Ductility
Also: Extensibility, malleability, formability
Ductility denotes the property of a material to deform plastically under tensile load without breaking.
Ductility is one of the most outstanding mechanical properties of precious metals and a decisive reason for their industrial as well as artisanal importance. It describes the ability of a solid to deform permanently and plastically under applied tensile stress — that is, to stretch, extend or bend — without tearing or breaking. The counterpart to ductility is brittleness: brittle materials such as glass or cast iron fail abruptly without appreciable plastic deformation.
How ductility arises
At the atomic level, ductility rests on the ability of the crystal lattice planes to slide against each other under shear stress. This process is called dislocation movement. Metals with a face-centred cubic (FCC) lattice structure — including gold, silver, platinum, palladium and copper — have particularly many slip planes (12 independent slip systems) and are therefore exceptionally ductile.
Ductility is usually measured via two parameters from the standardised tensile test (ISO 6892-1):
Elongation at break A = (Lf - L0) / L0 × 100 %
Reduction of area Z = (S0 - Su) / S0 × 100 %
- L0 = original gauge length, Lf = gauge length after fracture
- S0 = original cross-section, Su = cross-section at the fracture point
A high value in both quantities means high ductility.
Ductility of the main precious metals compared
| Metal | Elongation at break (typical) | Feature |
|---|---|---|
| Gold | > 40 % | most ductile metal of all; 1 g → ~3 km of wire |
| Silver | ~50 % | highest ductility among coinage metals |
| Platinum | 35–40 % | ductile, but tougher than gold |
| Palladium | ~30 % | similar to platinum, cold-workable |
| Copper | 40–50 % | basis of many alloys; high conductivity |
Gold is the most ductile known metal of all. From a single gram of gold, a wire around 3,000 metres long can be drawn; hammered into gold leaf, it reaches foil thicknesses of less than 0.1 micrometres — less than a thousandth of the thickness of a human hair.
Practical significance for precious-metal investors and processing
The high ductility of gold and silver has made both metals the preferred material for jewellery, coins and bars for millennia. For investors it is indirectly relevant:
- Coin minting: ductile metals can be struck without cracks. Fine reliefs on bullion coins such as the Krugerrand arise only through plastic deformation in the coining die.
- Alloys: pure fine gold (999) is very soft and ductile; alloying metals such as copper or silver raise the hardness at the expense of ductility. 585 or 750 gold is therefore more suitable for everyday use, but less deformable than fine gold.
- Counterfeit detection: tungsten has almost the same density as gold, but is much more brittle and harder. Mechanical tests (bend test, ping test) exploit this difference in ductility for authenticity testing — supplementing density measurement and X-ray fluorescence analysis.
- Recycling: the formability eases the recycling — scrap gold and scrap silver can be melted down and reshaped without material loss through breakage.
You can conveniently determine the current melt value of your precious metals online.
Influence of temperature and cold working
Ductility is not an unchanging constant: at low temperatures it decreases for some metals (cold embrittlement) and at high temperatures it increases. Cold working (e.g. rolling or hammering) increases the dislocation density in the crystal lattice and leads to work hardening — the metal becomes harder but less ductile. Subsequent annealing (recrystallisation annealing) largely restores the original ductility.
In brief
Ductility is the reason precious metals have been shaped, minted and recycled for millennia. Gold leads this property among all known metals and thus remains indispensable both for industrial processing and for the production of high-quality investment coins and bars.