The specific gravity test is one of the most scientifically rigorous authentication methods you can perform at home with inexpensive equipment. Based on the Archimedes principle — the same discovery that had a Greek mathematician running naked through the streets shouting "Eureka!" — this test measures a coin's density by comparing its weight in air to its apparent weight in water. It's particularly effective for silver authentication, where no common counterfeit material can match silver's specific gravity.
Equipment You'll Need
Digital scale: Accurate to 0.01g minimum. A 0.001g scale improves precision significantly but isn't required. These cost $15–$30. The scale must be stable enough to hold a cup of water without maxing out — a scale with a 200g or 500g capacity works well.
Cup or container: A small glass or plastic cup that fits on your scale's platform. It needs to be deep enough to fully submerge the coin with at least half an inch of water above it.
Water: Distilled water is ideal because its density is precisely 1.000 g/cm³ at room temperature. Tap water works but introduces a small error (typically 0.1–0.5% depending on mineral content).
Suspension method: You need a way to suspend the coin in the water while it sits on the scale. Options include a thin wire cradle (bent from a paperclip), a loop of thin fishing line or thread, or a small mesh basket. The key is that the suspension apparatus must displace as little water as possible.
The Principle
When an object is submerged in water, it displaces a volume of water equal to its own volume. That displaced water has weight, and it exerts an upward buoyant force on the object. By measuring the object's weight in air and its apparent weight in water, you can calculate its density (specific gravity).
The formula is:
Specific Gravity = Dry Weight ÷ (Dry Weight − Wet Weight)
Where "Dry Weight" is the coin's weight in air and "Wet Weight" is the coin's apparent weight when fully submerged in water.
Step-by-Step Procedure
Step 1: Weigh the coin dry. Place the coin directly on your scale and record the weight. For a 1 oz American Silver Eagle, this should read approximately 31.10g. Write this number down. This is your Dry Weight (DW).
Step 2: Set up the water bath. Place the cup of distilled water on the scale. Position the suspension apparatus (wire cradle or thread loop) so it hangs into the water from an external support — NOT from the scale itself. Some people build a simple bridge from a ruler across two books, with the thread hanging from the ruler down into the water on the scale. The suspension system must not touch the scale or the cup — only the water.
Step 3: Tare the scale. With the water, cup, and empty suspension apparatus all in place, press the tare/zero button. The scale should now read 0.00g. Any weight change from this point is purely from the coin being submerged.
Step 4: Submerge the coin. Carefully place the coin in the suspension apparatus so it hangs fully submerged in the water. Make sure no air bubbles are clinging to the coin's surface — gently swirl it or tap it to dislodge bubbles. The coin must not touch the bottom or sides of the cup.
Step 5: Record the apparent weight increase. The scale will now show a positive number. This is the weight of the water displaced by the coin, which equals the buoyant force. Record this as your Water Displacement Weight (WD).
Step 6: Calculate specific gravity.
SG = DW ÷ WD
Worked Example: Testing a Silver Eagle
Let's walk through a real test with numbers.
Step 1: Dry weight of coin = 31.10g
Step 2–3: Water bath set up, scale tared to zero.
Step 4–5: Coin submerged. Scale reads 2.97g (this is the weight of water displaced).
Step 6: SG = 31.10 ÷ 2.97 = 10.47
The expected specific gravity for .999 silver is 10.49. Our reading of 10.47 is within normal measurement error for a home setup. This coin is consistent with genuine silver.
Now let's say the same test on a suspect coin gives a dry weight of 31.05g (close enough to pass a weight test) but a water displacement of 3.46g.
SG = 31.05 ÷ 3.46 = 8.97
That's the specific gravity of copper (8.96), not silver. This coin is copper or a copper alloy with a silver plating. Fake.
Reference Specific Gravity Values
| Material | Specific Gravity | Notes |
|---|---|---|
| Gold (.999) | 19.32 | Very high — few metals come close |
| Tungsten | 19.25 | Dangerously close to gold |
| Lead | 11.34 | Higher than silver |
| Silver (.999) | 10.49 | Distinctive — nothing common matches |
| Silver (.900) | ~10.34 | Pre-1965 US coins |
| Copper | 8.96 | Common counterfeit core |
| Brass | 8.4 – 8.7 | Common counterfeit core |
| Nickel | 8.91 | Used in clad coinage |
| Zinc | 7.13 | Very light — easy to catch |
| Iron/Steel | 7.87 | Magnetic — caught by magnet test too |
| Aluminum | 2.70 | Far too light |
Why This Test Excels for Silver
Look at the table above. Silver's specific gravity of 10.49 sits in a gap between common metals. Copper (8.96) is 15% lower. Lead (11.34) is 8% higher. No cheap, readily available metal or alloy naturally matches silver's density. A counterfeiter would need to carefully blend metals to achieve SG 10.49, and any such alloy would likely fail the magnet test, ping test, or electrical conductivity test.
This is why the specific gravity test is considered one of the most reliable methods for silver authentication. A coin that passes both the weight test AND the specific gravity test is almost certainly genuine silver (or at least genuine in its metal content — it could still be a privately minted counterfeit using real silver).
The Tungsten Problem for Gold
For gold testing, the specific gravity method has a critical weakness. Tungsten's SG of 19.25 is within 0.4% of gold's 19.32. On a home scale with normal measurement error, a tungsten core inside a gold shell will produce a specific gravity reading indistinguishable from solid gold.
This is not a theoretical concern. Tungsten-filled gold bars and coins have been documented in the wild. A 1 oz gold bar with a tungsten core and genuine gold shell will weigh correctly, measure correctly, and show a specific gravity consistent with gold. It takes a Sigma Metalytics verifier, ultrasound, or XRF to catch it.
Common Errors and How to Avoid Them
Air bubbles: Tiny bubbles clinging to the coin's surface add buoyancy, making the coin appear less dense than it actually is. Always dislodge bubbles by gently agitating the coin after submerging. A drop of dish soap in the water reduces surface tension and minimizes bubble formation, but may slightly alter the water's density.
Temperature: Water's density varies with temperature. At 4°C it's exactly 1.000 g/cm³; at 25°C (room temperature) it's about 0.997. This introduces less than 0.3% error, which is acceptable for coin testing but worth noting if your results are borderline.
Coin touching the container: If the coin rests on the bottom or leans against the side of the cup, the reading is invalid. The coin must be freely suspended in the water, supported only by your wire, thread, or basket.
Scale instability: Water movement affects the reading. After submerging the coin, wait 5–10 seconds for the water to settle and the scale reading to stabilize. Take the reading only when the number is steady.
Wet hands: Water dripping from your fingers onto the scale, the coin, or the setup introduces error. Dry your hands between handling the coin and reading the scale. Keep a towel nearby.
Specific Gravity for Bars and Larger Pieces
The same principle works for bars, rounds, and other bullion products, but larger items require a larger container and a scale with higher capacity. A 10 oz silver bar weighs 311g and displaces about 29.6 ml (29.6g) of water. Your scale needs to handle the bar's weight plus the water and container without maxing out.
For kilo bars and larger, the test becomes impractical at home. The water container needs to be substantial, the scale needs several-kilogram capacity at 0.01g resolution (expensive), and the measurement error margins tighten. For large bars, electronic testing with a Sigma or professional assay is more appropriate.
Combining SG with Other Tests
The specific gravity test is most powerful as part of a multi-test authentication routine. My recommended sequence for silver coins: visual inspection first (5 seconds), ping test second (5 seconds), magnet slide third (5 seconds), then specific gravity if any doubt remains (2 minutes). If all four tests pass, the coin is genuine silver with very high confidence.
For gold coins: specific gravity, magnet slide, and visual inspection together, followed by Sigma Metalytics testing if available. The SG test alone is not definitive for gold due to the tungsten issue, but a coin that passes SG AND Sigma is genuine beyond reasonable doubt.