Learning Lost Foam

Finally rehabbed enough to trust my new knee to a pour. The technique of lost foam is still a work in progress probably because I keep pushing the boundaries of my technique. As you can see there are holes in the walls - on the inner left side at the top and middle and the other right side. This piece is a 7" cube, hollowed out, and the walls are roughly 3/8" thick. Typically voids occur when the thermal mass of the foam overcomes the aluminum and it freezes before vaporizing the foam.

The pour was made from the top to the four corners so I would expect voids at the bottom. Maybe the voids near the top are due to the colder temperature of aluminum at the end of the pour. Maybe - but why the voids in the middle? Fortunately it works artistically.