Tarnishing
Also: Tarnish, Tarnish Colours, Oxidation Tarnish
Tarnishing refers to the surface discolouration of metals caused by a chemical reaction with sulphur compounds or oxygen from the surrounding air.
Anyone who has stored silverware, silver coins, or jewellery for a longer period knows the phenomenon: the shiny surface first discolours yellowish, then brown, and finally deep black. This tarnishing is not corrosion in the classic sense, where the metal is destroyed, but rather an extremely thin chemical transformation of the very topmost atomic layers.
Causes and Chemistry
The most important trigger for silver is hydrogen sulphide (H₂S), a colourless trace gas released from rubber, wool, eggs, exhaust fumes, and even human skin. Concentrations as low as a few parts per billion (ppb) are sufficient to trigger a reaction:
4 Ag + 2 H₂S + O₂ → 2 Ag₂S + 2 H₂O
The resulting silver sulphide (Ag₂S) is electrically conductive, adheres firmly, and grows as a self-limiting protective layer. In parallel, oxygen alone can form a thin Ag₂O layer, which is however less intensely coloured. Carbonyl sulphide (COS) and sulphur dioxide (SO₂) from urban air additionally accelerate the process.
Which Metals Tarnish — and Which Don't?
| Metal | Tarnish Tendency | Main Product |
|---|---|---|
| Silver (Ag) | very high | Ag₂S (black) |
| Copper (Cu) | high | Cu₂O/CuO (reddish-brown), green patina (basic copper carbonate) |
| 925 Sterling Silver | high (Cu content accelerates it) | Ag₂S + Cu₂O |
| White gold (Ni-bearing) | moderate | NiO, abrasion of rhodium plating |
| Palladium (Pd) | very low | — |
| Platinum (Pt) | practically zero | — |
| Gold 999 | none | — |
Pure gold does not tarnish chemically due to its high standard potential (+1.50 V vs. NHE). However, alloys such as 333 or 585 gold can tarnish slightly due to the base metals they contain (copper, silver).
Effect on Collector Value and Coin Authentication
For investment and collector coins, the degree of tarnishing is decisive for value. Numismatists distinguish:
- Original toning — even, age-typical tarnishing that is sometimes desirable among collectors and proves authenticity.
- Patchy or aggressive tarnishing — significantly reduces the grade rating.
- Cleaned — tarnishing removed mechanically or chemically leaves scratches and permanently reduces a coin's value.
For coin authenticity checking, tarnishing is an important indicator: tungsten counterfeits with a gold coating often show atypical tarnishing patterns at edges and die lines.
Protection and Removal
Protective measures:
- Airtight storage (capsules, vacuum-sealed coin bags)
- Desiccants (silica gel) and sulphur-absorbing strips (Pacific Silvercloth)
- Do not store near rubber, wool, or eggs
- Wear gloves when handling (fingerprints = fatty acids + salt)
Removal is generally not recommended for investment coins. For everyday silverware, electrochemical cleaning with aluminium foil, baking soda, and hot water is suitable: the aluminium acts as a sacrificial anode and galvanically reduces Ag₂S back to metallic silver — without material removal. Metal polish, on the other hand, always removes some metal.
Those wishing to calculate the silver value of tarnished items: the melt value is completely unaffected by tarnishing, since the sulphide layer is extremely thin and is oxidised during melting.
In Brief
Tarnishing is an inevitable but superficial chemical reaction — triggered mainly in silver by sulphur compounds in the air. For investment bars and bullion coins it has no effect on metal value; for numismatic collector pieces, however, it can significantly affect trading value.