A refractory is a material that retains its strength at high temperatures. ASTM C71 defines refractories as "non-metallic materials having those chemical and physical properties that made them applicable for structures, or as components of systems, that are exposed to environments above 1000 °F (538 °C)".
The oxides of aluminium (alumina), silicon (silica) and magnesium (magnesia) are the most important materials used in the manufacturing of refractories. Another oxide usually found in refractories is the oxide of calcium (lime). Fireclays are also widely used in the manufacture of refractories.
Refractories must be chosen according to the conditions they will face. Some applications require special refractory materials. Zirconia is used when the material must withstand extremely high temperatures. Silicon carbide and carbon are two other refractory materials used in some very severe temperature conditions, but they cannot be used in contact with oxygen, as they will oxidize and burn.
Refractory materials are used extensively in the metal industries, along with glass melting and other heat treatment operations.
There are two common forms of refractories, bricks and monolithics. Bricks (also known as firebrick) are pre-sintered forms which can hold their shape. Monolithics are loose material which can be formed into complex shapes, or sprayed into place, and have to be sintered before use. Castable refractory cement is also commonly used.
Anti-abrasion refractory attached to pipes, chambers, etc, will require anchorage systems such as wire formed anchors or hexmetal to support the refractory linings.
Usually, refractories require special heat-up techniques to insure that their performance will be attained as designed, and to avoid thermal shock and drying stresses until the operational status is achieved.