The mold is a Collins aluminum.
It happened on every pour, but I only pured 3.
I didn't do any prep to the mold.
The lead is ingots I bought off Ebay.
I'm using a Lee pot, the smaller bottom pour model set and about #8 on the dial.
Sagacious explained it perfectly (about the detail). I will try all the great suggestions & report how it turns out.
Thanks to everyone.
Brad