Traditionally metal crowns have been made using the investment casting, or lost-wax, technique. This means sending an impression (taken in a rubber mold) of the tooth to be replaced, as well as the teeth next to it and the ones opposing it, off to a laboratory. The laboratory makes a model of the tooth and its adjacent teeth out of special type of plaster called dental stone or die stone.
The tooth to be crowned and its adjacent teeth are separated into two or more dies, as the models are called. These dies look just like the original teeth sitting in the jaw, just made out of stone. Next, a technician covers the die with paint to create a die spacer -- room between the crown and the existing tooth to allow for cementing the crown. Then, the technician uses wax to build up the die so that it the shape is correct and the fit works with the dies of the adjacent and opposing teeth. This buildup of wax is called a wax pattern and functions as a mold for the crown.
The wax pattern is attached to a stick known as a sprue, which ultimately allows the finished material to flow into the mold. The wax pattern and sprue together are dipped in plaster. This is the investment. After being attached to an investment ring and placed in a furnace, the wax melts. Melted metal, such as gold alloy, is injected into the ring as it spins in a centrifuge. A hollow metal shell of gold forms inside the plaster. After immersion in cold water, the plaster layer shatters, the sprue is removed, and the crown is polished. If the crown is a porcelain-fused-to-metal (PFM), a porcelain veneer is either milled (ground down from a larger piece of porcelain) or pressed and baked, then heat-fused to the metal crown.
Milling or pressing and baking have been the only methods used to create ceramic or porcelain crowns until recently. CAD/CAM (computer-aided design and computer-aided manufacturing) dentistry has made it possible to get one of these types of crowns much sooner. Instead of taking an impression, a dentist takes a picture of the prepared tooth to be crowned and in-office software creates a virtual crown in three dimensions. Then a computer uses this data to mill the crown, after which it is heat-treated for strength and colored to match the existing teeth. The whole process takes an hour or two, eliminating the need for getting a temporary crown and waiting weeks for the permanent one.
Even if you're getting a crown via the CAD/CAM method, it can still take some time. In the next section, learn about preparing for a crown.