What is Rhodium?
Rhodium is a silvery-white transition metal. It holds the distinction of being the world's most expensive precious metal. It has an atomic number of 45 and is about as nonreactive as gold. The only way to dissolve rhodium is with sulfuric acid. Part of rhodium's appeal comes from its high reflectance, almost unique among the metals. It is sometimes used as an expensive and flashy alternative to silver in jewelry, on which it is sometimes plated. Some of the most expensive consumer items in the world are made from rhodium.
Rhodium costs about six times as much as gold by weight. It is never found in mineral form, only being found in trace amounts within platinum or nickel ores. 80% of the world's rhodium comes from South Africa, and world production of the metal is only about 20 tons per year.
What is Rhodium Plating?
Rhodium is a silver-white hard metal often found in platinum ores. It is the most expensive precious metal. In its natural solid state Rhodium is far too hard to work, but it is used as an alloy in the smelting of platinum and palladium.
Rhodium plating is a metal deposition process used to coat materials with a decorative and protective layer of rhodium. Rhodium is a noble metal that imparts an extremely bright and hard wearing finish when applied as plating. When applied as a thin plate, it affords a durable finish of exceptional brightness. Rhodium finishes can greatly enhance the appearance and longevity of any metal to which they are applied.
The use of rhodium plating spread to white gold and sterling silver jewelry as a means of creating a highly durable, tarnish resistant surface.
The use of rhodium plating on base metal, “pot metal” and stainless steel jewelry findings and settings did not come into wide use until after World War II. Advances in technology eventually allowed even thinner layers of rhodium to be bonded with the surface of metals. While no vintage costume jewelry is “made of rhodium” or “set in rhodium”, costume jewelry of good quality was often rhodium plated.
The most frequently used method of applying rhodium finishes is the electroplating process. This involves submerging the recipient material in a heated bath of rhodium-based plating solution and running an electric current through the bath using the recipient as a cathode or negative electrode. This process causes rhodium from the solution to permanently bond to the recipient surface. Rhodium electroplating solutions typically consist of rhodium sulfate, sulfuric acid and water.
It is, however, important to be aware that rhodium plating will not last for an eternity. The lifespan will vary and will depend upon a number of factors including the amount of wear the jewellery receives, the original thickness of the plating, individual body chemistry, the quality of the plating and the colour of the underlying gold. The plating on an item that is subjected to a lot of wear or friction, such as a ring, may only last 6-24 months, whereas a necklace, brooch or earrings which comes in less contact with skin or the elements, or is simply worn less, can retain its plating for ten or more years.
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