Platinum alloys were commonly used in the U.S. jewelry markets until platinum became reserved for strategic uses in 1942.  During the following years, the use of platinum in the U.S. Jewelry markets declined and platinum manufacturing skills and experience were lost to many jewelry manufacturers. Renewed consumer interest in platinum jewelry has put a strain on the ability of manufacturers to satisfy increased demand for new platinum products.  Under these operating conditions, it is imperative that the best alloys are selected for a particular manufacturing and product application.

Platinum—ruthenium and platinum—iridium alloys have been commonly used in jewelry applications for many years, while platinum—cobalt alloys are relatively new in the United States.  Trials were undertaken to critically compare and contrast the behavior of a Pt—Co alloy with the behavior of Pt—Ir and Pt—Ru alloys in jewelry casting and finishing operations so that the best alloy could be selected for achieving Stuller's quality and customer service goals. 

Metallurgy
Most platinum alloys consist of pure platinum alloyed with only one other element. These other alloying elements include iridium, ruthenium, or palladium, which are also in the platinum group of metals.  In contrast to this simplicity, karat golds might typically contain up to four elements, some of which are not in the category of precious metals. In metallurgical terms, platinum alloys are simple binary alloys. Many karat golds are complex, quaternary alloys.

Melting temperature and freezing range are important characteristics of alloys and may be considered in any manufacturing process. The entire liquidus solidus melting range of Pt—Ir and Pt—Ru alloys is greater than the melting point of pure platinum.  This is uncommon behavior for alloys. The melting point of most metal systems decreases as the concentrations of alloying elements is increased. Platinum cobalt alloys have melting points and freezing ranges that are less than the melting point of pure platinum.

Platinum and its alloys can be fabricated by the same basic methods that are used to fabricate karat gold alloys.  Of course, consideration must be given to the much higher melting points of platinum alloys.

Investment Casting
Because platinum alloys melt at such high temperatures, special refractories must be used for molds and crucibles. Molds and crucibles used for casting platinum and its alloys are always on the ragged edge of failure. Any reduction in metal casting temperatures or increases in melting temperatures of mold and crucible materials are benefits for the platinum caster and represent opportunities to improve and increase process reliability.

Manufacturers worldwide are now using 95 platinum—5 cobalt alloy for investment casting. Cobalt is reported to improve platinum's fluidity, and thus leads to greater filling of the piece. The alloy is reported to be hard enough to make polishing an easier process and is reported to be the most popular European platinum casting alloy. Platinum—5% cobalt alloy is described as useful for casting a whole range of patterns from fine filigree to heavy section items and is more forgiving to changes in cross—section, thus reducing the chances for shrinkage and consistently producing quality castings regardless of part size. Pt—5% cobalt has a slightly lower temperature. This can allow use of slightly lower temperature conditions during casting and retard reactions between crucible and mold materials. All these factors make platinum—5% cobalt a logical choice for casting operations.

Evaluation Trials
Two different styles of jewelry mountings were cast under the same conditions in the three platinum alloys that have been used at Stuller Settings. The alloys were Pt—10% Ir, Pt—5% Ru, and Pt—5% Co. Castings were finished with the same characteristics.  Both styles of jewelry selected for these trials were of two piece construction.  The two—piece construction of both styles created opportunities to evaluate soldering procedures.

Casting Operations
Pieces were set—up on hexagonal, tapered sprue posts and invested with a two—part phosphate bonded silica investment.  The molds required for each alloy were prepared from the same investment mix. Molds were burned out with the standard schedule used for all platinum casting molds at Stuller.

Molds were cast in a vacuum centrifugal casting machine at the same conditions used for production parts.

Assembly Operations
Stone settings and mountings were assembled by methods that are routinely used in Stuller's manufacturing operations.

Pre—Finishing
Pieces were cut down in a centrifugal disc—finishing machine in two stages.  Initial tumbling was done with abrasive—filled plastic media. Pieces were tumbled for half an hour.  Vibratory tumbling in stainless steel media for four hours completed pre—finishing.

Resizing
Each assembled piece was resized to evaluate the results of inserting a piece of sizing stock into the palm—side of the ring shank as if the finger size were being increased.

Different joining methods were used at the two joints on each ring.  One joint was soldered using 1700ºC seamless solder, and the other joint was complete by fusion welding.

Final Finishing
Rings were prepared for final polishing by filing, buff—sticking and rubber wheeling operations with traditional jewelry finishing tools and supplies.  The rings were then lapped and buffed to develop the final finish.

Stone Setting
Rings that were to be stone—set were polished to their final finish before stone setting.  Stone seats were cut with a high speed bur and generous amounts of lubricant to prevent the platinum from galling and sticking to the bur. Stones were set into final position and prongs were pushed over the stone girdle.

Results and Discussion Casting Operations
Molds were broken out, trees were water blasted and pickled. After casting and cleaning, the Pt—Co had a duller surface. The Pt—Ru alloy was brighter, but slightly rougher.  The Pt—Ir alloy was brightest but contained visible shrinkage on close inspection. The Pt—Ir gave evidence of being the most unforgiving with regard to shrinkage in design with isolated heavy sections or drastic changes in cross sectional area.  Shrinkage was not grossly evident in the Pt—Co alloy castings.

Settings that were cast in Pt—Co alloy demonstrated the lowest levels of surface porosity on the under sides of "fish tail" settings.  The condition of the sprue post was used to evaluate the general surface quality of castings and was taken to be an indicator of tendencies for reactions between metal and investment material.  The surface of the Pt—Ru sprue was the roughest.  The Pt—Ir sprue post was also quite rough.  The Pt—Co alloy sprue post was the smoothest of all.  Sprue posts were sectioned length wise along their centerlines to evaluate the internal soundness and general structure of the sprue.

The Pt—Co sprue contained a well—formed shrinkage pipe along its centerline extended down into the interior of the cast tree. This was limited evidence of micro—porosity in the metal walls surrounding the shrinkage pipe. In contrast, both the Pt—Ru and Pt—Ir alloy sprues did not contain a massive shrinkage pipe.  In each alloy, the sprue post was riddled with micro—porosity from the centerline of the sprue post to the chill zone at the surface of the post.

Assembly Operations
The Pt—Co alloy developed a light oxide film during soldering as a result of some slight oxidation of the cobalt in the alloy.

Re—Sizing Operations
It is interesting to note that the fusion joint in the Pt—Co alloy appears to want to have a small confined sink from solidification of the fusion zone of the weld. This behavior seems to mirror the general solidification behavior of the alloy observed in investment casting.

Final Polishing and Stone Setting
Pieces were polished and stones were set.  All three metals behaved in the same way during these operations. No obvious differences in color or quality could be readily observed.

Conclusions
Careful monitoring at each stage of investment casting and finishing processes indicated that platinum—5% Co alloys tended to perform better than alloys of Pt—5% Ru and Pt—10% Ir. Raw castings were superior when cast in the Pt—Co alloy because of their smoother surfaces and reduced amounts of shrinkage and porosity.  Assembly operations 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.
 

V4N7

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

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