Ductility
Also: malleability, extensibility, formability
Ductility is the ability of a material to deform plastically under tensile stress without fracturing.
Ductility is among the most striking mechanical properties of the precious metals and a key reason for their industrial and craft importance. It describes the capacity of a solid to deform permanently under tensile stress — to stretch, draw out or bend — without tearing or breaking. Its opposite is brittleness: brittle materials such as glass or cast iron fail abruptly, with no appreciable plastic deformation first.
How ductility arises
At the atomic level, ductility rests on the ability of crystal-lattice planes to slide past one another under shear stress. This process is called dislocation movement. Metals with a face-centred cubic (fcc) lattice structure — gold, silver, platinum, palladium and copper among them — possess an especially large number of slip planes (12 independent slip systems) and are therefore markedly ductile.
Ductility is usually quantified through two measures taken 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 on both measures signifies high ductility.
Ductility of the main precious metals compared
| Metal | Elongation at break (typical) | Notable point |
|---|---|---|
| Gold | > 40 % | the 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 metal known. A single gram of gold can be drawn into a wire roughly 3,000 metres long; hammered into gold leaf, it reaches foil thicknesses below 0.1 micrometres — less than a thousandth of the thickness of a human hair.
Why it matters for investors and processing
The high ductility of gold and silver has made both metals the preferred material for jewellery, coins and bars for thousands of years. For investors it is relevant indirectly:
- Coin striking: ductile metals can be struck without cracking. The fine reliefs on bullion coins such as the Krugerrand arise only through plastic deformation inside the die.
- Alloys: pure fine gold (999) is very soft and ductile; alloying metals such as copper or silver raise the hardness at the cost of ductility. 585 or 750 gold is therefore more suited to everyday use, but less deformable than fine gold.
- Detecting fakes: tungsten has almost the same density as gold but is far more brittle and hard. Mechanical tests (a bend test, a ping test) exploit this difference in ductility for authenticity checking — alongside the density test and X-ray fluorescence analysis.
- Recycling: deformability eases recycling — scrap gold and scrap silver can be melted down and reshaped with no loss to fracture.
You can easily work out the current melt value of your precious metals online.
The effect of temperature and cold working
Ductility is not a fixed constant. At low temperatures it decreases in some metals (cold embrittlement) and at high temperatures it rises. Cold working (rolling or hammering, for example) 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, struck and recycled for thousands of years. Gold leads all known metals in this property, which keeps it indispensable both for industrial processing and for producing high-quality investment coins and bars.