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

• A temperature gradient in the brick which is caused by sudden heating or cooling.

• Compression in a structure of refractory due to expansion
  
  
• Variation in coefficient of thermal expansion between the surface layer and the body of the brick
  
  
• Variation in coefficient of thermal expansion between the surface layer and the body of the brick is due to slag penetration or due to structural change.

On sudden heating
 

On sudden cooling
      

Permanent Linear change (PLC) on reheating
In materials certain permanent changes occur during heating and these changes may be due to

• Change in the allotropic form
• Chemical reaction
• Liquid phase formative 
• Sintering reactions

 

 

These changes determine the volume stability and expansion and shrinkage of the refractory at high temperatures.

Thermal conductivity
Thermal conductivity of the bricks determines heat losses. Increase in porosity decreases thermal conductivity but at the same time decreases strength also.

Bulk density:
Decrease in bulk density increases volume stability, heat capacity.