Oxidation is a process that occurs naturally when a metal reacts to oxygen and other elements in the atmosphere, causing a change of colour on the metal surface. In sterling silver this manifests as a yellow to brown to black colour on the metal, depending on how long the metal has been exposed to these elements. This is also known as tarnish.
Low carat gold alloys may also tarnish slightly in time, but no other precious metal changes colour as drastically as sterling silver does, mostly due to the high copper content in a standard 925 alloy, which is especially susceptible to oxidation.
Note: Oxidation does not change the structure or purity of an alloy, it affects the surface only. Because of this, oxidation can easily be removed by dipping it in a chemical cleaning solution, or by rubbing or polishing the metal with the help of a polishing compound.
Deliberately oxidizing jewellery
Jewellers often exploit this tarnishing property of sterling silver by intentionally exposing a piece to chemicals that speed up the process of oxidation, causing a black patina on the surface.
When pieces are oxidized, and the oxidation is purposefully polished off from raised parts of the design, pieces acquire an antique look and feel. The dramatic contrast between black and bright is often used to highlight specific design features in contemporary jewellery pieces.
How to take care of partially oxidized pieces
Do NOT dip oxidized pieces in jewellery cleaner products. This will remove the black patina and ruin the intended effect of the oxidation.
In time, however, the bright parts of the piece will also start to discolour, which is also not desirable. The best way to restore the shine is to rub it gently with a cloth. Special jewellery cleaning cloths are available that works well for this purpose. These cloths are impregnated with polishing compound, which will remove the oxidation and restore the shine to the metal surface. Because only the raised parts of the design is exposed to the cloth, the dark recesses of the design will stay black, as the jeweller originally intended.
How to take care of wholly oxidized pieces
Sometimes a jeweller may opt to completely blacken a piece, without polishing off the oxidation in places. Pieces like these require no special care; they will always remain black as long as they are not rubbed, scratched, dipped in jewellery cleaner or polished.