Origin and effects of impurities in high purity gold

Chemical specifications for high purity gold have grown increasingly stringent as manufacturers strive to improve quality control. Once 999.9-fine grain and bullion bars were accepted without question; today sophisticated users insist on knowing impurity levels or, at least, the source of the gold. This paper demonstrates why. Using glow discharge mass spectrometry, concentrations of seventeen elements were measured in hallmarked bullion bars and grain from different sources. In 83 percent of the samples, gold bullion met or exceeded hallmarked purities, As expected, the principal impurity was silver followed by iron, copper and lead, At surprisingly low levels, some impurities can impact manufacturing processes, resulting in hard spots, embrittlement, blistering, and discoloration. After reviewing why various impurities are not entirely removed by the prevalent refining processes, this paper examines the effect of the significant impurities on manufacturing processes. Since the only means of dealing with excessive impurity levels is to refine contaminated metal, we conclude that manufacturers are exercising reasonable prudence by carefully evaluating incoming gold bullion in order to hold down overall production costs.