Ruthenium Plating — The Hard Dark Finish
Ruthenium Plating — The Hard Dark Finish
Electroplated ruthenium for dark grey to black surfaces in contemporary jewellery
Ruthenium plating is an electroplating process in which a thin layer of metallic ruthenium is deposited onto the surface of a precious-metal or base-metal substrate, producing a dark grey to near-black finish. The plating is harder and more wear-resistant than rhodium plating, the conventional white finish for white-gold jewellery, and is the standard option when a designer specifies a contemporary dark surface on silver, white gold, or yellow gold. Ruthenium plating is increasingly common in men's jewellery, in pieces with mixed-metal contrast, and in oxidised-look designs that aim for a darker palette than rhodium can provide.
The plating process
Ruthenium electroplating uses an aqueous bath of ruthenium salts, typically ruthenium sulphamate or ruthenium chloride complexes, with the substrate connected as cathode and a graphite or platinised-titanium anode supplying the circuit. Current density and bath temperature are controlled tightly because ruthenium deposits become brittle at greater thicknesses; the practical maximum is approximately 0.5 microns, with most jewellery plating in the 0.1 to 0.3 micron range. The substrate must be impeccably clean, and a strike layer of palladium or rhodium is sometimes applied first to ensure adhesion.
The deposited layer is metallic ruthenium with the characteristic dark silvery grey colour that appears nearly black in thin film. The colour is uniform and stable, and unlike chemical patinas on silver, ruthenium plating does not change colour over time in normal atmospheric exposure.
Comparison with related dark finishes
Black rhodium plating uses a rhodium bath with additives that produce a darker deposit; the result is less hard and less wear-resistant than true ruthenium, with a slightly different colour balance (more brown-black than grey-black). Oxidised silver — a chemical patina produced by exposing silver to sulphur compounds — is darker still but is much softer and wears off readily on contact surfaces. Physical vapour deposition (PVD) coatings produce harder ceramic-style layers in various colours, including black, but require specialised equipment and are more expensive than electroplating.
Ruthenium plating sits in the middle of the durability ladder for dark finishes: harder than oxidation, comparable to or slightly less hard than PVD, more durable than black rhodium. For most jewellery applications, ruthenium offers the best balance of cost, durability, and visual finish.
Wear and reapplication
Ruthenium plating wears in normal use, with the high-friction contact areas — the inside of ring shanks, the underside of bracelets, the back of pendant settings — typically the first to show wear-through. The base metal becomes visible as the plating thins, producing a contrast that is often the first signal that reapplication is required. Reapplication is straightforward and is a routine service offered by fine-jewellery workshops; the process is to strip any remaining plating, polish the substrate, and replate.
Care recommendations for ruthenium-plated jewellery favour gentle cleaning with mild soap and warm water, soft cloth wiping, and the avoidance of abrasive polishing compounds, ultrasonic cleaning at high settings, and chemical jewellery cleaners. The plating is hard but is also thin, and aggressive cleaning will accelerate wear-through.
In the trade
Designers and retailers should disclose ruthenium plating to buyers because the finish requires occasional reapplication and because the appearance differs from a bulk-coloured alloy. The trade convention is to describe such pieces as ruthenium-plated white gold or ruthenium-plated silver, with the substrate identified honestly. Pieces marketed as black gold are almost invariably ruthenium- or black-rhodium-plated rather than bulk black-gold alloy.