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Rhodium is a shiny silvery metallic white colored metal element with excellent light reflecting properties. The metal is hard and sturdy with a high melting point. It is one of the rarest precious metals and at the selling price of about US $80,000 per kg is the most expensive member in its class.
The name Rhodium stems from the Greek word rhodon which means a rose. It has been so named after the rose colored aqueous solutions formed by its salts.
William Hyde Wollaston discovered Rhodium in England in 1803 shortly after having discovered palladium. He successfully isolated it from its crude ore.
Very rare deposits of Rhodium in its elemental form do occur in Montana USA, but mostly Rhodium occurs in association with other platinum group metals in ores of nickel and copper. Chief sources of these ores are found in South Africa (which is the major source accounting for almost 60% of world’s Rhodium supply), in river sands of the Ural Mountains in Russia which ranks as the 2nd largest producer, and in North America, which include the copper-nickel sulfide mining area of the Sudbury, Ontario region of Canada.
The industrial extraction of rhodium is very complex and challenging as the metal occurs in ores mixed with other metals. It involves processes like assaying, solubilization, fractional distillation etc.
The atomic number of Rhodium is 45. It occupies Group Nine, Period Five and Block ‘d’ of the periodic table. It is categorized as a transition metal which belongs to the group of metals clustered around platinum in the periodic table and termed as the platinum group of metals. Rhodium, together with ruthenium, palladium, osmium, iridium, and platinum form the platinum group metals (PGM).
The main properties of Rhodium are that it is a lustrous metal with silvery white color and a high degree of reflectance. It is hard and sturdy and has a high melting point. Rhodium is not impacted by air at usual temperatures and hence does not get tarnished. It stays virtually unaffected by all corroding agents and so it is very resistant to corrosion. It is insoluble in most acids which includes aqua regia.
After the introduction of 3 way catalytic converters by Volvo in 1976, a significant portion of the produced metal (more than 85%) goes into the manufacture of catalytic converters for cars. The catalytic converters transform more harmful pollutants in car exhausts to less harmful ones.
One more major use of this metal is for producing alloys of platinum and iridium. These alloys have superior high-temperature strength and oxidation resistance and are made use of in furnace windings, phonograph-needles, pen-nibs, high temperature thermocouple and resistance wires, electrodes for aircraft spark plugs, bearings & electrical contacts.
Rhodium is also used to plate reflectors of searchlights for its brilliance.
Since Rhodium is the most expensive of all precious metals, it is used as an emblem of wealth and to confer honor when more common metals such as silver, gold or platinum are considered inadequate. In 1979, the Guinness Book of World Records gave Paul McCartney a rhodium-plated disc for being history’s all-time best-selling songwriter & recording artist.
Furthermore, considerable technical refinements to rhodium electroplating processes for industrial products have also allowed its broader commercial usage for a wide variety of decorative applications.
Rhodium is comparable in appearance and finish to highly polished chrome and it is the most tarnish resistant member among the platinum group of metals. A protective veneer of rhodium practically eliminates any chance of surface discoloring even when exposed to severe environmental conditions under usual temperatures. The hardness of plated rhodium is also highly resistant to incidental scratching and abrasion.
The aforesaid factors make Rhodium an ideal material for plating jewelry, tableware and other decorative objects. It is also used to coat some parts of high-quality fountain pens like those of Graf von Faber-Castell and Caran D’ache.
Rhodium is the most commonly used precious metal in the world of jewelry mostly for plating the jewelry items. Jewelry crafted out of pure Rhodium is very rare because it has a high melting point and low malleability both of which factors make designing of complex and intricate patterns a challenging task. Besides, the exorbitant cost of the metal also works as a dissuading factor to crafting of solid Rhodium jewelry.
Rhodium plating is imparted to platinum, white gold and sterling silver jewelry items for different reasons
Rhodium has a superior light reflecting property and brilliance than platinum and white gold. As such, platinum & white gold jewelry items are coated with rhodium to enhance the finish and luster.
Though silver has the very best reflecting property among all metals, it is also prone to quick tarnishing as a result of the spontaneous reactions with the gases in the air. Silver jewelry can also get easily scratched and damaged with the rigors of use. Hence silver jewelry is plated with Rhodium to shield it against tarnish and damage. Any jewelry piece with Rhodium plating will last for a long time.
But diligent care is required to protect the rhodium plating. You should stay away from ordinary jewelry cleaners based on harsh chemicals as they can undermine the rhodium plating and eventually remove it completely in the long run. You should instead clean them gently with soap or mild detergent and luke warm water. You can also decide to clean it with steam.
In spite of all the care and precautions, Rhodium plating does have a tendency to wear out with time and a trip to your jeweler is all that will be needed to get the re-polish which will be good as new.
The demand for Rhodium is consistent and high while the supply is tight. As such there will always be a demand pull on the price of Rhodium in the foreseeable future.