Strength 

It is the resistance of the refractory to compressive loads, tension and shear stresses.

In taller furnaces, the refractory has to support a heavy load; hence strength under the combined effect of temperature and load, i.e. refractoriness under load is important.

Specific gravity    

Specific gravity of the refractory is important to consider the weight of a brick. Cost of bricks of higher specific gravity is more that of lower specific gravity. But strength of bricks of higher specific gravity is greater than one with lower specific gravity.

Spalling   

Spalling relates to fracture of refractory brick which may occur due to the  

• Temperature gradient caused by sudden heating or cooling
• Compression in a structure of refractoriness due to expansion
• Variation in coefficient of thermal expansion between the surface layer and the body of the brick due to slag penetration or to a structural change.