Casting With Gemstones

Despite what you may think, several companies around the world are setting gemstones in waxes and casting them successfully.

The economic benefit of this process is readily evident. If you can set stones in wax and can cast with the stones in place, the stone-setting costs become more competitive with products from countries with lower wage rates.

Techniques for casting with gemstones (CWG) differ somewhat from the usual casting doctrines, so in-house experimentation is required. But the payoff is worth the effort.

This article deals with tested, proven techniques that have evolved over the past few years. Most manufacturers who do casting have the equipment required to properly perform CWG. However, model makers and casting and wax departments will be required to work together and share information for the process to be effective.

The goal in CWG is to produce quality pieces with undamaged stones cast securely in place. The techniques for doing this may seem controversial. However, they are being used with excellent results. And in the final analysis, success is what really counts.

Model Preparation
The CWG process begins with properly preparing the model. If the item is prong or bead-set (pave), the model is made so that the prongs or beads of the setting are fractionally smaller than the stone that is to be set.

The model maker must calculate the shrinkage factor that occurs when making a mold and then a wax. Depending on the type of rubber being used, there may be a range of between 0.5% to 10% shrinkage from model to wax. Since variations exist, some experimentation will have to be done to determine the shrinkage factor that prevails in your plant.

Most importantly, the setting must be prepared with pre-notched prongs to enable the stones to be seated properly. Your final aim is to arrive at a wax setting that is somewhat smaller than the stone itself.

Molds and waxes are made in the conventional manner, with care being taken so the wax settings remain undistorted.

Setting the Stones

After the waxes are made, you are ready to set the stones. Where necessary, select identically calibrated stones so they will fit the wax settings properly. If stones are too small, they will fall into the settings and not be held at all. If stones are too large, they won’t fit into the settings, or will cause the prongs to spread too much.

To set the stone, place the stone onto the prongs and gently push down so it spreads the wax prongs and slips into the pre-notched seat of the wax. The wax is flexible enough to open and accept the stone, and has enough physical “memory” to close around the stone and hold it securely.

It is wise to inspect the stone-set waxes to be certain stones are set straight and that enough prong covers the stone. A fine-pointed tool may be heated slightly and used to make adjustments to the wax if necessary. This should be done carefully and delicately.

Channel Setting

Channel setting in metal requires a great deal of skill and experience. However, when doing channel setting in wax, the task is rather easy. The model is prepared with the channel accurately arranged so the stones will slip into the prepared channel seat and allow for the “memory” factor of the wax to spring back and securely hug the stones.

The model should have generous seating for the stones, but the stones should not be loose in the channel. Therefore, upon inspecting the wax with the stones in place, additions of wax to the seating can be made with a fine-tipped, slightly heated tool.

After some practice, the average direct-labor cost for setting a stone in wax will be about 10 cents to 15 cents per stone, depending on whether it is prong, bead or channel set. This compares with a range of 50 cents for prong-set stones to $3 for channel set stones when setting a stone in the conventional way.

Treeing the Waxes
Waxes set with stones are best mounted on a tree. Results have not been satisfactory when they were mounted directly on bases.

The temperatures of the flask and metal used for casting gems in wax will be greatly different from the norm. Therefore, trees that fit into a six-inch or shorter flask are recommended. This allows for the molten metal to reach pieces more rapidly, with fewer chances of metal solidifying as it fills to the top of the tree.

All of the usual good practices for setting up trees should be used.

Investing is done in the normal way, using the water-to-powder suggested by the manufacturer.

Great care should be taken when mixing and vacuuming in order to eliminate trapped air and to obtain a smooth investment.

In the CWG process, the major concern is not to burn the stones that are invested in the flasks. Therefore, the burnout procedure veers sharply from the norm. Conventional burnout cycles go through various stages of elevated temperature changes to reach and dwell at 1350° F to 1400° F for several hours. However, these temperatures would be devastating to the stones with which you are casting.

So, after investing, allow the investment to harden for at least 1½ hours. Remove the rubber bases and place them into a steam de-waxer. In addition to environmental concerns, there are two reasons for doing this: Less wax remaining in the flask while burning out in the oven will create a minimal amount of carbon; and reduced carbon in the flasks allows for the lower burnout temperatures that must be used to protect, the stones.

After the flasks have been thoroughly de-waxed in the steam cabinet, and while they are still warm, they are placed in an oven preheated to 275°F. The subsequent burnout cycle is as follows:
300°F for two hours
500°F for two hours
800°F for 10 hours

It is of utmost importance not to exceed 800°F, since that could damage the stones. Therefore, temperature controls and gauges must be checked and calibrated for accuracy.

Similarly, casting will be done with flask temperatures no higher than 800°F.

Melting and casting equipment differs widely from one plant to another. In addition, the wide range of alloys that are used governs variances in the melting and pouring temperatures that are used. Therefore, hard-and-fast rules for the pouring temperatures of metal cannot be given here. The only concrete rule is never to exceed a flask temperature of 850°F.

Experiment with alloys that permit the metals to flow more easily at lower temperatures. Your supplier of gold or alloys can assist you.

All principles for quality casting must be followed. Cleanliness of the metal must be maintained, and at least 50% fresh metal should be used. Since 14k gold usually flows better than 10k, it is somewhat easier to work with.

Casting can be done by either centrifugal methods or with a vacuum-assist system. Better results generally are achieved with the vacuum-assist method, where a crucible and an immersed pyrometer are positioned in the crucible above the flask to allow for accurate metal temperature readings and controls.

In conventional casting, you have the latitude to raise the temperature of the flasks up and down by several hundred degrees. That’s not the case with CWG. You may lower the flask temperature from 800°F if you wish, but it is far too risky to go above 850°F. To solve these problems, accurate metal-temperature controls must be used.

In conducting initial experiments without stones, you will find the lowest metal temperature you can cast with and still get a complete fill. For sustained success, accurate metal temperatures must be recorded, and be able to be duplicated.
To solve the problem of non-fill, several techniques may be employed. Since flask and metal temperatures have to be kept at their lowest points, an increased tree thickness will aid in getting metal to the thinner jewelry items without losing too much heat too quickly.

Additional assistance can come from increasing the thickness of the sprue to the jewelry piece.

Vacuum-assist equipment must be able to achieve a high vacuum in a short period of time and be capable of evacuating a large amount of air per minute. This will help the metal fill all the cavities in the flask without having to super-heat the metal.

When casting is finished, allow the flask to cool in air without quenching. Flasks must be cool to the touch of your bare hand before removing the investment. Quenching the flasks while they’re still hot will shatter or cloud the stones.

Finishing items with stones cast in place may take a little more care than usual. Before subjecting the stone-cast pieces to finishing, the jewelry items should be inspected to assure that stones are set properly and held securely, and will not pop out while polishing or washing.

If needed, stones can be tightened in their metal settings by conventional means. When polishing media such as tripoli and rouge. Avoid using abrasive materials that may damage the set stones. Care should be taken not to polish off the prongs that are holding the stones in place.

Some companies using CWG say they’ve been successful at casting with baguette diamonds, princess-cut stones, and even invisible settings. It also has been observed that better-quality diamonds are less prone to damage than stones with inclusions.

Stones to Set
Stones that are suitable for setting in wax are diamond, sapphire, ruby, and garnet. Cubic zirconia also can be used in CWG, as can some other laboratory-grown colored stones.

Emerald, opal, topaz, tourmaline, turquioise, pearl, lapis, peridot, jade, coral, aquamarine and amethyst cannot be used. They will crack, burn up or change color from the heat. In any case, experiment with small, less costly stones of a given type.

To make CWG work and to expand its possibilities, experiments must be conducted in your own plant. For example, through some masterful techniques, a few companies have developed proprietary methods of placing stones in molds and injecting wax into the molds. This creates a wax with stones set in place, and eliminates having to hand-set stones into the wax.

This article has presented an overview of casting with gemstones. This process deviates from the usual ways casting and stone-setting are done, and some of the recommendations may be controversial. However, the methods described here have been proven to work well.

Jack Weinraub is the former owner of a large casting and stamping company, and also was vice-president of operations for MichaelAnthony Jewelers. For the last 10 years he has been a technology and management consultant, and recently served on MJSA’s “Ask the Expert” panel at the Expo New York show. He can be contacted at: 5665 Aspen Ridge Circle, Delray Beach, FL, 33484; (407) 496-1955. | 800-759-9997 | (fax) 800-616-9997