Don’t pour molten metal into a cold mold.
Maggie writes, “OMG! Does this only happen if there is some trapped moisture in the ingot?”
No, it’s the same problem that glassworkers have. The thermal expansivity of aluminum is high. The cold outside parts shrink faster than the hot inside bits, squeezing it. The now brittle cold outside then pops like a metal balloon overfilled with hot liquid metal. Notice how it sounded like a balloon?
Maggie : “That is fascinating. Thanks! I shall not be trying this experiment at home.”
Yes. Please do not.