Originally Published AJM Magazine
IN THE SHOP Section APRIL 1999
by Daniel Ballard
“To Avoid Shattering Bracelets and Polka-Dot Rings, Know Your Alloy”
Picture this: An 18k rose gold bracelet is cast, set with high quality diamonds, and polished to a high shine. Then the customer wants a different clasp. Back to the shop it goes. Heat is applied to melt the solder around the unwanted clasp and the bracelet shatters, breaking some diamonds in the process.
What went wrong here?
Plenty, but it all comes down to one thing: the jewelry manufacturer didn’t understand the nature of his alloy. After two decades of working for a refiner, I’ve witnessed plenty of cases in which manufacturers with the best intentions produced the worst results, all because they didn’t know their metals.
Let me give you a few examples. One day I was shown some cracked and broken gold that a jeweler had unsuccessfully tried to roll out into sizing stock. The gold came from a cast tree that was re-melted and poured into an ingot.
The problem: Casting alloys for lost-wax casting contain additives, generically known as deoxidizers; they protect the molten gold from surface flaws as it reacts with the gypsum investment. Silicon, the most common casting additive, causes brittleness. But for fabrication, the gold needs to be softÃ‘which means no additives allowed. (There not needed, anyway; since the gold will be poured into a steel or graphite mold, there’s no reaction with investment to consider.) If you will be fabricating, do not use casting gold, and vice versa. Always tailor the metal to the task.
Another customer once asked, “Why did this gold cast so well at first, only to become dark, porous, and difficult to finish in later castings?” Well, there probably wasn’t adequate fresh metal in the alloy. Fifty percent new metal is the practical minimum for fine jewelry. When you reduce the fresh percentage too far, you are really just shifting increased cost from the metal purchase to the polishing/repair part of the process. Or you’re just sending it to your refiner (in this case, me) in the form of failed castings. Good for my commission, bad for your bottom line.
I worked with one customer who was making the switch from torch casting to induction casting. One major oversight: Both the manufacturer of the casting machine and the customer believed they could start casting their gold as they always had. The result? Well, I wound up at the customer’s factory, reformulating alloys every few casts until we finally achieved good results. That took three days and 15,000 Dwt of karated gold. A very valuable lesson.
If you’re going to change casting methods, talk to your alloy/gold supplier. Alloys for low-tech torch casting are different than those made for new $50,000-and-up super casting machines. A torch operates in the air, where oxygen can get to the copper and other metals in the gold. A torch caster can adjust his flame, his gas mix, and his timing as he goes along. Because the new induction machines all do a great job of isolating the gold from the oxygen, we don’t need as much deoxidizer as we do for a torch casting alloy. Another important point: The new machines are programmed to very specific timing and temperatures, so the alloy must be extremely consistent.
Then there was the customer who had yellow rings with random white spots that resisted polishing. I immediately saw that he hadn’t used different crucibles for different colors or karats of gold. White and yellow gold do not cast well together. If you cast yellow gold in a crucible used for white, you will get solid beads of white gold through the casting. Unintentionally two-toned polka dot jewelry will be the result. Not a popular style.
And, finally, about that horror story at the beginning of this article. The bracelet shattered because, quite simply, rose gold is difficult to use in 18k. For reasons that metallurgists alone really understand, 18k rose gold often cracks or even shatters when heat is applied for assembly, repair, or ordinary sizing. If you can’t encourage your customers to order 14k gold, see if they’ll at least tolerate a paler rose color with a higher silver content. If the rose alloy is less than 10 percent silver (gold excluded) there will be trouble in 18k. If we get to about 15 percent or 20 percent silver, we get fair results in 18k.
Jewelry makers cut corners at a cost, and that’s especially true with alloys. So the next time you’re ordering metal, ask yourself a few questions: Are you trying to use the same type of alloy for everything? Are you using the same metal again and again? Have you made major changes in methods? And here’s the big one-am I using the correct alloy for my purposes? Knowing how and why your alloys do what they do, you’ll make better jewelry—and not have to go through any “shattering” experiences.
Daniel Ballard is the national sales manager for Precious Metals West/Fine Gold in Los Angeles, and handles customer technical assistance.