The measure of an material's ability to store heat is known as its specific heat.
There are several ways to express specific heat.
The international standard (SI) measures specific heat as the amount of heat -- measured in joules -- required to raise one kilogram of the material by one degree celsius.
More commonly, it is expressed in calories as the amount of energy required to raise one gram of material by one degree celsius. When this unit of measure is used, specific heat is similar to specific gravity in that the measure is a direct comparison of the material with water (because 1 calorie is the specific heat of water).
Understanding specific heat is useful when evaluating refractories in kiln and furnace design. The specific heat of any material can vary based on the starting temperature. When comparing materials, make sure the specific heat of each one was measured at the same temperature.