Tarnishing
Also: tarnish, tarnish colours, oxidative tarnish
Tarnishing is the surface discolouration of metals caused by a chemical reaction with sulphur compounds or oxygen from the surrounding air.
Anyone who keeps silver cutlery, silver coins or jewellery for a longer period knows the phenomenon: the shiny surface first turns yellowish, then brown and finally deep black. This tarnishing is not corrosion in the classic sense, where the metal is destroyed, but an ultra-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. Even concentrations of a few parts per billion (ppb) are enough to start 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 accelerate the process further.
Which metals tarnish — and which do not?
| 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) | Ag₂S + Cu₂O |
| White gold (Ni-bearing) | medium | NiO, wear of the rhodium plating |
| Palladium (Pd) | very low | – |
| Platinum (Pt) | practically zero | – |
| Gold 999 | zero | – |
Pure gold does not tarnish chemically because of its high standard potential (+1.50 V vs. NHE). Alloys such as 333 or 585 gold can however tarnish slightly due to the base metals they contain (copper, silver).
Effect on collector value and coin inspection
For investment and collector coins, the degree of tarnish is decisive for value. Numismatists distinguish:
- Original toning — even, age-typical tarnish, sometimes desirable to collectors and evidence of authenticity.
- Blotchy or aggressive tarnish — significantly reduces the grade.
- Cleaned — mechanically or chemically removed tarnish leaves scratches and permanently devalues coins.
For the coin authenticity check, tarnish is an important indicator: tungsten counterfeits with a gold coating often show atypical tarnish patterns at the edges and stamping ridges.
Protection and removal
Protective measures:
- Airtight storage (capsules, vacuum-sealed coin bags)
- Desiccants (silica gel) and sulphur-absorbing strips (Pacific Silvercloth)
- No storage next to rubber, wool or eggs
- Gloves when handling (fingerprints = fatty acids + salt)
Removal is generally not advised for investment coins. For everyday silver, 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 loss. Polishing paste, by contrast, always removes a little metal.
If you want to calculate the silver value of tarnished objects: the melt value remains entirely unaffected by tarnish, since the sulphide layer is extremely thin and oxidises when melted.
In brief
Tarnishing is an unavoidable but superficial chemical reaction — mainly triggered on silver by sulphur compounds in the air. For investment bars and bullion coins it has no effect on the metal value; for numismatic collectibles, however, it can significantly influence the trading value.