Specific Gravity
Also: Relative density, Density ratio, spec. gravity
Specific gravity (also: relative density) indicates how much heavier a substance is compared to water (density 1.0 g/cm³) and is one of the most reliable indicators for verifying the authenticity of precious metals.
Specific gravity (technical term: relative density) is a dimensionless quantity that describes the ratio of the density of a substance to the density of pure water at 4 °C (= 1.0 g/cm³). In practice, the term is often used interchangeably with density in g/cm³, since the numerical values are identical. For precious metal buyers and testers, this figure is indispensable: a genuine gold bar feels noticeably heavier than a forgery made from a lighter metal of the same size — specific gravity reveals the deception.
Formula and Measurement
Density is derived from mass and volume:
ρ = m / V
ρ = density in g/cm³
m = mass in grams (dry weighing)
V = volume in cm³
Volume is difficult to measure directly for irregular objects (coins, scrap gold). The most elegant method is the Archimedean principle: weigh the piece dry, then submerged in water. The difference equals the weight of the displaced water — from this the volume is derived, and from the volume the density. Our Authenticity Verification Calculator carries out this calculation step by step.
Specific Gravity Comparison
| Metal | Density (g/cm³) | Note |
|---|---|---|
| Osmium | 22.59 | densest stable element |
| Iridium | 22.56 | second densest element |
| Platinum | 21.45 | heavier than gold |
| Gold (999) | 19.32 | most important reference value |
| Tungsten | 19.25 | dangerous counterfeiting metal |
| Palladium | 12.02 | significantly lighter than gold |
| Silver (999) | 10.49 | readily measurable |
| Copper | 8.96 | base metal in alloys |
| Aluminium | 2.70 | immediately noticeably light |
The proximity of tungsten (19.25 g/cm³) to gold (19.32 g/cm³) makes tungsten forgeries difficult to detect by density testing alone — additional methods such as ultrasound or X-ray fluorescence are required here.
Influence of Fineness on Density
Alloys deviate from the values of pure metals. The lower the fineness, the greater the influence of the alloying metal on overall density:
- 750 gold (18 carat): approx. 15.5–16.5 g/cm³ (depending on alloying metal)
- 585 gold (14 carat): approx. 12.9–14.0 g/cm³
- 925 silver (sterling silver): approx. 10.36 g/cm³
Anyone wishing to calculate the melt value of scrap gold needs to know the fineness — and anyone who does not know the fineness can narrow it down through a combined density and weight measurement. The Coin Weight Checker automatically compares target and actual weight for common coins.
Practical Authenticity Testing
- Dry weighing with a precision scale (resolution 0.01 g or better).
- Submerged weighing in distilled water (using a thread or basket, no air bubbles).
- Calculate volume: V = (dry mass − submerged mass) / 1.0 g/cm³
- Calculate density: ρ = dry mass / V
- Compare result with the target values in the table above.
A deviation of more than ±0.5 g/cm³ from the target value is a serious warning sign. For coins, it is also advisable to carry out a coin dimensions check (diameter and thickness).
Key Takeaway
Specific gravity is one of the oldest and most reliable physical methods of materials testing — Archimedes is said to have recognised the underlying principle as early as the 3rd century BC. For precious metal owners, the rule is: anyone who combines a scale and a glass of water can expose many forgeries free of charge and without a laboratory.