solid, the bar is notched and broken off in the appropriate lengths. Valcambi’s permanent mold and continuous casting lines work much like those in an automated production metalcasting shop, with robots perform- ing the work that would normally be done by human employees. “We were the first company to have


a fully automated unit to produce gold bars,” said the Valcambi spokesperson. Because little premium is placed on the geometric design of bullion


bars, producers of the products focus on controlling a few casting process parameters to find success—alloy temperature and consistency, mold material/temperature and mold re- lease agent. The ideal casting temperature of


gold is 2,080F (1,138C). According to Arvay, going over that temperature (superheating) is not advisable; falling below it is worse. “If you don’t hit that temperature, there is a good chance you will get


insufficient fill, as the metals cool too quickly,” Arvay said. The molds used for casting precious


metals generally are either cast iron or graphite, with some proprietary blends used by different manufacturers. The molds must be heated prior to pouring and coated with the appropriate mold release agent. According to Arvay, iron molds are less expensive than graphite molds and last longer.


Preparing the Metal The thought of alloying a precious


metal seems heretical. Why would you want to change the composition of a metal that is worth the greatest amount at its most pure? But refining the alloys to their purest form is itself a trick. “If anything, we de-alloy,” Arvay


said. “We remove any of the base metals through refining.” Traditional alloying also occurs,


just as it does in commercial casting. Introducing small amounts of certain elements can either make precious metal alloys more castable, or elements can be used to produce a piece of bul- lion with a certain color. Rose gold, for example, is gold alloyed with copper. White gold has large amounts of pal- ladium. De-oxidized silver is actually an alloyed silver that contains boron, silicon and other metals that are less attractive to oxygen molecules. With the cost of gold in particular so


high, casters of precious metals must be careful to maximize the alloys used in their processes. Metal loss, a common concern in most (if not all) commercial metalcasting facilities, is unacceptable in the world of precious metal bar production. And pre-measuring prior to pouring is critical. “It’s all about material preparation


beforehand,” said the Valcambi spokes- person. “We measure out metal shot to the exact weight you need before melt- ing and pouring it. You have to meet exact standards in weights, measure and fineness.” Refining scrapped precious metals


Valcambi, one of the world’s largest producers of cast gold bars, produces the bullion in both the permanent mold and continuous casting processes in an automated facility.


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also is an exacting process. Silver must undergo extensive refining (typically performed by an outside firm) to over- come its propensity to attract oxygen during melting. De-oxidized versions of the metal can alleviate this step. Gold also can be purchased in a de-oxidized state or with other additives that make casting it more economical and less prone to scrap production.


MODERN CASTING / September 2010


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