The lost-wax casting process is easy to picture if you think of it as replacing wax with precious metal.
Building a wax sprue tree
The lost-wax casting process is easy to picture if you think of it as replacing wax with precious metal. To begin, you must build a wax sprue tree—a series of wax models adhered to a central wax cylinder called the main feed sprue. Each wax model must be attached to a sprue (small metal cylinder) that will be attached to the main feed sprue. (Sometimes pieces require multiple sprues to ensure fill.) This entire wax sprue network will eventually be replaced by metal.
Removing air bubbles
Once the wax sprue tree is complete, it is adhered to a rubber base and placed inside an open-ended stainless steel flask (cylinder). At this point in the process, the caster is ready to make a mold around the wax tree. To do so, he begins by preparing a batch of casting investment, a mixture of a special plaster and water that resembles a creamy white slurry. This is a precise process that must be completed in a short timeframe. The investment is vacuumed to remove air bubbles, poured into the flask, and vacuumed again to ensure that all air is removed. (Trapped air can lead to metal castings with rough surfaces.) After sitting for about two hours at room temperature, the flasks are ready for the burnout process.
The burnout cycle
This is the part of the casting process where the wax is “lost.” Removing the wax from the flask requires subjecting it to heat. The prepared flasks are loaded into a gas-fired oven in which three essential steps occur: the investment is dried and cured; the wax is melted and burned out of the investment mold, leaving a cavity that will later be filled with molten metal; and the mold is heated to the correct temperature necessary to accept the molten metal for casting. Timing and temperature are key to achieving a clean burnout. The burnout cycle generally takes about 16 hours and requires oven temperatures as high as 1,350°F/730°C.
At the end of the burnout cycle, the flasks are ready to be filled with molten metal. All Cascade Star jewelry is made with 18k gold. All 18k gold alloys contain 75 percent pure gold; the remaining 25 percent comprises a number of elements used to enhance the metal color and provide good working properties. Some consumers are surprised to learn that white gold is not naturally white. The alloying elements added to the gold, which may include palladium, nickel, zinc, and silver, act as bleachers to change the color of the yellow gold to the bright, lustrous white that you see in our finished jewelry pieces.
The resulting cast tree
For casting, the 18k white gold casting grain (which is the shape of small granules) is placed into a crucible, which is a small ceramic or graphite container shaped like a bowl, and heated to 2,012°F/1,100°C, at which point it becomes molten and is ready to pour into the flask. The casting grain can be heated with a gas-powered torch or electricity. The latter method requires the use of either a resistance melter, which uses a high-temperature wire that is coiled around the outside of the crucible and heats it through radiation, or an induction melter, which uses radio frequency energy to heat the metal in the crucible.
Just as there are different methods for melting metal, there are different methods for casting. The two methods for casting gold are centrifugal casting and vacuum casting. In centrifugal casting, the caster sets the flask into a drum that contains an articulated arm, which spins around the machine at high speeds. He places the necessary amount of casting grain into the crucible located next to the mold and torch-heats it until it is molten. When the metal is heated, the arm is released to spin, forcing the metal into the mold with centrifugal force. Because this method is so powerful, it enables casting of detailed jewelry pieces, which have many small parts that must be filled with metal.
In vacuum casting, the metal is melted by torch or through induction (depending on the equipment being used) and is driven into the investment mold by gravity and pressure differential. Vacuum is applied to remove trapped air and gas from under the metal as it enters the mold cavity, resulting in better fill and high quality castings.
Although the process of melting metal, filling the flask, and waiting for it to solidify may sound complicated, it takes minutes—sometimes less—to complete. The result: A cast tree that looks exactly like the wax sprue tree at the beginning of the process, only now it’s made of gold.