The process of investment casting is utilized for jewelry production throughout the world. In our design of nickel-free white golds a few years ago, we insisted on an alloy formulation that would be compatible with conventional gypsum investment casting. We settled on palladium as a replacement bleaching agent despite an elevated liquidus about 115C higher than comparable nickel formulations. Regardless of higher absolute casting temperatures and elevated investment flask temperatures, we did not see the detrimental phenomenon known as metal/mold reactions during experimental testing or production. The mechanism involves cast quality deterioration as a result of the thermal decomposition of standard gypsum investment. A review of previous studies suggested a number of factors contribute to the phenomenon. We confined our study to 14K nickel and palladium bleached white golds in an effort to determine the range of commercial investment casting conditions that contributed to the reaction. This involved casting a standard set of models for both formulations through a large range of metal superheat, investment flask temperature and casting atmosphere in a primary test series. A secondary set focused on the effect of metal recycling and investment contribution to the issue. We also wanted to evaluate how various processing contaminants such as carbon and sulphur contributed to the issue. Physical properties evaluations provided objective backup for the interpretive models used to highlight any metal/mold reaction. Building on the work of Dieter Ott and Chrisoph Raub from FEM in Germany, we sought an understanding of the causes and detrimental effects of metal/mold reactions on the critical white gold component in many investment cast jewelry articles.

Author: Greg Normandeau