First - use a good marble mold, makes life so much easier
Second - Cold/hot seals are actually a range but basically the colder the punty is, the lighter the bond. I usually warm my marble and get my punty very hot, then wait a sec and attach. The waiting allows me to see the color and wait for the perfect moment to attach for the seal type I want.
Example - warm marble with white-hot punty will take a piece of marble with it (partial hot seal)
2nd example - glowing hot marble with cooler punty will leave a piece of punty on the marble (partial hot seal)
3rd example - warm marble and cooler punty (clean break due to cold seal)
Third - Crack off your old punty while the new one you attached is still soft and mobile. This prevents the new one from breaking off from the shock.
Last - Yes, your puntys are welding to the marble if you get the punty side too hot which makes them difficult to remove. Only heat one side of the marble at a time then switch punty and work another side. Some people do as many as 5-6 switches like that to get it perfectly round.
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