🧪 The Physics of Archimedes Specific Gravity Testing
Over 2,200 years ago, Greek mathematician Archimedes discovered that an object immersed in water experiences a buoyant upward force equal to the weight of the water it displaces. In modern bullion investing, this hydrostatic law remains the most foolproof, non-destructive test to expose counterfeit gold and silver coins.
Specific Gravity = Dry Weight (g) ÷ [ Dry Weight (g) − Submerged Weight (g) ]
Or simply: SG = Dry Weight (g) ÷ Displaced Water Weight (g)
Why Fake Coins Cannot Beat Physics: The Tungsten Dilemma
Counterfeiters face an inescapable law of elemental physics: Density = Mass ÷ Volume.
- If a fake coin uses a low-density metal (like copper, brass, or lead) and matches the genuine coin's dimensions, it will weigh far too light on a digital scale.
- If the counterfeiter adds thickness or diameter to make the fake coin match the genuine weight, digital calipers will instantly expose the oversized dimensions.
- Tungsten fakes: Tungsten matches pure 24K gold (19.25 vs 19.32 g/cm³), but it cannot mimic 22K Crown Gold (American Gold Eagles and Krugerrands, SG 17.77 g/cm³). A tungsten-core Gold Eagle will fail specific gravity testing by registering dangerously high density!
Standard Metal Specific Gravity Benchmark Table
| Metal / Coin Alloy | Fineness | True Specific Gravity | Acoustic Ping Velocity | Magnetic Property |
|---|---|---|---|---|
| 24K Pure Gold (Maple Leaf / Buffalo) | 99.99% Au | 19.32 g/cm³ | 3,240 m/s | Diamagnetic (Repels) |
| 22K Crown Gold (Eagle / Krugerrand) | 91.67% Au | 17.77 g/cm³ | 3,310 m/s | Diamagnetic (Repels) |
| .999 Fine Silver (Silver Eagle) | 99.90% Ag | 10.49 g/cm³ | 3,650 m/s | Diamagnetic (Eddy Slow) |
| 90% Constitutional Silver (Morgan / Peace) | 90.00% Ag | 10.34 g/cm³ | 3,720 m/s | Diamagnetic (Eddy Slow) |
| Tungsten (Common High-End Fake Core) | Pure W | 19.25 g/cm³ | 5,220 m/s | Paramagnetic (Slight pull) |
| Lead (Pb) | Base metal | 11.34 g/cm³ | 1,210 m/s (Dull thud) | Diamagnetic (No ring) |
| Copper / Brass (Common Cheap Fake Core) | Cu-Zn alloy | 8.50 – 8.96 g/cm³ | 4,700 m/s | Diamagnetic (Too light) |
Lenz's Law & The Neodymium Magnet Slide Test
Silver possesses the highest electrical conductivity of any element on the periodic table. When a rare-earth Neodymium (N52) magnet is placed on a 45-degree angled pure silver bar or coin, the moving magnetic field induces circular electric currents inside the silver (called eddy currents). According to Lenz's Law, these eddy currents generate an opposing magnetic field that acts like an invisible brake. The magnet will glide down the coin in slow motion, taking 2 to 3 seconds. If the magnet drops instantly or sticks, the coin is a fake.
Frequently Asked Questions
No. Distilled water is non-reactive and does not harm coin luster. Simply blot the coin gently with a soft microfiber towel or let it air-dry naturally. Never rub or wipe the coin dry.
Yes! Even microscopic air bubbles clinging to the coin's surface or suspension thread will add buoyancy, causing the scale to read a lower displaced weight and giving an artificially high specific gravity. Always tap the coin gently under water to dislodge any bubbles before recording the reading.
The "Triple Test" method: 1) Precision digital scale weight, 2) Digital caliper diameter and thickness, and 3) Acoustic ping resonance. Passing all three simultaneously is virtually impossible for a counterfeit.