An alumina crucible is a type of crucible made from alumina, or aluminum oxide — the same material used to produce aluminum metal. Crucibles are used in chemistry laboratories as containers for extremely hot chemical compounds, and are made from materials with high temperature resistance. The ceramic form of alumina is commonly used because of its low cost, strength, and ability to withstand temperatures as high as 3272°F (1800°C). Alumina crucibles come in different shapes and varieties and have applications in laboratory research, academia, and industry.
Aluminum oxide is refined industrially from bauxite ore, which contains aluminum. After several purification processes, a substance called calcined alumina is obtained, which contains mostly alumina with some impurities, mainly sodium oxide. Calcined alumina is often used to make crucibles. High-purity alumina can also be manufactured using similar processes and is often preferred in industrial laboratories due to its greater resistance to extreme heat.
Alumina is a refractory material, meaning that it can withstand high temperatures without breaking down. The melting point of alumina is approximately 3760°F (about 2071°C). Since an alumina crucible is not made of pure alumina — although a high-purity container can approach 100% alumina content — it cannot withstand temperatures quite this high, but can usually be trusted above 3000°F (about 1649°C).
A wide variety of shapes and sizes of refractory containers exist for different uses. For example, the high-form alumina crucible is a common fixture in many academic laboratories. A high-form alumina crucible looks like a deep, narrow cup and is often used to melt or heat chemicals over a Bunsen burner — a lab appliance with a gas flame — or in an oven. Rectangular and cylindrical options are also sold commercially. All crucibles are used with a lid made from the same material, and are handled with tongs and gloves to prevent burning or other accidents.
The refractory containers used in academic or research laboratories are usually small, with a diameter less than the size of the human hand, reflecting the small amounts of chemicals they are required to hold. An industrial alumina crucible, which may be used for melting metal and creating alloys, can be much larger and must be installed in the furnace with sufficient space and exterior support for use. Industrial containers may also be selected based on the thickness of their sides, since thicker crucibles tend to be more durable and have a longer life.