With the right information, it is possible to increase profits by casting stones in place. You can increase productivity by reducing stone-setting time at the bench, bringing your product to the customer faster.
Wax design. The first—and one of the most important—operations is properly setting the stones into the wax. Close tolerances are required here; it is crucial that there is enough wax to hold the stones in place. At the same time, the wax should be designed so that, when the wax is treed and invested, there is enough investment to securely hold the stones in place after the wax is melted away.
One example of a poor design for this type of casting is a closed-channel design with princess-cut stones set girdle-to-girdle. As the wax melts away, there is investment on top of the stones to hold them but none underneath to secure them. When a model like this is cast, some or all of the stones will likely fall out of place during the process. Stones can end up lodged in other areas of the design or somewhere in the shank.
Stones that are to be channel-set should be arranged with a small amount of space between them. Some jewelers slip a corner of paper or the tip of a scalpel blade between the stones; this provides enough space to allow for shrinkage and any slight shifting of the stones during the casting process but is typically not noticeable in the finished casting.
Preparing the wax. Clean and inspect the waxes before placing stones into them. The stones can be locked into the wax using a warm wax pen, but take care to leave no wax or wax residue on the stone before the item is cast. Any wax residue left on the surface of the stone will leave a void in the investment after burnout, causing metal to flow over the surface of the stone during casting. This type of defect can usually be easily cleaned off the surface of the stone with a sharp graver, but it costs valuable time and slows down production.
Investing the stone-in tree. Investing a tree with stones in place is slightly different from investing a tree without stones in place. For best results, start with a premium investment such as WestCast Caster’s Choice or Kerr Satin Cast 20. High-quality investment helps ensure better surface quality and helps reduce finishing time and effort. Many manufacturers add 2% (by weight) of boric acid to the investment powder during the investment process. The boric acid envelops the stones, reducing the effects of oxidation.
The investing process is a critical area of casting that is often overlooked. First, flasks and rubber bases should be cleaned to remove any residual investment. This will ensure even drying of investment and will help prevent investment inclusions in your castings.
Second, be aware of the temperature of the water and of the investment. The higher the water temperature, the faster your investment will set. In the winter, your water can be more than 20°F cooler than it is in the summer. If investment is stored in a cold area, the investment can be very cold as well, which slows down the investment setup. Longer setup times can lead to separation of water and investment, which can, in turn, cause fins or irregular surface qualities.
For the most consistent results, both the water and the investment powder should be between 70°F and 75°F. Adhering to this temperature range, you should see consistent results in both setup time and casting quality. Furthermore, it is good practice to time each step of the investment process, from adding the water to the investment to the hardening of the investment.
Third, pay close attention to the quality of your water. Water that worked well yesterday may today have more chlorine or other chemicals due to shifting environmental regulations. These differences can adversely affect the outcome of your investment. For consistent results, use a good filtration system or choose distilled or deionized water.
Fourth, accurately weigh your investment and carefully measure your water, following the manufacturer’s recommendations. Mix thoroughly, observing all recommended safety precautions.
All of these factors contribute to preventing defects and ensuring profitable results for your process.
Wax burnout. A different burnout schedule should be used for stone-in-place processes. When casting stones in place, the flask should not be heated to above 900°F (as opposed to the conventional type of casting process in which the flask is usually taken up to 1,350°F). A typical burnout schedule for casting stones in place might be:
The flask never really reaches a true burnout temperature in order to fully cure the investment. By holding the flask at the upper end of the temperature range for a longer period of time, you achieve a cleaner burnout and maximum strength of the investment.
Casting. Casting can be done in vacuum-assisted systems, in centrifugal motion systems, or in closed systems that use an inert gas to shield the metal from oxidation. Ideally, a closed casting system is best, but you can achieve very good results with the less sophisticated casting equipment as long as the casting schedule and other precautions are followed.
After casting, the flask should be left to air-cool slowly to room temperature. Once you can pick up the flask with your bare hands, you can remove the surrounding investment from the newly cast tree. Important! Never quench a flask containing stone-in-place castings. Quenching subjects the castings to an extreme temperature shift that can break the stones or cloud them with a frosty or smoky surface.
A water-blast cabinet is one of the most efficient methods for removing the investment from your tree. If you wish, you can tap on the sides of the flask to help break the investment free from the flask, but do not tap on the button. Any direct force on the button can directly affect the entire tree and may damage the stones.
Finishing. Once removed from the tree, the stone-in castings may be magnetic pin-finished or tumbled, depending on the types of stones you are handling and how fragile they are. Castings also may be hand-finished and textured as required for your design.
Take your time and be very familiar with the steps you plan to take as you move into stone-in-place casting. Good results take patience and attention to detail. Before you know it, you’ll be enjoying the convenience, cost-effectiveness, and profitability of successful stone-in-place casting in your business.