Term denoting a material's property of not being homogeneous and not being dense, but containing pores filled with air or foreign matter. These pores can be open or closed. Porosity is specified in percent by volume and/or (in accordance with ASTM B 276) by frequency distribution. The groups

• A-porosity: pores smaller than 10 mm
• B-porosity: pores between 10 mm and 20 mm

C-porosity: inclusions of foreign matter are broken down in five frequency classes. Face materials with closed pores (e.g. aluminium oxide) are gas-tight, but those with open pores (e.g. carbon graphite, electrographite) must be given an impregnation to satisfy the requirement on gas-tightness.