A graphite crucible is a container typically used for melting non-ferrous — or non-iron — metals such as gold, silver, aluminum or brass. Graphite, a form of carbon, is often used for crucibles because of its high temperature resistance and low reactivity with molten metals. A graphite crucible typically is made of graphite and clay that is shaped and fired to very high temperatures to form a solid container.
The purpose of a graphite crucible is to provide a non-reactive vessel that will survive the high temperatures needed for metal melting and processing. Crucibles provide a stable container that does not react with the metals or materials being burned at high temperatures. Graphite is carbon that might resemble flakes or sheets. When mixed with clay, graphite can be shaped into vessels or containers that are heat-resistant and durable. Like many ceramic containers, graphite crucibles can be brittle and should be handled with care.
There are different types of crucibles that vary primarily in their shape. Common shapes include “bilge,” which are barrel-shaped; and “A,” which are cup-shaped. “A” crucibles might be less expensive because the open cup shape is easier to manufacture than a barrel shape.
Using a graphite crucible correctly includes conditioning and storage. It is important for a new graphite crucible to be conditioned before it is used. The crucible should be placed in an oven at 500 degrees Fahrenheit (260 degrees Celsius) for two hours and then allowed to cool slowly in a dry area. This drives moisture from the crucible and prevents cracking. Crucibles should be stored away from moisture and should be re-conditioned if they get wet.
A graphite crucible should be cleaned completely when it is done being used. Metal should not be allowed to remain in the crucible to cool. Residual metal can expand and crack the crucible when it is reheated. Crucible tongs should be used that match the shape of the crucible. To prevent cracking, the tongs should hold the crucible without putting pressure on the sides.
Graphite crucibles can be damaged by improper use or handling. They should not be used for metals containing iron, because iron will react directly with the carbon in the crucible and will change the metal composition or destroy the crucible. Solid metals should be loosely packed in a crucible before it is heated. The metals might expand, and a tightly packed crucible can crack or fail. Water or damp metals should never be added to molten metal in a crucible, because that might cause a steam explosion or result in failure of the crucible.