Surface treatment: gilding oxidation and patination

The form is only half the story. The final surface treatment is often what determines whether a piece of jewellery appears warm and golden, cold and silver-gleaming or dark and dramatic. As a goldsmith and silversmith you work with several different techniques to give the metal its final expression — and some of them require you to know both the chemistry and the safety behind the process.

Gilding applies a thin layer of gold onto another metal, often silver or a cheaper alloy. The most common method today is electrolytic gilding — also called electroplating — where the workpiece is immersed in a bath and a weak electric current causes gold to deposit evenly on the surface. The method is widespread because it uses very little gold compared to other techniques.

Historically, gilding was also performed as mercury gilding, also called fire gilding: gold was dissolved in mercury to an amalgam, applied to the object and then heated so the mercury evaporated leaving a thick layer of gold. The method gave a durable result, but the evaporation of mercury is seriously health-damaging, and the technique is therefore largely not used in modern workshops — electrolytic gilding and newer methods such as PVD (physical vapor deposition) have taken over.

Where gilding adds a new metal, oxidation and patination instead work by inducing a chemical reaction in the metal's surface itself. Silver can be oxidized so it darkens in indentations and details, which gives contrast and highlights patterns and engravings. Patination is used similarly to give a surface an aged, coloured or textured appearance — often with chemicals that react differently depending on the alloy's composition.

Surface treatment is not just about appearance. Cheap alloys of brass or copper under a coating often contain nickel and nickel allergy is one of the most common contact allergies in Denmark. If the coating wears off the skin can come into direct contact with a metal that triggers a reaction — another reason to know the materials under the surface not just on top of it.

• 01Electrolytic plating uses a bath and low current and is today the standard in the trade.
• 02Mercury gilding has been abandoned due to the health risk from mercury vapours.
• 03Oxidising and patination create colour and contrast through a chemical reaction in the surface.
• 04Chemicals for baths, cleaning and staining require safety data sheets, ventilation and protective equipment.

Regardless of which surface treatment the task requires, the rule is the same: know the chemistry, know the risk, and always use the right protective equipment. It is this treatment that gives the jewellery its final, visible expression — but it must never cost you your health.