The term either describes the heat resisting properties of a material, or the heat resistant objects such as tuyères, crucibles, and furnace and hearth linings. A refractory material must have the following properties.
A high melting point
It is likely that much of the history of metallurgy has been controlled by the improvement of the properties of refractories. Although most copper alloy metallurgy can be carried out in the temperature range in which common clay-base refractories are adequate, the same is not true for the production of steel and cast iron.
Chemically inert with respect to the charge
At high temperatures the slag produced can attack some furnace linings extremely aggressively. This was particularly true in the 19th and 20th centuries with the development of the basic steel making. The normal silica refractories were attacked and destroyed rapidly by the basic (calcium-rich) slag needed to reduce the phosphorus content of the steel.
Mechanical strength and dimensional stability at high temperatures.
In the case of crucibles, the material must be able to support the weight of the molten metal and be able to be picked up without excess deformation at the pouring temperature. In the case of furnace linings they must be able to support the weight of the material above them without slumping.
Thermal shock resistance
Crucibles, in particular, have to with stand large thermal shocks when the crucible is removed from the furnace for pouring. Furnace linings have to withstand thermal stresses imposed by thermal gradients within the furnace lining.
Thermal conductivity
Ideally crucibles and muffles should have high conductivity, whereas, furnace linings should have low conductivity. However, conductivity of crucible refractory was not factor that would have been considered important until recently, the other factors being much more important. Similarly, for furnace linings the requirement for low thermal conductivity was not an overriding factor, although reducing the overall thermal losses through the furnace walls would have been important.
The difference between the thermal conductivity of modern refractories and those used in the past has not always been considered in some experimental reconstructions of early
smelting processes. The use of different refractory materials can have a major effect on the thermal losses and thus on the blowing and rate of fuel use required to run a smelt.
The earliest refractories were made from local clays. These may have been modified to improve their properties usually by the addition of quartz rich sand, but where available graphite was used. Eventually, some clays (high kaolin fire-clays) were discovered to make particularly good refractories. So that production became centralized in a few centres and the crucible exported over large distances as is shown by the import of large number of Hessian crucibles, manufactured in what is now Germany, into England during the 17th and 18th centuries. The introduction of the blast furnace required the use of refractories with high silica content to withstand the higher temperatures involved. These were either in the form of natural high purity sandstones with low iron content, or later silica bricks that contain more than 96% silica. To deal with basic slag or higher temperatures refractories based on magnesite MgO or dolomite (Ca,Mg)CO3 were used.