Oxidised Finish on Silver Alloys

When silversmiths talk of an oxidised finish on silver they typically mean the deep blue-black sulphide layer on the surface of a silver alloy created by treating it with a chemical compound such as Liver of Sulphur. This is different to a true silver oxide; that is oxygen bonded with silver, which is a difficult chemical compound to form.

Liver of Sulphur is a mixture of potassium polysulphide, potassium bisulphide, potassium sulphide and potassium thiosulphate. It is produced by reacting potassium carbonate with sulphur and immersing a silver item in it allows the formation of a thick sulphide film on the surface of silver items, typically a deep blue-black colour although other finishes are possible by removing the pieces after shorter times or by mixing the chemical with substances other than water to obtain a solution with a different pH (acid content) which would give a different rate of reaction.

Liver of Sulphur is available in different forms; the solid chemical (lump or flake) is affected by moisture and light and needs to be stored in dark dry conditions. The ready made solution has a limited shelf life as it is affected by bright light and needs to be stored in the dark.

So, you may ask yourself, how can a sulphide coating on the surface of a silver alloy be called an oxidised finish? Well this goes back to some basic chemistry and the definition of what an oxidative reaction is and what a reducing reaction is.

In chemistry oxidation is a word which originally meant combination with oxygen gas. However so many other chemical reactions were seen to resemble reactions with oxygen that the definition was broadened to refer to any reaction in which a substance or species loses electrons. This is remembered by use of the mnemonic:

OILRIG – Oxidation is Loss (of electrons), Reduction is Gain (of electrons).

If we think of the tarnishing reaction where silver combines with a ‘free’ sulphur (from either hydrogen sulphide (H2S) and sulphur dioxide (SO2) in the atmosphere) to form a silver sulphide we are looking at a reaction where each silver atom gives up one electron to bond with the ‘free’ sulphur atom.

2Ag + [S] → Ag2S


As the silver atoms are losing electrons they are undergoing an oxidative reaction in forming the silver sulphide at the surface. This is why use of Liver of Sulphur to get a black surface finish can correctly be called an ‘Oxidised Finish’ on silver.

To quote some meerkats, “Simples”!