Specific Gravity
Also: Relative density, Density ratio, spec. gravity
Specific gravity (also: relative density) indicates how much heavier a substance is compared with water (density 1.0 g/cm³) and is one of the most reliable metrics for testing 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 synonymously with density in g/cm³, since the numerical value is identical. For precious metal buyers and testers the quantity is indispensable: a genuine gold bar feels significantly heavier at the same size than a counterfeit made from a lighter metal - specific gravity reveals the fraud.
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
For irregular objects (coins, broken gold), the volume is difficult to measure directly. The most elegant method is the Archimedean principle: you weigh the piece dry, then submerged in water. The difference equals the weight of the displaced water - from this follows the volume, and from that the density. Our Coin Authenticity Check carries out this calculation step by step.
Specific gravities compared
| 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 counterfeit metal |
| Palladium | 12.02 | significantly lighter than gold |
| Silver (999) | 10.49 | easily measurable |
| Copper | 8.96 | base metal in alloys |
| Aluminium | 2.70 | immediately noticeably light |
The closeness of tungsten (19.25 g/cm³) to the gold value (19.32 g/cm³) makes tungsten counterfeits hard to expose by a density test alone - here additional methods such as ultrasound or X-ray fluorescence are necessary.
Influence of fineness on density
Alloys deviate from the pure metal values. The lower the fineness, the more strongly the alloying metal influences the overall density:
- 750 gold (18 carat): approx. 15.5-16.5 g/cm³ (depending on the alloying metal)
- 585 gold (14 carat): approx. 12.9-14.0 g/cm³
- 925 silver (sterling silver): approx. 10.36 g/cm³
Anyone wanting to calculate the melt value of scrap gold needs the fineness - and anyone who does not know the fineness can narrow it down via 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 fine scale (resolution 0.01 g or better).
- Submerged weighing in distilled water (thread or basket, no air bubbles).
- Calculate volume: V = (dry mass - submerged mass) / 1.0 g/cm³
- Calculate density: ρ = dry mass / V
- Compare the 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, the additional coin size check (diameter and thickness) is recommended.
In brief
Specific gravity is one of the oldest and most reliable physical methods of material testing - Archimedes is said to have recognised the basic idea as early as the 3rd century BC. For precious metal owners the rule is: anyone who combines scales and a glass of water can expose many counterfeits free of charge and without a laboratory.