Mineralogy
"Hardness" is determined by comparison with other minerals. In the Mohs scale, a standard set of minerals are numbered in order of increasing hardness from 1 (talc) to 10 (diamond). A harder mineral will scratch a softer, so an unknown mineral can be placed in this scale by which minerals it scratches and which scratch it. A few minerals such as calcite and kyanite have a hardness that depends significantly on direction. Hardness can also be measured on an absolute scale using a sclerometer; compared to the absolute scale, the Mohs scale is nonlinear.
Hot hardness
In materials engineering and metallurgy, hot hardness or red hardness (when a metal glows a dull red from the heat) corresponds to hardness of a material at high temperatures. As the temperature of material increases, hardness decreases and at some point a drastic change in hardness occurs. The hardness at this point is termed the "hot" or "red" hardness of that material. Such changes can be seen in materials such as heat treated alloys.