What 925 means

The year silver went up

In 1697 England had a problem with its money: too many silver coins were being melted down. The answer was to change the silver, not the coins. The standard for silverware was raised to 95.84% pure silver, to protect the coinage from the melting pot, and a seated figure of Britannia was struck on the new metal to show it.

It did not last. In 1720 the old standard came back, with the higher one kept as an alternative. That old standard was sterling, 92.5% silver. Three centuries on it is still the silver most jewellery is made from, and the number stamped inside a silver ring today, 925, is the same promise in modern figures.

What the number means

925 means that at least 925 parts in every 1,000 are pure silver. It is a minimum, not an average: a piece marked 925 has been made, and if hallmarked has been tested, to contain no less.

The other 75 parts are usually copper. Pure silver is a beautiful white metal, but it is soft, and the UK Assay Offices explain that precious metals are normally mixed with "lesser metals" to give them strength and durability. Copper does that job for silver. A supplier's technical guide puts it simply: standard silver "is alloyed with 7.5% copper to increase its hardness." The result is hard enough to hold a clasp, a setting or a crisp edge, and still white enough to look like silver.

"Sterling silver", "925 silver" and "925 sterling" all mean the same thing. If you want to see how much harder the copper makes it, the numbers are in How strong is each metal?

A very old standard

Sterling was a legal requirement long before anyone thought of writing it as 925. The Goldsmiths' Company Assay Office records that in 1300 a statute of Edward I required silver to be "of sterling standard (92.5% pure silver)", which is where the long story of British hallmarking begins.

The full history, from the leopard's head to the four assay offices, is told in The story of the hallmark.

Why sterling tarnishes

Leave a silver chain in a drawer for long enough and it will darken. That film is tarnish, and it is mostly silver sulphide, formed when silver meets sulphur compounds in the air and in sweat. Even a low concentration of hydrogen sulphide is enough, and the classic jewellery textbooks note that tarnish forms faster as the air gets warmer and more humid.

Tarnish creeps in by stages. In its first, thinnest layer the silver looks faintly yellow, almost as if it had been gilded. As the film thickens it turns brown, then a dirty blue, then bluish black, and finally deep black. The copper in sterling plays a part too, forming sulphides and oxides of its own, and pure silver tarnishes less quickly than sterling does.

A 925 stamp or a hallmark?

Here is where the small print matters. In the UK it is an offence for a business to describe something as silver unless it has been hallmarked by an assay office, or is light enough to be exempt. And "describe" is wider than you might think: the UK Assay Offices' guidance says a description can be spoken, written, or in the form of a maker's fineness mark on the jewellery itself. A 925 stamp is a description.

The exemption for silver is 7.78 grams. Below that, a piece does not need a hallmark, and it can carry a simple 925 stamp instead. Where silver is combined with base metal, the 7.78 grams is measured against the total metal weight, base metal included.

At 7.78 grams or more, a stamp on its own is not enough. A UK hallmark has three compulsory parts:

• The sponsor's mark. The registered initials of the person or company that sent the piece to be tested.
• The fineness mark. 925 for sterling, inside a surround whose shape shows the metal is silver.
• The assay office mark. A leopard's head for London, an anchor for Birmingham, a rose for Sheffield or a castle for Edinburgh.

A traditional symbol for sterling, the lion passant, can be added if the sponsor asks for it, and so can a date letter. Neither is compulsory.

Two more rules are worth knowing. Silver made before 1950 can be sold as silver without a hallmark if the seller can show its age and standard. And anything that merely looks like silver must say so plainly: the guidance suggests wording such as "silver tone" or "silver coloured" for pieces that contain no silver at all.

The other silver standards

Sterling is the best known silver standard in Britain, but not the only one. UK law recognises four, and the same four are recognised under the International Convention on Hallmarking.

Britannia still has its admirers among silversmiths. A supplier's technical notes say its extra silver makes it "more easily worked", that it work hardens less quickly, and that it is less prone to firestain, the dark shadow copper can leave under the surface when silver is heated. Fine silver is softer still, which is why you see it used for pieces that are shaped by hand and then left alone, and less often for rings and clasps that take daily knocks.

Anti-tarnish sterling

If copper helps silver to tarnish, the obvious idea is to change the copper. That is what a family of newer alloys tries to do. The best known is Argentium silver, invented by the master silversmith Peter Johns at Middlesex University in London. His research began as an attempt to get rid of firestain, using germanium, and the alloy turned out to resist tarnish as well.

Argentium comes in two silver contents: 935, which is 93.5% silver with copper and germanium, and 960, which is 96%. Because hallmark standards are minimums, 935 is comfortably above sterling, and 960 is above the Britannia standard of 95.84%.

How much difference does it make in real life? An independent study presented at the Santa Fe Symposium in 2006 put Argentium and several other tarnish-resistant sterling alloys through indoor, outdoor and accelerated tests alongside ordinary sterling. In one comparison, the as-received sterling tarnished slightly more than the Argentium. But across all the tests, the authors found that traditional sterling performed as well as or better than the tarnish-resistant alloys in most cases, and they warned that laboratory tests did not seem to reflect real-life wear. The honest summary is that these alloys can help, and that care and storage still matter for every kind of silver.

Three numbers, one promise

The men who melted coins in 1697 left behind an unexpected legacy: a second silver standard, a figure of Britannia, and a reminder that the number matters. Turn over a silver ring today and 925 says what it has said, in one form or another, since 1300. Not pure silver, but silver made strong enough to wear, in exactly the measure the law requires.