Ductility
Also: malleability, stretchability, formability
Ductility refers to a material's ability to deform plastically under tensile stress without breaking.
Ductility is one of the most outstanding mechanical properties of the precious metals and a decisive reason for their industrial as well as craft importance. It describes the ability of a solid to deform plastically and permanently under applied tensile stress — that is, to stretch, elongate or bend — without tearing or breaking. The opposite of 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 one another 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 (BS EN 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 indicates high ductility.
Ductility of the main precious metals compared
| Metal | Elongation at break (typical) | Special 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 metal known. 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 one thousandth 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 thousands of years. For investors it is indirectly relevant:
- Coin striking: ductile metals can be struck without cracking. Fine reliefs on bullion coins such as the Krugerrand are created only through plastic deformation in the striking die.
- Alloys: pure fine gold (999) is very soft and ductile; alloying metals such as copper or silver increase hardness at the expense of ductility. 585 or 750 gold is therefore more everyday-durable, but less deformable than fine gold.
- Counterfeit detection: tungsten has almost the same density as gold, but is far more brittle and hard. Mechanical tests (bending test, ping test) use this difference in ductility for authenticity testing — as a complement to density measurement and X-ray fluorescence analysis.
- Recycling: the formability makes recycling easier — scrap gold and scrap silver can be melted down and reshaped without material loss from breakage.
You can conveniently determine the current melt value of your precious metals online.
The influence of temperature and cold working
Ductility is not an unchanging constant: at low temperatures it decreases for some metals (cold embrittlement) and increases at high temperatures. Cold working (e.g. rolling or hammering) raises 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 and thus remains indispensable both for industrial processing and for the manufacture of high-quality investment coins and bars.