Stainless steel vs sterling silver

A patch test with a surprise

In 1994 a group of researchers published the results of a simple experiment in the journal Contact Dermatitis. They took people already known to be allergic to nickel and patch tested them with small samples of metal: some nickel-plated, some ordinary stainless steels such as 304 and 316L.

The nickel plating did what everyone expected. 96% of the patients reacted to it. The stainless steels, which contain nickel too, produced no reactions at all. The difference was not how much nickel was in the metal, but how much came out of it.

That result sits at the heart of any honest comparison between steel and silver. The two metals look alike, and both make good jewellery, but they get there in completely different ways.

At a glance

What each metal is

Sterling silver is at least 925 parts of pure silver in every 1,000. The rest is usually copper, added because pure silver is too soft to wear well. The full story is in What 925 means.

Stainless steel is iron with at least 10.5% chromium, according to the British Stainless Steel Association. The chromium is the clever part. It forms a thin layer of oxide on the surface, called the passive layer, and that layer stops further corrosion. It is extraordinarily thin, 1 to 5 nanometres, and if it is damaged it will normally repair itself, provided there is enough oxygen around.

Two grades turn up in jewellery again and again:

• 304 (European number 1.4301). The classic "18/8" steel: about 18% chromium and 8 to 9% nickel in typical steelmakers' analyses.
• 316L (1.4404). About 17% chromium, 11% nickel and 2% molybdenum in a typical analysis. Molybdenum is added for extra corrosion resistance, and the L means low carbon.

You will sometimes hear the word passivation. It is the name for helping that protective layer form properly, and steel makers recommend passivating treatments after jobs such as welding that can disturb the surface.

Tarnish and corrosion

This is the difference most people notice first. Silver reacts with sulphur compounds in the air and in sweat to form silver sulphide, the film we call tarnish. It begins as a faint yellow and, left alone, works its way through brown and blue to black. It is a surface film, and usually polishes off.

Stainless steel's passive layer means it does not tarnish in the same way. It is not invincible, though. The British Stainless Steel Association notes that the passive state can be broken down, and then the steel can corrode. That is one reason 316L, with its molybdenum, is often chosen where pieces will spend a lot of time against skin or water.

Hardness and weight

Hardness decides how readily a piece dents and how quickly it loses a crisp edge. On the Vickers scale, where a diamond is pressed into the metal and the dent measured, research on sterling silver found it at about 66 when fully softened, rising to about 142 after heat treatment. A widely quoted reference value for 316L stainless steel is about 140. In everyday terms, steel is the tougher customer, and a steel ring tends to shrug off knocks that would leave their mark on silver. How strong is each metal? has the full comparison.

Weight runs the other way. Sterling silver has a density of about 10.3 grams per cubic centimetre and stainless steel about 7.9. So the same design made in sterling weighs roughly 30% more than it would in steel. Whether that reads as pleasingly substantial or simply heavy is up to you.

Nickel: content is not release

Here is the point that confuses almost everyone. 304 and 316L both contain nickel, and 316L actually contains more than 304, so steel described as "nickel free" usually isn't. Yet the 1994 study found that people allergic to nickel did not react to them.

The reason is that an allergy is triggered by nickel that comes out of the metal and onto the skin, not by nickel locked inside it. In the study's tests in acidic artificial sweat:

• Nickel-plated metal released about 100 micrograms of nickel per square centimetre per week.
• Low-sulphur stainless steels, including 304 and 316L, released less than 0.03.
• A high-sulphur grade, 303, released about 1.5, and 14% of the allergic patients reacted to it.

The researchers put the difference down to corrosion resistance. 304 and 316 stayed passive; the plating and the 303 did not. A follow-up study in 1997 reached the same conclusion. The 1994 paper also warned that the usual spot test for nickel reveals nickel present in a metal, not how much of it escapes.

The law is written the same way. Under UK and EU REACH rules, jewellery that is in direct and prolonged contact with the skin, such as necklaces, bracelets, rings and earrings, must not release more than 0.5 micrograms of nickel per square centimetre per week. Posts that go through a piercing have a stricter limit of 0.2. And if a piece is coated over nickel, the coating has to keep the release under 0.5 for at least two years of normal use.

Traditional sterling is silver and copper, with no nickel added. If you have a nickel allergy, the research on 304 and 316L is reassuring, but a doctor or pharmacist is the right person to advise on your own skin.

Hallmarks

Hallmarking in the UK covers four precious metals only: platinum, gold, palladium and silver. Steel can't be hallmarked, and a business may not describe it as "silver". If the colour is the point, the UK Assay Offices' guidance suggests wording that makes it plain, such as "silver tone" or "silver coloured".

Sterling silver over 7.78 grams must be hallmarked before it can be sold as silver. Lighter pieces are exempt and often carry a simple 925 stamp. And when a piece combines silver with steel, it can still be hallmarked, but only on the silver part, with "+METAL" struck beside the hallmark to show it isn't silver throughout.

Repairs and resizing

Silver's softness, a drawback in daily wear, is a gift at the workbench. A silver ring can be stretched up a size on a mandrel, or cut, a piece added or removed, and soldered closed again. Broken chains and bent clasps can usually be mended.

Stainless steel is a different matter. The hardness that helps it survive daily life makes it far harder to cut, stretch and join, and many jewellers simply don't resize it. If a steel ring no longer fits, a new ring is often the practical answer, so it is worth getting the size right first time.

Plating on each

Both metals are often plated, usually with gold or rhodium, and the rules are different for each.

• On silver. A hallmarked silver piece may be plated with silver, palladium, gold, platinum, ruthenium or rhodium. If plated after hallmarking, the coating must not be more than 2 micrometres thick, and base metals, nickel included, are not allowed as a layer underneath. Gold plated silver is sold with wording such as "18ct gold plated silver".
• On steel. Gold plated steel can't be hallmarked, so it is described as gold plated, never simply as gold. If nickel is used as a layer under the plating, the finished piece still has to meet the nickel release limit for at least two years of normal wear.

Plating wears with time on any metal. When it does, silver can be re-plated by a jeweller; on steel, the answer depends on the maker.

Two metals, two kinds of honesty

Go back to that patch test. The nickel in the steel was real, and so was the result: no reactions. Steel earns its place by being tough and stubbornly unchanging. Silver earns it differently, with a hallmark that says exactly what it is, a softness that lets a jeweller remake it, and a surface that changes with you and polishes back to white. Neither is the better metal. They are simply honest about different things.