In recent years, we undertook a series of studies to compare various working properties of three common platinum alloys, platinum iridium 90/10, platinum ruthenium 95/5, and platinum cobalt 95/5. Trials were undertaken to critically compare and contrast the behavior of the three alloys in jewelry casting and finishing operations.

Investment Casting
The first issue addressed was whether one alloy demonstrated superior casting properties relative to the others. In short, the answer is a definitive yes. Platinum-cobalt 95/5 is a superior alloy for investment casting. 

Platinum-cobalt 95/5 produced castings with better surface characteristics, controlled shrinkage, and less porosity overall. Castings produced with the platinum-iridium and platinum-ruthenium alloys under the same condition, resulted in castings that were deemed unacceptable by our standards. The surface of the Pt-Ru sprue was the roughest, indicating a strong driving force for metal-mold reactions. The Pt-Ir sprue post was also quite rough, while the platinum-cobalt exhibited a surface comparable to investment cast 14k yellow gold. Obviously the improved fluidity, achieved with the addition of cobalt in platinum combined with the lower melting point, contributed significantly to the results.

During the finishing and stone setting phases of the trials, subtle differences were noted but were deemed significant enough to be a deciding issue between the three alloys. Assembly operations which included sizing, were easily accomplished in spite of a tendency by the Pt-Co alloy to develop a light film of cobalt oxide.  This film was easily removed in subsequent finishing and polishing operations.

Pt-Co alloys are slightly magnetic but this feature did not interfere in any way with jewelry manufacturing operations.  Finally, the CIELAB color coordinates of Pt-5% Co, Pt-5% Ru, and Pt-10% Ir alloy were measured in a polished and finished condition The numerical values for the color coordinates indicate that these alloys are indistinguishable from one another when observed by a normal human eye.

There are some differences in working with the platinum cobalt alloy over the others (most notably the inability to fuse platinum iridium or platinum ruthenium to it.) However, these differences should not pose significant obstacles to the bench jeweler, provided that they are aware of them.

Knowledge and training appear to be the overriding factors in the acceptance of platinum cobalt alloys for investment casting. This brings to mind a saying, "Once you know the nature of the beast, it's easily tamed."
 

V5N4

A Review of Cast Platinum Jewelry Fabrication Methods
Gregg Todd, Dennis Busby,
Dena Landry, Mart Linscomb,
Greg Gilman
Stuller Settings, Inc

This is an abbreviated version of the original work. For full technical details, please consult the original paper.