2009-Jewelry Alloys: Hardness & Hardenability

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2009-Jewelry Alloys: Hardness & Hardenability

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Mechanical and physical properties for silver and gold alloys have been used for at least 140 years to select working processes and find their limits. Hardness was once an adequate indicator to a gold/silversmith or a craft jeweler; no longer. Batch production and new alloys have complicated the assumption that jewelry alloys are always ductile, malleable and sufficiently wear resistant. Platinum group alloys, white golds, stain-free silvers, stainless steels, titanium and niobium have expanded the jewelry alloy spectrum greatly. We now need a better understanding of hardness test data. The history of hardness testing is used to show different measures of hardness, and instrument settings give significantly different results. A penetration hardness impression involves local work hardening, which depends on alloy composition and on the latent hardness to create an equilibrium stress reading. So the softest and fastest work hardening alloys give a measured hardness higher than their true latent hardness. Hardness tests are surface tests at a point; they may not represent the average hardness of the material. Surface/interior differences may depend on electroplating, porosity, reactions in melting and casting. Some polishing, machining and surface working operations produce a thin, hard layer. Some alloys are prone to shrinkage, gas porosity or coring, which may not need to be eliminated but cause at least micro-hardness variations. A good understanding of work hardenability is very important in jewelry manufacture and the hardness test is a very useful, cost-effective tool. A standard penetration hardness value is a good indicator of the state of a specific jewelry alloy. But hardness tests are empirical; they do not accurately simulate life performance or further processability across a wide range of materials. There is no need for further hardness test development to suit jewelry alloys. The industry does need more exact contextual reporting and wider exchange of test data to better appreciate the meaning and limitations of hardness data.

Author: Dr. John C. Wright

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