Tarnishing
Also: tarnish, toning, oxide film
Tarnishing is the surface discolouration of metals caused by a chemical reaction with sulphur compounds or oxygen in the surrounding air.
Anyone who has stored silver cutlery, silver coins or jewellery for any length of time knows the sight: the bright surface first turns yellowish, then brown, and finally a deep black. This tarnishing is not corrosion in the classic sense, where the metal itself is eaten away, but a wafer-thin chemical conversion of the very top atomic layers.
Causes and chemistry
The chief culprit for silver is hydrogen sulphide (H₂S), a colourless trace gas released by rubber, wool, eggs, exhaust fumes and even human skin. Concentrations of just a few parts per billion (ppb) are enough to set the reaction going:
4 Ag + 2 H₂S + O₂ → 2 Ag₂S + 2 H₂O
The silver sulphide (Ag₂S) that forms conducts electricity, clings tightly and grows as a self-limiting protective film. Oxygen on its own can also build a thin Ag₂O layer, though that is less strongly coloured. Carbonyl sulphide (COS) and sulphur dioxide (SO₂) in urban air accelerate the process further.
Which metals tarnish — and which do not?
| Metal | Tendency to tarnish | 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 speeds it up) | Ag₂S + Cu₂O |
| White gold (nickel-bearing) | moderate | NiO, wear of the rhodium plating |
| Palladium (Pd) | very low | – |
| Platinum (Pt) | practically nil | – |
| Gold 999 | nil | – |
Pure gold does not tarnish chemically because of its high standard potential (+1.50 V vs. SHE). Alloys such as 333 or 585 gold, however, can tarnish slightly thanks to the base metals they contain (copper, silver).
Effect 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-appropriate tarnish that is sometimes prized by collectors and speaks to authenticity.
- Patchy or aggressive tarnish — markedly lowers the grade.
- Cleaned — tarnish removed mechanically or chemically leaves scratches and permanently devalues coins.
For coin authenticity checks tarnish is a useful clue: tungsten forgeries with a gold coating often show atypical tarnish patterns along rims and strike edges.
Protection and removal
Preventive measures:
- Airtight storage (capsules, vacuum-sealed coin bags)
- Desiccants (silica gel) and sulphur-absorbing strips
- No storage next to rubber, wool or eggs
- Gloves when handling (fingerprints = fatty acids plus salt)
Removal is generally inadvisable for investment coins. For everyday silver, electrochemical cleaning with aluminium foil, baking soda and hot water works well: the aluminium acts as a sacrificial anode and reduces Ag₂S back to metallic silver galvanically — with no loss of material. Polishing paste, by contrast, always removes a little metal.
For anyone wanting to calculate the silver value of tarnished items: the melt value is entirely unaffected by tarnish, since the sulphide layer is extremely thin and simply oxidises away during melting.
In brief
Tarnishing is an unavoidable but purely superficial chemical reaction — driven mainly on silver by sulphur compounds in the air. For investment bars and bullion coins it has no bearing on metal value; for numismatic pieces, though, it can move the trading value considerably.