How strong is each metal?

A diamond and a dent

In 1921 two engineers at Vickers Ltd, Robert Smith and George Sandland, came up with a neat way to measure how hard a metal is. Press a tiny diamond, shaped like a pyramid with a square base, into the surface with a known force. Lift it away, measure the little square dent it leaves, and work out the hardness from the force and the size of the dent. A small dent means a hard metal. A big one means a soft metal.

The result is the Vickers hardness number, written HV, and metal makers still quote it on their data sheets today. It is also the most useful single number for anyone wondering how a ring or bracelet will stand up to daily life, because hardness is, quite literally, resistance to being dented.

The numbers side by side

Hardness figures for jewellery metals come from different makers and laboratories, and they rarely line up perfectly. So rather than a single number for each metal, the chart below shows a range: from the softest condition each source gives, usually annealed (fully softened by heat), to the hardest, usually after the metal has been rolled, hammered or heat-treated.

Two things stand out. First, the ranges overlap a great deal: a well worked 22 carat gold can be harder than a soft 14 carat. Second, the order isn't simply "more gold, softer". In these particular alloys, annealed 9 carat is a little harder than annealed 14 carat, and 18 carat is the hardest of the four. That is the nature of alloys. What a metal is mixed with matters as much as how much of it there is.

Why the same metal can be soft or hard

Pure metals tend to be soft. Gold is the extreme case: the World Gold Council's Christopher Corti describes it as the most malleable of all pure metals, and gives 24 carat gold a hardness of only about 20 to 30 HV when annealed. Fine silver is in the same soft family. Adding other metals makes an alloy harder, which is a large part of why jewellery is made from sterling and carat gold rather than from the pure metal.

Then comes the workshop. Every time a metal is rolled, drawn or hammered, it gets harder, which metalworkers call work hardening. Heating it again, annealing, softens it back. A gold supplier's data sheets show what that means in practice: one 9 carat alloy goes from 115 HV annealed to 225 HV after being rolled to 40% thinner. Sterling silver can also be hardened by heat treatment, from about 66 to about 142 HV in one study.

So a hardness figure always belongs to a particular alloy in a particular condition. A cast ring, a ring made from rolled strip, and a ring that has been hammered and burnished can all be the same alloy and still behave quite differently.

Metal by metal

• Fine silver. The softest metal here, about 25 HV when soft. Lovely to shape by hand, and it marks easily.
• Sterling silver. The copper roughly doubles to triples the hardness of fine silver, from about 66 HV annealed, and heat treatment can take it higher. Annealed, it is still softer than steel and the carat golds above.
• Stainless steel. About 140 HV for 316L in the reference value we found, at the hard end of this list, and it stays that way without any special treatment.
• Brass. Anywhere from about 55 to 180 HV depending on how it was rolled and treated, so it can be soft or quite hard. It usually appears in jewellery plated, so the plating wears long before the brass does.
• 9 and 14 carat gold. Hard wearing alloys, around 100 to 115 HV annealed in the alloys above, and much harder once worked.
• 18 carat gold. The hardest of the four golds in these figures: 135 HV annealed. Another published 18 carat yellow alloy gives about 150. The richer colour doesn't come at the cost of softness.
• 22 carat gold. Noticeably softer: 75 HV annealed in one supplier's alloy and about 52 in another. It suits pieces that won't take hard knocks.
• Platinum. It depends heavily on the alloy. Platinum Guild International gives about 130 HV for 950 platinum with ruthenium and 135 with cobalt, but only about 80 for 950 platinum with iridium, which it says can give castings that are "quite soft".

What it means for everyday wear

A softer metal is not a weaker choice, but it does wear differently. Corti puts the problem with very soft gold plainly: in service, items "are liable to distort, fracture and scratch and wear heavily". Alloying, he notes, is one of the main reasons gold jewellery is made from carat golds.

In practice, that leads to some sensible rules of thumb:

• Rings and bracelets work hardest. They meet desks, handles and other jewellery all day. A harder alloy, or a sturdier design, helps most here.
• Pendants and earrings have an easier life. Softer metals such as fine silver and 22 carat gold suit them well.
• Scratches are not the end. On silver, gold and platinum, surface scratches can usually be polished out by a jeweller. Each polish removes a little metal, so it is best done when needed rather than as a habit.
• Thickness matters too. A thin, delicate ring in a hard metal can still bend; a sturdy band in a softer one may last a lifetime.

The dent that tells the story

Smith and Sandland's diamond is still pressing into metal in laboratories a century later, and the little dents it leaves explain a great deal: why sterling needs its copper, why 22 carat feels so buttery, and why a steel ring looks the same after a year of hard wear. A hardness number can't tell you which metal to love. It can tell you what to expect from it.