I had forgotten about the tin muffin pans!
I had one do that to me once as well. I'd forgotten about it. Good tip.
As for pot material, I'd stay away from aluminum sauce pans.
I stole an old one from the kitchen and after about a year, the bottom began to sag bad enough that it would not sit flat on the coleman camp stove I'd been using. I doubt it would have ever broke open or melted, but it did deform badly. I went to a steel pan and shortly after went with an electric melting pot.
My ingot making pot has always been a large cast iron pot with a lid. I thought about using the stainless pot that came with the turkey fryer burner but it was so hand to user for.... well, frying turkeys... who'd have thought?