Blowhole. An indentation or pore on a cast metal surface caused by a bubble of air or gas escaping during solidification. The effect of several such minor cavities creates the typical porous surface of casting by certain methods, particularly any casting that is not vacuum degassed. Uneven porosity, too many cavities or an excessively large blowhole would be cause for rejection during inspection. A blowhole is to a cast surface what a pinhole is to a struck surface. If the cavities are excessive or unsightly the piece should be rejected.