This the third entry in a four-part series on the process of custom designing a piece of jewelery.
Now that the wax model or CAD representation has been accepted by the customer, it is time for casting.
Casting is a process by which a liquid material is poured into a mold (or a flask) that contains a hollow cavity of a desired shape, and then is allowed to solidify. Casting is a 6000 year old process. The oldest surviving casting is a copper frog from 3200 BC. For more information see Wikipedia.
The most common method of casting is by lost wax. First a channel called a sprue is attached to the wax model. It is also made of wax. On more complicated pieces, multiple sprues of varying diameters are used. Next the model is place upside-down inside a flask and a heat resistant plaster is poured all around the piece — encasing every minute detail. A vacuum is used to make sure all the bubbles are removed. After the plaster sets, it is put into a kiln and heated in a controlled manner to melt out the wax. Thus the term lost wax casting: a perfect negative of the piece is made and the wax destroyed.
After proper heating in a kiln to remove the wax, the flask is ready to be filled with a precious metal. After the metal cools down and hardens, the flask is submerged in water to loosen the plaster and out comes the piece, sprues and all.
In addition to lost wax, there are several other casting methods. Here are a couple:
Vacuum casting is used with silver and gold alloys only. A vacuum is created under a flask and molten metal is poured into the opening. The vacuum helps the metal flow into all the spaces.
Centrifical casting is used with most metals including platinum. This can be quite exciting to watch because a special machine is used to shoot the molten metal into a flask at very high velocities to ensure a complete casting.
The final step is to cut off the sprues and finish the piece. Finishing is the subject for the fourth and final part of this series.