Platinum Casting

From design to final finish, the art of casting platinum requires patience, preparation, and many modifications.

Ed Friedman still remembers the first time he cast platinum. And it still makes him wince. “I used a special platinum investment, and I was completely unfamiliar with its working characteristics,” says Friedman, a senior instructor at the Revere Academy in San Francisco. I Invested one flask, and after it dried I noticed a few hairline cracks In the surface. They didn’t look serious, and I decided to go ahead and cast. Talk about a disaster: The molten platinum blew right through the flask” burned a hole in the quarter-inch-thick aluminum flask plate, and splashed against the outer well If not for the well, it would have hit me.”

As Friedman later discovered, he had improperly dried the platinum investment, which had a phosphoric-acid binder requiring longer setup time. The weakened investment couldn’t withstand the high temperatures required for platinum casting. The result: an ounce of lost platinum and one damaged centrifuge.

This “exercise in frustration,” as he puts it, inspired Friedman to learn all he could about platinum. Ultimately, he decided, it’s not so much the difficulty of the metal but the “lack of knowledge about platinum that creates most of the problems beginners face.”

This view of platinum as being different rather than just difficult prevailed, without exception, among all the people interviewed for this article. And with good reason: Platinum requires modifications in almost every area of casting. Most of these changes revolve around the fact that platinum melts at a much higher temperature than gold and cools, or solidifies, rapidly. Also, platinum can be easily contaminated by various materials, resulting most commonly in brittleness (which becomes apparent when prongs snap off during setting!).

So, in an attempt to make getting started easier, this article will look at what it takes to cast platinum. It will address every area, from design to final polish, and-with the help of several experts-explain what modifications are necessary to ensure success.

THE DESIGN

Designer Michael Bondanza, principal of Michael Bondanza Inc. in New York City, has become especially known for his 18k and platinum pieces. He explains that designing for platinum requires special considerations that aren’t necessary when designing for gold production.

“Generally, whatever design you can make in platinum you can make in gold, but not the other way around,” he says. “There are lots of parameters to consider, and these become part of the design.

“The first such parameter is size limitation. If I’m designing something that I intend to cast in platinum, I restrict it to a size of about 1 inch by 1/2 inch or smaller for general production. I keep the thickness of those pieces to around 1 mm, although sometimes I may go up to 1 1/2 or 2 mm. If I’m making a ring, the shank sometimes goes up to 2 mm thick, but then I will cast the ring in several pieces. The majority of my platinum pieces are constructed of [at least] two pieces and sometimes as many as 10.”

Bondanza goes on to explain several other important considerations. “Wall thickness is vital,” he says. “Platinum is difficult to cast when the design has variations in wall thickness. For example, if you go from 1 mm up to 3 mm and then back down to 1 mm, you’re asking for porosity problems. I limit wall thickness to 1 mm, and I measure it very accurately. If I make a ring or a bracelet, I’ll cut the item in half and detail it very carefully, making it as thin as possible. Weight is also a consideration. Platinum is very heavy and adds a lot of weight very fast.”

SPRUEING

Because platinum starts to cool immediately after being removed from heat, it has to enter the flask fast. For that same reason, sprues should be kept short; otherwise, the platinum could freeze in the sprue before it completely fills the mold cavity.

“Sprues [for platinum] are generally shorter and heavier than for gold, and often simpler,” Bondanza says. “I used to [use] many thin sprues [with complicated branching], but now I find I use fewer, thicker sprues and it works much better. I make sure all the joints are really smooth, so the platinum flows easily. “Generally, if I use a #8 sprue on gold, I use a #6 sprue on platinum. It depends on the pieces you’re sprueing, of course: Thicker pieces require thicker sprues.”

Above left: For casting plat- inum , a “double broken” cen- trifugal arm-in which both weight and crucible ends are hinged-can reduce turbu- lence. Right: When designing his platinum and 18k jewelry, Michael Bondanza tries to limit the wall thickness to 1 mm.

Platinum also must flow through the tree quickly. For this reason, even the largest platinum casting machines limit the flask size-which in turn limits the tree size-to about 3.5 inches in diameter and 4 inches high.

INVESTING

There are basically two types of platinum investment to choose from: Those that include a phosphoric-acid binder, and those that don’t. The phosphoric-acid type generally gives a smoother surface finish, but it takes a long time to set up. The non-acid investment sets up quickly, but the surface finish can be poor and the reject rate much higher.

Several phosphoric-acid investments are available, most of them modeled after the popular Japanese investment being marketed in the United States as Supra. This investment imparts a smooth surface finish, which minimizes metal waste during polishing and also reduces finishing time. According to Friedman, “Supra Investment gives the best surface without a doubt, plus it sets up hard and well. But it does take a long time.”

Because of the long setup time, many casters use Supra for their regular casting and keep a non-acid type of investment, such as R&R Astrovest or Kerr Platinite, for rush jobs. These investments set up within an hour or two, but they generally produce castings with rougher surfaces that will take longer to polish. With Supra and most other phosphoricacid investments, the acid is shipped in a separate bottle along with the investment powder. The acid must be mixed with water in a specific ratio to make a “binder solution,” which is then mixed with the powder to form a slurry. Because it has the consistency of wet cement, the slurry is neatly impossible to mix uniformly by hand. An electric Kitchen Aid-type mixer can do the job, but its bowl shouldn’t be completely filled (othetwise the motor will burn out).

You should mix just enough investment for one or two flasks at a time. (Of course, production casting houses can use larger mixers for greater production.) Mixing can take up to 10 minutes, after which the bowl is vacuumed as usual. The slurry is poured into the flasks and the flasks vacuumed. After the flasks dry (usually 16 to 24 hours or longer), they can be burned out.

To ensure a phosphoric-acid binder dries propetly, you should not mount waxes or trees onto a rubber sprue base. Instead, melt the bottom edge of a Wax Sprue Cone (available from suppliers specializing in platinum casting equipment) and attach it to a base of absorbent paper. Attach the waxes or tree to the cone, then position the flask over the tree and, with melted wax, attach it to the absorbent paper as well. Put the whole onto a small board and invest. After investing, slide the flask off the board onto a 2-inch-thick layer of newspapers. The absorbent paper base will transfer liquid from the investment to the newspaper for faster drying.

DENTAL INVESTMENTS

For casting just one or two items at a time, dental investments are becoming increasingly popular. John Henkel, president ofJ.A. Henkel Co. Inc. in Brunswick, Maine, swears by dental investments for good surface finish and fast setup. “My casting flask is the size of a film can,” he says. “I use Deguvest F, a dental investment that sets up in about 15 minutes. It [finishes] its exothermic reaction in about 45 to 60 minutes. I put it into a 500°F kiln for 1 hour to dump the wax and remove water, then crank [the kiln] up to 1,600°F-and I’m casting 1 1/2 hours later! To remove the dental investment, I let the flasks air cool, then use a high pressure water jet to remove about 90 percent of the investment. I don’t want hydrofluoric acid in my shop, and dental investment can be removed …with a dental probe.”

SAFETY PRECAUTIONS

Before you move to the burnout phase, make sure you have all safety precautions in place for platinum casting. Properly rated eye protection is a must: Without it, you can sustain permanent eye damage when welding or melting platinum. Goggles with a #6 lens should be worn when welding. A #9 or even #lllens is needed for melting, although both lenses are so dark they can obscure the work area until the melted metal is bright enough for illumination. For this reason, many casters wear a #6 lens but never stare directly into the metal when it has melted completely.

In addition, the high oven temperatures for platinum burnout require a long pair ofheat -resistant gloves. The gloves’ sleeves should go up to and even over the elbow (unless you like the hairless arm look!).

BURNOUT & FLASK TEMPERATURES

Generally, your top temperature for burnout will be about 1,600°F to 1,700°F (your investment manufacturer can usually provide details for each particular powder). You can use any kind of oven for burnout. But for high production, sooner or later you will want to invest in a gas oven built especially for platinum castirig. Robert Lane, a manufacturer of gas ovens, explains how his platinum burnout ovens differ from standard models: “Standard ovens for gold are built with a double burner, one on each side of the oven, but for platinum we add a third burnel in the middle. Heat flow is more even, and the oven easily reaches the high temperatures required for platinum investments.

“The platinum ovens also have extra insulation to hold the heat inside without getting hot on the outside. For safety reasons, we use a Honeywell electronic ignition system, so there is no pilot light or gas flow until the temperature controller is activated. But most important, whether you cast gold or platinum, you will get a better burnout with a gas oven than with an electric oven. When wax is burned out it turns to carbon. During burnout, you need high temperatures and plenty of oxygen to get rid of the carbon so you have no inclusions, contamination, or investment reactions when you cast. Heat cycles on and off in both gas and electric ovens, but in a gas oven, when the burner goes off, oxygen is pulled in from the room air through the burner vents.”

Following burnout, many platinum casters these days lower the flask temperature for casting. “I normally cast thin pieces at 1,000°F to 1, 100°F and thicker pieces at about 900°F, and I’m getring really good fills,” says Friedman. “I have found that the hotrer you cast, the more surface problems you tend to have …The hot metal hits the investment and burns in, producing a rough surface. A lower flask temperature allows the platinum to solidify quickly.”

CASTING ALLOYS

Like everything else in this industry, people have their own personal preferences about alloys and can argue convincingly to support them. So what’s a beginner to do? Experiment with several different types, maybe several different metal suppliers, to find the one that works best.

The most popular platinum alloy in the United States has been 90 percent platinum and 10 percent iridium, although 95 percent platinum, 5 percent cobalt has also made a strong showing. Some people say that the platinum-cobalt alloy looks slightly bluish, but just as many people would disagree. Still others would tell you that the platinum-cobalt alloy is slightly magnetic due to the cobalt. But this poses a problem only in that it can attract metal filings, which, if not removed prior to welding or heating, will contaminate the platinum and result in brittleness.

According to James Mangold, plant manager at Aurafin Kurgan Inc. in Bethel, Connecticut, this magnetism is nothing to worry about. “The platinum-cobalt alloy is not so magnetic that someone wearing a ring will have metal utensils clinging to it,” he says. “The magnetism is very slight, and with standard platinum housekeeping techniques like thorough cleaning and pickling before welding, there won’t be a problem with contamination.” Mangold says his company has had great success casting with platinum-cobalt. On the other hand, Edward Friedman prefers the platinum-iridium alloy “because it can be easily fused if there are any pits. It also seems to be the most forgiving of all the alloys, and color matching is easier.” A new alloy called Plat/SK, named after its inventor, designer and metallurgist Steven Kretchmer, is being marketed by Hoover and Strong in Richmond, Virginia. This alloy is over 95 percent pure platinum, with the remaining ingredients “proprietary.” Cretchmer says he specially formulated the alloy to cast and polish easier, and just generally be easier to work with than any other platinum alloy on the market. Initial reports about Plat/SK look promising. Time will tell.

CASTING MACHINES

Platinum is traditionally cast by centrifuge. The standard vertical centrifugal unit has heavy duty springs in the base to throw the platinum into the flask quickly. Using this machine can be a little awkward and unnerving, since casters must hold the torch and release the spring-Ioaded handle at the same time. (A second pair of hands makes the job much easier.)

Friedman teaches his students at the Revere Academy that, if production and speed are not factors, they can use a standard horizontal centrifuge normally used for gold or silver. He reports satisfactOry results for up to three small pieces (about 2 ounces of platinum) in one flask.

However, Henkel believes the vertical machine is safer to use. “If molten platinum leaves the crucible, you don’t want it flying horizontally at you,” he says. “Molten platinum is over 3,000°F, and if it hits you, the damage will be much more serious than [with] gold or silver. A vertical machine will direct the metal up or down-not into your face.”

In either case, the platinum must first be torch melted. John Henkel says his biggest surprise when he first started casting platinum was the amount of heat the metal took. “1 ruined a few eatly pieces because of insufficient torch heat and flask heat,” he says. “Now I use a hydrogen torch with about 40lbs. oxygen. It’s very clean, very hot.” In fact, the amount of heat and the thrust of the hydrogen torch can actually push the metal out of the crucible. This also holds true for a propane torch: Because of the high oxygen pressure, you’ll need to control the angle of the torch to keep the metal in the crucible.

HIGH PRODUCTION CASTING MACHINES

Torch melting does not lend itself to high production platinum casting. For this, you’ll want an induction melting/ casting machine. These machines range from about $11,000 to over $50,000, depending on the features included. The lower end models usually don’t have an optical pyrometer. Fortunately, platinum is relatively easy to cast by eye. When the metal color and the crucible color are the same (i.e., when you cannot tell where the crucible ends and the metal begins), it’s about time to cast. But if you’re looking for predictable and repeatable results, you need an optical pyrometer.

Another desirable feature is a “double broken arm” where the centrifugal arm is hinged on both the weight end and the crucible end. It has been well documented that this arm design reduces turbulence by directing the metal into the center of the sprue hole.

Adjustable speed for both the acceleration of the arm and its final rpm also offers advantages. It’s a natural assumption to think you should always set the arm speed to the fastest setting. But, surprisingly, this is not always true: High speeds can actually contribute to surface roughness, flashing, and crazing, depending on the size and the design of the piece. You’ll need to do some experiments to determine the optimal speed for casting your parts. As the demand for platinum casting machines has increased, so has the development of new units that include all of the above features at much lower prices. Check with your supplier for a comparison chart of their machines.

CRUCIBLES

Platinum is normally cast in fused quartz or silica crucibles, which are more expensive than graphite and do not last as long. However, unlike graphite, the melting point of quartz or silica is very close to that of platinum.

After the first cast, the crucible surface will look shiny, as if it were glazed with I flux-an indication that the surface has melted slightly. This is a concern not only for wear, but also for contamination: Silica from an overused crucible can be absorbed into the platinum. For this reason, some casters use a new crucible for every cast.

You can make these crucibles last longe. if you cast as soon as the metal is ready. You can also paint the inside of the crucible with zirconium oxide paint, which forms a barrier between the crucible and the platinum. Although it does not prevent the crucible from wearing out, it does significantly reduce the likelihood of contamina- tion if you use your crucible for more than one melt.

INVESTMENT REMOVAL

Platinum investment without phosphoric acid behaves just like the standard gold investment. However, phosphoric-acid platinum investment hardens like cement after burnout. You can remove a good amount of it by quenching the flask in water after casting. You can remove even more ftom the tree by using a water blast cabinet with a high pressure pump. But there will always be some investment left in the settings and undercuts.

To fully remove platinum investment, the tree must be soaked in hydrofluoric acid (available ftom most chemical supply houses). Because this acid emits fumes even when cold, good ventilation is a must. Also, you should always obtain a Material Safety Data Sheet ftom the supplier. (There is a new platinum investment remover ftom Keystone Industries [800-333-3131] called “Super Strip It”; it contains hydrofluoric acid but comes with its own neutralizer, so you can treat it yourself and dispose of it fairly easily.)

Mix the acid in a 1:1 ratio with water. Always add the acid to the water to prevent acid splashes, and pour it very slowly. A tremendous amount of heat is generated when adding hydrofluoric acid to water, so proceed cautiously and wear gloves, an apron, and full face protection (including eyewear). Also, use a plastic container, since the acid will react with glass or metal. To hasten cleaning, you can place the container in an ultrasonic cleaner full of water-but don’t turn on the heat! Also, never put hydrofluoric acid directly into your ultrasonic cleaner.

Once the investment has been re moved, you can clip off the sprues as you normally would (although you’ll really appreciate a pneumatic sprue cutter over a hand cutter) and proceed to polishing. You can use standard tools for cutting and sawing platinum, but you can expect the blades to wear out a lot faster because platinum is so hard.

POLISHING

The standard compounds used for gold and silver don’t perform properly on platinum. The best compounds on the market today are those developed in Japan specifically for platinum. They are more expensive than domestic compounds, but they make finishing much easier and faster.

Use a Beartex wheel or a similar fiber wheel to remove rough surfaces. Follow this with Gray 800 Compound on a stitched treated buff to remove the Beartex marks. Next, use White 1500 Compound, which performs the same function as Tripoli on gold. Follow with either White 8000 Compound, for a bright white polish, or Carrot Compound, for a bright white finish with a subtle luster. (These four Japanese compounds are available in a kit form for trial purposes.)

Other available compounds include White 4000 and White 6000. Some manufacturers use these two grits as intermediate steps, and some end the polishing process with them. Again, it’s a matter of personal preference.

WELDING

To fill holes in your platinum castings, use a wire drawn from the same alloy to ensure a perfect color match. Insert the wire into the hole and melt it with a torch until the hole is filled. For high production, laser welders provide neat, invisible seams and make quick work of any welding job.

If you use a torch, the best gas for platinum is either hydrogen or propane. You can also use natural gas, but avoid acetylene; acetylene contains a lot of carbon, which can easily contaminate the platinum.

If, because of design constraints, you must use platinum solder, use 1700. (Platinum solders are named for their melting points in Centigrade degrees.) Keep in mind that solders with lower melting temperatures will be faintly visible as gray lines, so reserve those only for solder seams that will not show.

ADDITIONAL INFORMATION

To learn more about platinum casting, try the following resources: – Platinum Guild International USA, 949-760-8279 – The Revere Academy, 415-391-4179 – The Orchid Web site, http://www.ganoksin.com

Also, if you plan to attend the 1999 MJSA JCK Expo New York (March 28-30), be sure to register for the fifth annual Platinum Day Symposium, which will take place March 27. (For more information, call MJSA at 800-444-6572 or 401- 274-3840.) Finally, check with your supplier for specifics about your casting equipment and its use. The more you know, the more success you’ll have. As Edward Friedman says, it’s the “lack of knowledge about platinum that creates most of the problems beginners face.”

Elaine Corwin is vice president of technical services at Gesswein in Bridgeport, Connecticut. She can be reached at 800-544-2043, ext. 287, or via e-mail at ecorwin@gesswein.com.

AJM, August 1998
www.gesswein.com
Elaine Corwin

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