Casting ingots and milling it out into usable metal stock can be a difficult and frustrating process. There are many variables.
To clarify, you’re not using an ingot mold, you’re melting scrap metal open faced onto a charcoal block. That path works, but it tends to make bumpy ingots that can be harder to hammer and mill out with a rolling mill.
That said I have a friend who makes what he calls bread loaf ingots. He melts a small blob of scrap silver or gold on a charcoal block and while it is still red hot and malleable, he lightly squishes it with tweezers to make a bread loaf shaped piece of metal that he can roll out in his rolling mill. I’ve watched him do it and he makes it look really easy. I’ve never seen any one else do this. I’ve tried his technique with marginal success, but I’m sure if I kept on practicing I’d get it. I always use two part steel ingot molds for making ingots to mill out. That’s what works best for me.
Here’s a recent thread with a slightly similar question that might have some helpful info for you.
Best of luck and keep asking questions!!
Jeff