Aluminum Oxide

Also referred to as alumina or in its crystalline form as corundum, aluminum oxide occurs naturally in abundance and is used in many industries. It is very hard making it suitable for use as an abrasive and its high melting point makes it an excellent refractory material. It does not conduct electricity, but its most common use is in the production of aluminum metal.

Rubies and Sapphires

Aluminum oxide takes its colors from impurities in the crystals. Gem quality crystals are more often known as rubies and sapphires. The red color of rubies is created by traces of chromium in the crystals, while the blue color in sapphires is caused by various impurities including iron and titanium. Chromium also accounts for the laser properties of rubies. The hardness of corundum makes it ideal for use in cutting tools.

Alumina Prevents Corrosion

Aluminum metal is very reactive to oxygen and a very thin layer of alumina forms on the metal within 10 seconds of exposure to oxygen. This coating of aluminum oxide actually prevents oxidation and is responsible for the metal's weather resistant properties. For certain applications aluminum undergoes a process called anodizing which actually enhances the thickness of the alumina coating and makes it harder.

Manufacturing Aluminum

Prior to the late 1800s, aluminum was used in jewelry and was valued at the same rate as platinum. Although it is the most common metal on earth, no simple way to refine it had been discovered. In 1883 a college student, Charles Martin Hall, used alumina powder melted with cryolite and subjected to an electric current for two hours to produce aluminum metal. The inexpensive process allowed the metal to be commercially manufactured for the first time.

Pure Aluminum Is Not Found In Nature

Although abundant, aluminum is not found in nature in its pure state. It combines with other elements to form compounds like aluminum oxide. Since 8.2% of the Earth's crust is composed of aluminum, aluminum oxide is one of the most common natural compounds. While pure aluminum is a metal, corundum is considered a ceramic and has few of the properties found in pure aluminum.

Additional Commercial Uses

Aluminum oxide is used with activated charcoal filters to purify water and remove fluoride, calcium and sodium. It is mixed with clay to create commercial ceramic materials for electric insulators and even ceramic dinnerware. Tiny pure crystals of alumina are compressed tightly together to make medical grade ceramics. The ceramics are used to make joint replacements because the only substance harder than aluminum oxide is diamonds and joint replacements must be durable.

It is fortunate that aluminum oxide is abundant since it has so many uses in the modern world. Inexpensive aluminum metal is used to contain and to cook food. Most people eat from ceramic plates made with alumina. The hardness of corundum makes ideal for many uses and its insulating properties are ideal for coating the aluminum and steel wires that carry electricity to homes and businesses.