The effects of hydrogen content, local solidification time and strontium modification on microporosity were studied. Two critical fraction solids were associated with the formation of microporosity. The first, the critical fraction solid for pore growth, is the point at which preexisting baseline porosity begins to grow due to hydrogen evolution. It varied with hydrogen content and strontium modification. The second, the critical fraction solid for nucleation, is the point at which shrinkage type porosity nucleates. It did not vary with hydrogen content or strontium modification, and remained constant at approximately 75--80% solid.