By Shawna Kulpa
Gemstone setting is an art form, and a very precise one at that. It’s also led many jewelers to tear their hair out. After all, there may be few things more stressful than forcing a metal prong over a stone while worrying that you’re applying just the right amount of pressure—too little, and the stone won’t be secure; too much, and the stone can break.
Could stone-in-place casting be the answer?
Just like it sounds, the process involves casting models with the stones already set in place. It’s not that common here in the U.S., but according to J. Tyler Teague, owner of JETT Research in Ashland City, Tennessee, “it’s probably the dominant way they’re set outside the U.S.”
However, it’s not as simple as just taking a wax model and popping gemstones in just prior to casting, with everything coming out of the machine perfect. As Teague cautions, “It’s not just a walk in the park.”
To maximize the chances of success, the process requires special materials and procedures: It isn’t suitable for all types of projects, nor can it be done with all types of gemstones (only a handful are suitable, actually).
Stone-in-place casting also requires jewelers to come at the design in a slightly different way, as the engineering of the piece may need to be altered. But when done right, it can result in time and cost savings that make the effort worth it in the end.
At first glance, it would seem the big advantage of stone-in-place casting is the ease with which stones can be set. After all, it’s much easier to push wax prongs over a gemstone than metal ones. And the ease with which stones can be set means they’re also less likely to be damaged in the process.
“There’s no hammering or forcing,” says Ajit Menon, director of technology at SIPI Metals Corp. in Chicago. “You’re just pushing the stone down into the seat within the wax. There’s not a lot of pressure, so there’s less chance of chipping or breaking the stone.”
Menon also notes that if the stone isn’t set properly on the first go, it’s much easier to fix in wax.
“It’s difficult to pull out a stone already set in metal,” he says. “If the same were to happen in wax, it’s a lot easier to take out stones. There’s no need for any excessive force.”
Not only does the technique have the potential to save time, there’s also the resulting labor cost savings. While someone will still have to set the stones, the ease of setting them in wax means “it’s more cost effective to do it at the front end of the process than having people setting stones after the fact,” says Michael Stover, director of technical services at United Precious Metal Refining in Alden, New York.
In fact, Teague says that after a month or two of training, “a good wax setter can do about five times the number of stones a day than a [metal] setter.”
However, despite the time and cost savings that this process offers, the benefits of stone-in-place casting ultimately come down to whether the design actually warrants it.
One consideration is the number of stones in a piece. “Stone-in-place casting is actually only for those high labor-intense jewelry styles with 50 or 100 stones. A channel-set piece with lots of stones or pavé with many tiny stones, those are styles that go well for this process,” Menon says. “There’s no benefit to stone-in-place casting if you have only a couple of stones” in a design.
Not all gemstones are suitable for stone-in-place casting. The limited few include diamonds, natural rubies, sapphires, some garnets, most synthetics, and cubic zirconia. Natural emeralds are also a possibility, but they need to be of really nice quality and free of rutile imperfections.
He believes that using the process for pieces with just one or two stones isn’t worth the headache because of the risk it poses. “[After casting] the stone is never the same quality as you got it,” he says. “There will always be some degradation of the gemstone during the stone-in-place casting process, though how much depends on many factors and is debatable.” Possible degradations include loss of luster and brilliance, cloudiness, or even a complete change in color in the case of colored gemstones.
Another consideration is whether the design includes stones and metals that can actually withstand stone-in-place casting, and whether it has been engineered to provide the necessary support for the stones. All of these factors can dramatically affect the success of the casting, as we’ll see.
Due to the high temperatures needed during the burnout cycle, not all gemstones are appropriate for stone-in-place casting. The stones most likely to survive the process are diamonds (both natural and lab-grown), natural rubies, sapphires, some garnets, and most synthetics, including cubic zirconia. Natural emeralds are a possible option, depending upon how lucky you feel.
“Emerald can be wax set if it doesn’t have rutile imperfections,” says Teague. “It has to be a really nice quality emerald, and [you should] expect to have to re-oil the stone afterward.”
Menon concurs. “Some people have tried emerald, but many have failed. And anything below [emerald] on the Mohs hardness scale is unlikely to survive the heat involved in the stone-in-place casting process.”
However, that doesn’t mean that any of these stones will always work. Stones with flaws are not likely to come through the process unscathed. “You’re only as good as what you start with,” warns Stover.
It’s also important to be aware that some stones, particularly those that have been irradiated to improve color, may emerge from the process with a slightly different shade. If a stone has been irradiated, it’s possible to repeat the process to get back the desired color, but there’s no guarantee.
In addition to the type of stone being used, jewelers also need to be wary of a stone’s shape. Some shapes are more problematic than others.
“Baguettes are longer and more sensitive to cracking due to metal shrinkage force,” says Menon. “They require proper clearance within the channel or seats to compensate for the metal shrinkage.”
“The worst are marquise and baguettes because of the shape,” says Teague. “Anything with points can be bad because they break easily due to uneven thermal transfer. The best shape for wax setting is round.”
Stone-in-place casting is possible with most silver, gold, and brass alloys. It’s not an option with platinum or palladium alloys, simply because of those metals’ higher melting temperatures. “There’s no way to protect stones from that heat,” says Stover.
(He notes that while he’s read accounts of jewelers who’ve successfully cast stones with platinum alloys, he believes that the knowledge needed to accomplish such a feat is beyond the scope of most jewelers.)
Menon notes that stone-in-place is possible with 9k and 10k low palladium white-gold alloys. However, he cautions that diamonds might not be able to handle the heat involved, though “CZs could survive.”
Aside from having a lower melting temperature, the alloy should flow well. “You want a low melting alloy with high fluidity and low shrinkage, flowing in and filling those tiny prongs,” says Menon.
Most metal suppliers will indicate which of their alloys are best suited for stone-in-place casting. If in doubt about an alloy, don’t hesitate to ask your supplier about it.
Teague is a firm believer that, ultimately, success or failure with stone-in-place casting comes down to engineering and how well the piece is designed from the start.
“You have to engineer the part to be wax set,” he says. And one of the first things that designers need to consider is how the stone will be supported during casting once the wax burns away. If the piece isn’t properly engineered, you risk failure.
For example, a stone could come loose during casting. “The metal comes in and washes it downstream and fills in the hole where the stone used to be,” says Teague. “It looks like the stone is still there and is covered with metal.”
According to Teague, success or failure with stone-in-place casting comes down to how well a piece is engineered for the process. Stones need to be properly supported so they don’t come loose during casting. They also shouldn’t touch when set in wax, as the metal will contract as it cools and pull the stones closer together, which could result in chipping or cracking.
The spacing of those stones is also important. For those casting a lot of stones next to each other, such as in a channel setting, Stover recommends leaving a small space between stones to prevent them from touching.
“You don’t want them to touch,” he says, “because when the molten metal shrinks as it solidifies, it compresses the stones.” That compression can then lead to cracking.
Menon agrees. The stones should be a “little bit loose” when they’re set in wax, especially for baguettes, he says, so the setting will “tighten with metal.” For those experiencing a problem with stones cracking because they’re being compressed during casting, he recommends leaving small gaps around the stones: “Increase the clearance to eliminate this problem.”
Finally, designers need to keep in mind how the stones will look once cast in the finished piece.
“One of the most important criteria to keep in mind is to have an open area below the stone,” advises Menon. “Light has to enter through the bottom to enhance the brilliance of the [stone].”
In addition to improving the appearance of the gemstone, leaving an open area behind the stone makes it possible to clean the metal in that area. “The metal can oxidize [and if you don’t leave an opening] you can’t get in there to clean,” says Menon. “It will make the gemstone look dull and dark, like it has lost its brilliance.”
Sprueing and gating are always important in casting. They’re even more important for successful stone-in-place casting, thanks to the lower temperatures that must be used to protect the stones during burnout. Extra gates should be included when casting with stones in place to help get the metal where it needs to go as quickly as possible.
Designers also must revise the way they sprue models for stone-in-place casting. That’s because gemstones (diamonds, in particular), especially when present in large quantities, can have a chilling effect, pulling heat away from the molten metal, which can lead to non-fills and defects such as shrinkage porosity
“Diamonds are a heat sink,” says Teague. “They suck heat out of metal really fast.”
To counteract that chilling effect, designers may need to add another sprue or thicken up existing sprues to improve metal flow to the areas closest to the stones.
Those casting with diamonds in place will need to use investment formulas specifically created for the stone-in-place casting process. Available from most industry investment suppliers, these formulas differ from standard jewelry investments: most include boric acid, which strengthens the mold and helps to protect the diamonds during burnout. Like other forms of carbon, diamonds can burn when exposed to sufficient heat and oxygen. The boric acid acts as a flux for the diamonds, creating a barrier that blocks oxygen from reaching them during burnout.
Though casters could add boric acid to a standard investment to make it better suited, Stover says they’re better off buying a stone-in-place–specific investment, since the boric acid will already be factored in at the correct amount.
“Buy an investment that’s for stone-in-place casting and no adjustments will be needed other than to use a lower burnout temperature,” he says. “The overall benefit here is [that] the protection has been thoroughly mixed within the investment and adjusted to specific parameters. Most suppliers also add a colorant, which tints the mix and invested molds a different color to avoid mixing non-stone-in-place molds with stone-in-place molds.
(He also offers a reminder that casters should “adhere to all the recommendations involving mixing proportions, temperature controls, and process water to avoid fluidity, working time, and strength fluctuations.”)
The protection offered by the boric acid is a double-edged sword, though. “It’s widely known that the additive...creates a harder mold,” says Stover. That may protect the stones, he says, but it also makes the investment “much harder to remove.
“Protecting the stones is the goal,” he continues, and changing the investment to make breakout easier “would risk that protection. It’s the necessary evil in the equation.”
That’s particularly true for anyone who decides to increase the investment’s protective power by adding more boric acid.
“There’s no need for an additional coating of boric acid if you’re using a stone-in-place casting investment, which has it included,” says Menon. While he admits that adding more boric acid to the investment would be added protection for the stones, it would be bad for the investment itself as it would become too hard and require a longer burnout cycle to remove all of the moisture within the investment.
“It’ll take longer to reach burnout temperature,” he says, “and it hardens the investment so bad you’ll have to hammer it out.”
Just as you need particular alloys and investment to maximize your chances of success when casting with stones in place, your burnout schedule will also need to be adjusted. Diamonds, in particular, require lower than normal burnout temperatures. To avoid damaging stones, Stover recommends following the burnout specifications of your stone-in-place casting investment to the letter.
“1,100°F to 1,200°F [593.3°C to 638.9°C] is the target temperature,” says Stover. “You might be able to go higher, but no one wants to be accused of burning out thousands of diamonds.”
Since normal burnout temperatures are usually around 1,300°F [704.4°C], there’s a risk that using these lower temperatures will result in an incomplete burnout.
“Proper airflow is needed and always requires longer dwell [hold] times at the upper portion of the cycle to obtain a good burnout,” he says. “This is important because leftover carbon residue may cause gas porosity issues and metal/mold reactions.”
Residual carbon left in the mold during an incomplete burnout can also result in diamonds becoming cloudy. While it might be possible to remove the cloudiness with repolishing, it won’t always work.
“If it’s just on the surface, it’s recoverable,” says Menon. “You can remove the diamond and repolish it,” although that will result in the diamond going down in size. (He notes that the lower quality diamonds tend to turn cloudy faster than better quality stones.)
In addition to a loss of brilliance and shine, diamonds can also burn and become discolored. And when the diamonds get overcooked, they’re really cooked—there’s no recovery for them.
Like other forms of carbon, diamonds can burn when exposed to sufficient heat and oxygen. To protect them during stone-in-place casting, use a specially formulated investment that contains boric acid: The boric acid creates a barrier that blocks oxygen from reaching the diamonds during burnout.
It might seem odd to worry about the effects of the burnout temperature on stones, given they’ll be exposed to much higher temperatures from the molten metal during casting. However, it’s all about the time of exposure, says Menon.
“Exposure of the gemstone to the metal temperature is less significant than in the burnout oven,” he explains. “Burnout is around 10 to 15 hours. Contact with molten metal is only a few seconds.”
To help minimize the risk of damage, Menon recommends limiting stone-in-place casting to small stones only, ideally nothing measuring more than 0.25 cm2 of a single exposed area. “The bigger the stone, the more chances of it being damaged because there’s more surface area to absorb the heat,” he says.
Also, despite the proliferation of 3-D printers, stone-in-place casting is still best when used in combination with wax models. This is because many of the printers use a photopolymer resin material that requires much higher burnout temperatures. If someone is intent on using 3-D–printed models, they should choose a material for the model that’s wax-based and thus has lower burnout temperatures.
“The wax content in the material has to be high so you can completely burn it out,” says Teague. “Almost all resins require such a high temperature to burn out [that] it would damage the diamonds.”
Stones may not only be damaged during the high heat of an extended burnout but also by quenching. Many stones suitable for stone-in-place casting are liable to shatter due to the drastic temperature change that occurs when the flask is quenched immediately after casting.
For most alloys, casters can set aside the flasks and allow them to slowly air cool, but there are alloys for which that extended cool-down period is not feasible.
“Some high-copper gold alloys, such as 18k red gold, will harden and become brittle as they sit in ambient air for more than five minutes,” Menon warns. “If you let it sit for half an hour, the metal will be brittle like glass.”
But if a casting can’t be quenched right away because the diamonds could shatter, and it can’t be cooled gradually or else the metal might shatter, what’s a caster to do?
According to Menon, some casters have found a workaround where they will take flasks and put them in a shallow pan of water within two minutes of removing them from the casting machine. The metal won’t overly harden as heat is slowly pulled away through the button, and because the flask isn’t being quenched, the diamonds won’t shatter or crack.
“Let it sit in the pan of cold water,” says Menon. “Once the flask is cool to the touch, you can take the tree out of the flask.”
Stover notes that some jewelers may experience a similar problem with casting nickel white golds because the metal is essentially annealing while it’s cooling. As a result, many people will “do smaller trees when working with high nickel alloys,” he says. “Anything they can do to lower the overall heat retention.”
When the time does finally arrive to break out the casting tree, Stover recommends jewelers “break flasks out over a separate container to catch any stones that might have come loose.”
In addition, he recommends keeping scrap metal from stone-in-place castings separate from the rest of your shop’s scraps. “It might become contaminated faster due to unburned residue in the flask because of the lower burnout temperature.”
As with any jewelry-making process, there are risks involved with stone-in-place casting. While it’s not always suitable for the job at hand, it’s a viable option for anyone casting designs that feature a lot of smaller stones. And by doing their homework, designing with the process in mind, and investing in the right materials for it, casters will be well on their way to achieving stone-in-place success.