What rhodium plating is

A metal named for a rose

In 1803 the chemist William Hyde Wollaston was working on a commercial venture to sell pure platinum. He dissolved ordinary platinum in acid, took out the platinum and palladium, and was left with a beautiful red solution. From it he grew rose-red crystals, and from those he eventually produced a new metal.

He named it after the colour: rhodium, from the Greek rhodon, meaning rose coloured. There is a quiet joke in that. The metal itself is not pink at all. It is a hard, shiny, silvery white, and today it is the finish on a great deal of the whitest jewellery you will see.

What rhodium is

Rhodium is one of the platinum metals, a small family that it shares with platinum and palladium, and it turns up in nature alongside them. The Royal Society of Chemistry calls it the rarest of all the non-radioactive metals. World production is about 30 tonnes a year, and most of it, around 80%, goes into catalytic converters for cars.

The rest goes into jobs that need a bright, durable surface. Rhodium coats optical mirrors and headlight reflectors, and it is used for electrical contacts because it is highly resistant to corrosion. The qualities that suit a headlamp, brilliance and a surface that doesn't tarnish, are the same ones a jeweller wants.

Jewellery is rarely made of solid rhodium. Instead it is applied as a plating: the piece is lowered into a solution containing rhodium, and an electric current lays a thin, even film of the metal over the surface.

Why white gold is plated

Pure gold is yellow, and no amount of mixing makes it truly white. White gold is gold alloyed with white metals, such as nickel, palladium and zinc. That pales the colour, but GIA notes that white gold will still look faintly yellow unless it is plated with rhodium. The more gold in the alloy, the warmer it looks: 18 carat white gold, with more gold in it, can appear warmer than 14 carat.

So most white gold is finished with rhodium. GIA gives three reasons: a whiter look, a better shine, and some protection against scratching. Under the plating, the ring is still white gold, and its hallmark shows its gold content exactly as it would without the plating.

Why silver is plated

Silver is white already, so the reason here is different. Silver tarnishes when it meets air and moisture, slowly darkening unless it is polished. GIA says rhodium plating protects silver from tarnishing, and gives it a bright, mirror-like shine.

There is a small change of colour too. Silver has a soft, slightly warm white; rhodium is brighter and cooler. A rhodium-plated silver piece can look much like white gold, which is part of its appeal.

How thin it is

Very. Rhodium plating on jewellery is measured in tenths of a micron, and a micron is a thousandth of a millimetre. One leading maker of plating solutions, Umicore, says its decorative rhodium bath for jewellery, watches and spectacle frames deposits layers 0.1 to 0.3 microns thick without cracking, with about 0.3 microns as its maximum.

For comparison, the American definition of gold vermeil asks for at least 2.5 microns of gold. A typical rhodium finish is a small fraction of that. It is hard, about 800 to 900 on the Vickers scale according to the same maker, but hardness doesn't make a layer that thin last for ever.

Rhodium and the hallmark

In the UK, plating a hallmarked piece doesn't change what it is. The UK Assay Offices' guidance lists which metals may be plated onto which, and rhodium is on every list.

Three conditions come with it. If the plating is applied after hallmarking, it must not be more than 2 microns thick on any part of the piece. Any precious metal used for plating must itself meet the minimum UK standard. And base metal is not allowed at all, either as a layer underneath or on the surface: the guidance names nickel and copper, and says assay offices will reject pieces where they find it.

So a rhodium-plated white gold ring is hallmarked as gold, and a rhodium-plated silver pendant is hallmarked as silver. The rhodium is a finish, not a change of metal.

How it wears, and re-plating

A layer a few tenths of a micron thick is not permanent. GIA is plain about it: rhodium plating wears away gradually, and a ring will need to be re-plated now and then. On white gold, the sign is colour. As the rhodium thins, the slightly yellowish white of the gold underneath starts to show through.

Everyday life speeds it up. GIA notes that regular exposure to water and the chemicals in some soaps and shampoos can dull a rhodium finish, and suggests taking white gold off before you shower. Abrasives, including those in some metal polishes and toothpaste, can wear metal away, and chlorine and bleach can damage some metal alloys.

The good news is that rhodium can simply be put back. A jeweller can re-plate a piece, restoring the bright white finish, and GIA suggests asking whether your jeweller will do this when you buy. How often a piece needs it depends on how it is worn, and on how warm the metal underneath is to begin with, so there is no fixed timetable.

The rose behind the white

Wollaston went looking for platinum and found a red solution instead, and named his new metal for the colour of its crystals. Two centuries later, the same metal makes the whitest jewellery whiter, a few tenths of a micron at a time. It wears, as all coatings do. But it can always be renewed, which is more than can be said for most finishes.