Plating, explained

A surgeon's solution

In 1840 a Birmingham surgeon named John Wright found that silver dissolved in a solution of potassium cyanide could be laid down on another metal with an electric current, beautifully and evenly. Two Birmingham businessmen, George and Henry Elkington, bought his process and patented it. Their works went on to build the electroplating industry in Birmingham, and from there it spread around the world.

Most plated jewellery is still made the same basic way: a piece hung in a liquid, a current switched on, and a skin of precious metal growing on the surface, a few atoms at a time.

How electroplating works

Electroplating needs four things.

• The piece to be plated, which becomes the cathode: it is wired to the negative side of the power supply.
• An anode, wired to the positive side. It is either a block of the plating metal, which dissolves to top up the bath, or an inert conductor that does not dissolve.
• The electrolyte, a solution containing the plating metal as positively charged ions.
• A direct current from a power supply.

When the current flows, metal ions in the solution are drawn to the piece, pick up electrons at its surface and turn back into solid metal, coating it. The arithmetic was worked out by Michael Faraday in the 1830s: the amount of metal deposited is proportional to the electric charge passed, which is current multiplied by time. Double the current, or double the time, and in principle you double the metal.

In practice the coating is not perfectly even. Edges and points closer to the anode tend to collect more metal than recesses further away, and how evenly a bath plates, its "throwing power", depends on the solution, its temperature and the current used.

None of this works on a dirty surface. Before plating, a piece is cleaned thoroughly so the new metal can grip, and anything left on it, from polishing compound to fingerprints, shows up later as a weak spot.

Thickness, in microns

Plating is measured in microns, properly micrometres, written µm. One micron is a thousandth of a millimetre. The layers on jewellery are a few microns at most, and often a fraction of one, which is why they are described by rules rather than by eye.

Notice that the two countries pull in opposite directions. American rules set minimums, so that a name like "heavy gold electroplate" or "vermeil" promises a decent layer. UK hallmarking rules set a maximum, so that a plating on silver can never be thick enough to pass for the precious metal underneath.

Gold, rhodium and silver

• Gold plating gives the colour of gold to silver or to base metals such as brass or steel. Its carat is stated alongside the word "plated": UK rules require gold plated silver to be described as silver, with "gold" followed directly by "plated", as in "18ct gold plated silver ring". The full rules are in Gold hallmarks explained.
• Rhodium plating is the bright, cool white finish on most white gold and on much silver jewellery. Rhodium is one of the platinum group metals and a minor by-product of platinum refining. It is too difficult to work to make jewellery from, but as a plating it is intensely white and one of the most reflective of metals. UK rules allow gold, platinum, palladium and silver to be plated with rhodium.
• Silver plating is the oldest of the three in industry, the Elkingtons' original product. It is used over base metals to give the look of silver, and silver itself may be plated with silver.

The full list of platings allowed on hallmarked metals is short and precise. Silver may be plated with silver, palladium, gold, platinum, ruthenium or rhodium. Gold may be plated with gold, platinum or rhodium. Any precious metal used for plating must itself be at least the UK's minimum standard of fineness.

Strike layers

Not every metal plates well straight onto every other. Some combinations bond poorly, so platers often begin with a strike, sometimes called a flash: a very thin first deposit with good adhesion that acts as a foundation for the main layer. A strike is typically less than a tenth of a micron thick and is laid down from a bath with a low concentration of metal at a high current density. It is slow, so once it is in place the piece moves to a faster bath for the main plating.

A strike can also act as a go-between, compatible with both the base metal and the final finish. In industrial plating a nickel strike is a common choice for that job. On jewellery, nickel brings a problem of its own.

Why there is no nickel under a hallmark

The UK Assay Offices are blunt about this. Base metal may not be used for plating on precious metal articles, "whether as an interlayer or as a surface treatment", and that includes nickel, copper and alloys such as Miralloy. If an assay office finds it, the articles are rejected. The guidance adds that the practice tends to be more evident on pieces made outside the UK, and encourages importers to tell their suppliers before an order is placed.

There are two good reasons for the rule. A hallmark is a promise about the precious metal, and a hidden layer of base metal undermines it. And nickel is the commonest cause of metal allergy from jewellery, so UK and EU law limit how much nickel jewellery can release onto skin. American rules make a similar point about honesty: sterling silver covered with nickel and then gold plated may not be called vermeil unless the base metal layer is disclosed.

PVD, the vacuum alternative

Physical vapour deposition, or PVD, coats metal without a liquid bath. The process takes place in a vacuum at high temperature, where the coating material is turned to vapour, by methods such as cathodic arc evaporation or sputtering, and condenses onto the piece. A paper at the Santa Fe Symposium on jewellery manufacturing described it as a durable, cost-effective and environmentally clean alternative to electroplating, with excellent wear resistance and good tarnish resistance.

The gold colour of much PVD-coated steel comes from titanium nitride, which is not gold at all, though real gold can be deposited this way too. Watchmakers use PVD widely: Longines, for example, describes a black chromium carbide coating up to a single micron thick on steel. A coating of titanium nitride with no gold in it is a colour, so in the UK it is described as "gold tone" or "gold coloured", not as gold.

How plating wears

Plating is a skin, and skin wears. Every brush against a desk, a sleeve or another ring takes away a little, and the high points and edges go first because they take the most contact. Rings and bracelets lead hard lives. Pendants and earrings get off lightly.

That is why thickness matters. A thicker layer has more metal to lose before the colour underneath shows through, which is exactly what the American "heavy gold electroplate" and "vermeil" minimums are protecting, and why the law asks a coating to keep nickel off the skin for at least two years of normal use. Laboratories test this by simulating wear and corrosion before measuring nickel release, under the European standard EN 12472.

What lies under the plating matters too. Plating reproduces the surface it covers rather than filling it, so scratches and file marks show straight through, and a carefully finished surface gives a brighter, longer-looking result. When a plated finish does wear, a jeweller can strip, clean and re-plate it.

A thin skin, carefully made

John Wright's cyanide solution made silver affordable to people who could never have bought it solid, and the industry it started in Birmingham still works on the same principle. A good plating is not about how it looks on the first day. It is about what was done before the piece went into the bath, how much metal came out with it, and how honestly it is described afterwards.