If there is ANY chemical water left or if there is any wax residue left in the plaster there is likely to be pitting, boiling, gas released from the mold and even a violent BURP or explosion like a geyser that will blow the metal right back out the top at worst or just bubble away at less worse. Both are real bad. If we had a chart with time on one axis and heat BTU's being absorbed on the other axis, the line across would be near straight. BTU HEAT absorption and TIME are again consistent but as the aluminum soaks up the heat it seems to stay more nearly hard and then get like a softer putty. Time passes, BTU's still are absorbed but the metal remains putty like for a longer time.
Minutes pass but all of a sudden checking shows it's liquid.
Do NOT overheat now as the melting curve has just taken a squared jump up the scale of viscosity. And as the metal sits in the mold it again cools for a longer time than seems normal before it suddenly starts to get putty like and then harden. It cools to a harder stage again in a reversed and squared chart line. Had you given it another 10 to 15 minutes of soaking up the heat it would have only further extended the period of time it would sit in the mold as a liquid. SO here's the rub.