Welcome to the casting forum !!!!!!!sit back & enjoy & join in ifin ya can or want . Been castin long ????
Sounds as if some lube of some type was leachin out & cookin off , do you put anything on ya molds for storage ???
Alloy temp hi enuff???
The last Lee mold I purchased it took 2 sessions to "cook" out the machinin lube & that was after boilin it for 15min.