I don't know about zinc, I've tried to get the stuff to mix and pour but can't do it. It pops up to the top of the pot looking like gray cottage cheese. I always set my PID controller to 800 degrees but rarely if ever achieve it but do run in the 780's. Being cheap I'd like to get the zinc to mix in to get more bullets for plinking purposes - too hard to do what I want for hunting. I just can't believe what we are seeing here is zinc. It looks like the metal was cooling too fast in layers before the cavities could fill out and that was probably a cold mold issue I'm betting. Some people are using hot plates for pre-heating molds and will tell you that dipping a mold in molten metal could warp it. They may be right but I started casting as a broke college kid so cheap was the way I operated and dipping the mold in the pot is free whereas a hot plate costs $'s. I DO like to set the mold on top of the pot for maybe a couple minutes some times before dipping it into the metal so it's not so drastic a temp shock.
Heck I had it full blast as hot as it would go. Lol
Ah, yeah I forgot about those combo conical/round ball molds since I'm not a traditional ML shooter. The only combo molds I have make a gas check/non-gas check version of the same bullet.