Ductility

Ductility is one of the most outstanding mechanical properties of precious metals and a decisive reason for their industrial and artisanal importance. It describes the ability of a solid to permanently deform plastically under applied tensile stress — that is, to stretch, elongate or bend — without tearing or breaking. The counterpart to ductility is brittleness: brittle materials such as glass or cast iron fail abruptly without any significant plastic deformation.

How Ductility Arises

At the atomic level, ductility is based on the ability of 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 — which includes gold, silver, platinum, palladium and copper — have a particularly large number of slip planes (12 independent slip systems) and are therefore exceptionally ductile.

Ductility is typically measured using two parameters from the standardised tensile test (DIN EN ISO 6892-1):

Elongation at break A = (Lf - L0) / L0 × 100%
Reduction in area Z  = (S0 - Su)  / S0  × 100%

• L0 = original gauge length, Lf = gauge length after fracture
• S0 = original cross-section, Su = cross-section at the point of fracture

High values for both parameters indicate high ductility.

Ductility of the Most Important Precious Metals Compared

Gold is the most ductile known metal of all. From a single gram of gold, a wire of around 3,000 metres in length can be drawn; when hammered into gold leaf, it reaches foil thicknesses of less than 0.1 micrometres — that is less than one 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 thousands of years. For investors, it is indirectly relevant:

1. Coin minting: Ductile metals can be struck without cracking. Fine reliefs on bullion coins such as the Krugerrand are only produced through plastic deformation in the minting die.
2. Alloys: Pure fine gold (999) is very soft and ductile; by adding alloying metals such as copper or silver, hardness increases at the expense of ductility. 585 or 750 gold is therefore more suitable for everyday use, but less formable than fine gold.
3. Counterfeit detection: Tungsten has almost the same density as gold, but is considerably more brittle and harder. Mechanical tests (bend test, ping test) exploit this difference in ductility for authenticity verification — supplementing density testing and X-ray fluorescence analysis.
4. Recycling: Malleability facilitates recycling — scrap gold and scrap silver can be melted down and reshaped without material loss through breakage.

The current melt value of your precious metals can be calculated conveniently online.

Effect of Temperature and Cold Working

Ductility is not an unchangeable constant: at low temperatures it decreases in some metals (cold brittleness), and increases at high temperatures. 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 why precious metals can be shaped, minted and recycled for thousands of years. Gold leads this property among all known metals and thus remains indispensable both for industrial processing and for the production of high-quality bullion coins and bars.