Testing Platinum Enhanced Gold

Following the release of its Sterling with Platinum alloy, ABI Precious Metals in Carson , California , has developed yellow gold alloys that contain platinum. Having had success casting the Sterling with Platinum, we were curious to see how the new yellow gold alloys measured up.

ABI sent us three samples of Platinum-enhanced Gold casting grain: 14k, 18k, and 22k. We tested the metal to see how it would perform under real shop conditions, and if the addition of platinum in the alloy made the metal different to work with than typical yellow gold alloys.

As there are many variables in the casting process, and therefore each caster's experience with an alloy may vary, we decided to examine three qualities that are critical to our trade casting shop: the ability of the metal to fill patterns, surface quality and color of the castings, and reusability of the metal.

The Procedure

We decided to cast multiple test trees of each alloy. Using 3 inch by 5 inch perforated flasks, we set up the patterns in the investment we normally use for casting yellow gold. Casting was done in an induction machine with pressure over vacuum, using nitrogen as the cover gas. We cast the first tree of each sample following the alloy supplier's recommendations for flask temperature (800°F to 1,000°F/427°C to 538°C) and metal temperature (1,850°F to 1,875°F/1,010°C to 1,024°C for 14k, 1,875°F to 1,925°F/1,024°C to 1,052°C for 18k, and 1,925°F to 1,950°F/1,052°C to 1,066°C for 22k). We expected these alloys to behave in a manner similar to that of Sterling with Platinum, which requires the use of higher metal and flask temperatures to compensate for its fast freeze rate, so we oversprued and ramped up to the highest casting temperature recommended by ABI. However, in subsequent casts, we pulled back and were eventually casting at temperatures we would normally use for most standard yellow gold alloys.

To examine fill, we created a wax test grid for each of the trees. To assess surface quality and color, we cast a flat washer shape for which we already had a mold. We chose this shape because it's easy to read and similar to our clients' pieces in surface and weight. In addition, we cast sample rings, pendants, and small belt buckles similar to the kinds of pieces we cast for our customers.

Lastly, to test reusability, we regrained the metal between each cast.

The Results

For all three alloys tested, the patterns filled incredibly well at low casting temperatures that ranged between 1,750°F and 1,850°F/954°C and 1,010°C. This was comforting to know, as the higher temperatures we started out with posed a threat to the investment; at these lower casting temperatures, we were fairly sure that we would not have surface defects caused by investment breakdown.

Of all the alloys tested, our favorite was the 14k. The sample castings had a rich yellow color right out of the investment, similar to the good color you can expect at breakout from a de-ox alloy. In addition to having good color, the 14k pieces were in-credibly hard as-cast compared to traditional 14k alloys. This hardness made for quick and easy finishing by hand and with mass finishing equipment. Although we did not use a Vickers hardness test and our results are based on hands-on experience, we felt the hardness of the 14k when clipping the parts from the main sprue. (The company reports that the 14k alloy has an as-cast Rockwell hardness of 88, and can be heat-treated to 94.) The 14k alloy's extreme as-cast hardness could be very useful for certain jobs, such as mechanical pieces and complex catches. Also, the alloy's hardness and color after casting would lead me to believe that it would be suitable for stone-in-place casting, but we did not test that at this time.

The 18k trees also filled well with good surfaces, very little porosity, and a lemon yellow color. I lapped the washer, took a close look, and liked what I saw; it was very clean and bright right out of the investment. However, the finished castings were nowhere near as hard as the 14k alloy.

Although the 22k tree filled well with good surfaces, it likewise lacked the degree of hardness we were so impressed with in the 14k. But perhaps most interesting was the metal's color—a deep gold, almost orange hue much unlike the rich golden color of traditional 22k and 24k yellow gold alloys. Since many of our conventional customers desire that rich, depletion-gilded look synonymous with high-karat gold, we could not fill their needs with this alloy. In our assessment of the 22k alloy, it didn't appear that the addition of platinum affected the alloy in any substantial way.

On the other hand, the otherworldly orange 22k color may be desirable to a designer or custom jeweler who is looking to create a unique art jewelry piece. It could be a selling point for someone who doesn't mind spending a little more for a gold alloy that costs approximately $50 per ounce more than traditional 22k yellow gold. Since casting the alloy does not require special casting equipment, it shouldn't present a problem for a small shop doing open vacuum casting.

As for reusability, all three of the sample alloys recycled nicely in our controlled atmosphere graining machine. We used 50 percent new metal as recommended by ABI between each cast. At the end of the casting tests, we recycled all the metal successfully. The grain was bright and consistent every time.

The Conclusion

These alloys could be useful for casters doing open or torch melting in their shop. Although the 14k was by far our favorite alloy of the group, particularly because of its extreme as-cast hardness, the 22k could pose some interesting opportunities for art and custom jewelers. I look forward to seeing where ABI will go next in the refinement of its Platinum-enhanced Gold.

Photo by Gary Dawson

Supplier's Note:

We are finding that our Platinum-enhanced Golds are outperforming standard karat golds in many ways, especially in hardness. We suggest heat-treating at 700°F for two hours and air cooling. Contact us directly with any technical questions: 1-800-878-2242.