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The Role of Mercury in Gold Refining

Save for those in the metal refining profession, not many people realize that mercury and gold share a special “bond” – namely how readily they bond to one another. If you take a look at a periodic table, you’ll notice mercury and gold are next to each other. The difference between their atomic structures is only one electron, so that helps explain the close relationship they’ve formed throughout refining history.

Mercury dissolves many other metals, and is especially effective on gold and silver (platinum is actually quite resistant to this phenomenon). When mercury dissolves another metal, the resulting mixture or alloy is called an amalgam – which is useful for many aspects of precious metal refining.

Most pre-19th century placer mining operations relied on mercury to retrieve the maximum amount of gold. By mixing mercury with crushed ore, even the finest particles of gold could be collected. After heat-treating the resulting slurry, miners could separate the mercury and gold for processing.

Even artisans have used mercury and gold to create stunning effects. Ormolu – a type of mercury-gilding – involves coating a piece (usually porcelain) in an amalgam, then using extreme heat to burn away the mercury, leaving a layer of gold. However, the process creates toxic fumes, so ormolu has been banned since the mid-19th century. Electroplating rose to take the place of ormolu, but many artisans argue that genuine ormolu pieces have a richer color and beauty – making them very valuable among collectors and pawnbrokers.

Dentists are probably the most familiar with mercury. Since the 1800s, silver/mercury amalgams have been the material of choice for restorative dentistry. They have become less popular in recent times (amalgams have been banned from Swedish dental offices since 2008), but amalgams are still more effective than other composite materials at resisting wear and bacterial growth.

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