Tarnishing
Also: Tarnish, Tarnish colours, Oxide tarnish
Tarnishing refers to the surface discolouration of metals through chemical reaction with sulphur compounds or oxygen from the surrounding air.
Anyone who keeps silver cutlery, silver coins or jewellery for a longer time 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 a wafer-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 gases and even from human skin. Concentrations of just a few parts per billion (ppb) are enough 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 colour-intense. Carbonyl sulphide (COS) and sulphur dioxide (SO₂) from urban air additionally accelerate the process.
Which metals tarnish - and which do not?
| Metal | Tarnishing 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 nil | - |
| Gold 999 | nil | - |
Pure gold does not tarnish chemically owing to its high standard potential (+1.50 V vs. NHE). Alloys such as 333 or 585 gold can, however, tarnish slightly through the base metals they contain (copper, silver).
Influence on collector value and coin testing
For investment and collector coins the degree of tarnish is decisive for value. Numismatists distinguish:
- Original toning - even, age-typical tarnish that is partly desired by collectors and proves authenticity.
- Patchy or aggressive tarnish - considerably reduces the condition grade (grading).
- Cleaned - mechanically or chemically removed tarnish leaves scratches and permanently devalues coins.
For the Coin Authenticity Test, tarnish is an important clue: tungsten counterfeits with gold plating often show atypical tarnish patterns at the edges and struck rims.
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 inadvisable 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.
Anyone wishing to calculate the silver value of tarnished objects: the melt value remains completely unaffected by tarnish, since the sulphide layer is extremely thin and oxidises during melting.
In brief
Tarnishing is an unavoidable but superficial chemical reaction - triggered mainly on silver by sulphur compounds in the air. For investment bars and bullion coins it has no influence on the metal value; for numismatic collector pieces, however, it can significantly affect the trading value.