / 0.9167 = 33.93\) grams. The additional 2.83 grams of copper and silver impart scratch resistance and durability in circulation.
- Classic US $20 Double Eagle (Saint-Gaudens / Liberty Head): Struck from 90% gold and 10% copper. The coin has a gross weight of 33.436 grams, containing exactly 0.9675 troy ounces (30.093 grams) of pure gold.
Real-World Refiner & Scrap Settlement Case Studies
Case 1: Estate Jewelry Melt Turned into Sovereign Bullion
A family inheriting an estate collection of tangled 10K and 14K chains weighed the lot at 145.0 grams. After removing stone clasps (6.0g stone weight), the net scrap weighed 139.0g (60g of 10K and 79g of 14K). Pure fine gold recovery calculated to \(60 \times 0.4167 + 79 \times 0.5833 = 25.00 + 46.08 = 71.08\text{ grams}\) (2.285 oz t). At a spot price of \$2,650/oz t and a 98% refinery settlement, the lot yielded \$5,937 net cash, or enough fine gold to mint two full 1 oz Gold Eagles plus two British Half Sovereigns.
Case 2: The Trap of Unassayed Dental Gold
A coin dealer bought 50 grams of yellow dental crowns assuming standard 16K (66%) content. However, modern dental restorative alloys frequently substitute palladium and silver alloys with gold contents as low as 40% to 52%. When the refinery assay returned 48.2% gold and 12% platinum-group metals, the net settlement was \$700 lower than the dealer anticipated. Professional buyers now utilize portable XRF analyzers before quoting scrap purchases.
Case 3: Inquartation Recovery by an Independent Artisan
A custom studio jeweler collected 100 grams of mixed bench filings and sprues. Because the lot contained 18K and 14K scrap (averaging 68% gold), direct nitric acid boiling was blocked by passivation. By melting the scrap with 200 grams of pure scrap silver shot (inquartation to 22.6% gold), the resulting granulated shot reacted vigorously in 68% nitric acid. The residual gold sponge washed, melted with borax, and yielded 67.8 grams of 99.5% pure casting grain for new custom bands.
Common Pitfalls in Calculating Scrap Gold
- Kitchen Scale Avoirdupois Error: Measuring scrap on a postal or kitchen scale in ounces yields 28.35g per ounce rather than the 31.10g required for troy ounces. Selling 10 ounces of scrap using kitchen scale figures forfeits almost an entire ounce of precious metal.
- Lead Solder Contamination: Old repaired jewelry often contains lead-based soft solders. If melted directly in a graphite crucible, lead forms low-melting eutectic phases with gold, rendering the entire ingot brittle as glass and causing cracking during rolling or coin striking.
- Gold Plating Deception (HGE / GF): Stamps such as "14K HGE" (Heavy Gold Electroplate) or "1/20 14K GF" (Gold Filled) contain less than 5% gold by weight. Inexperienced collectors who mistake gold-filled jewelry for solid 14K suffer massive financial losses at the refinery melt.
Frequently Asked Questions (FAQ)
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