Practically speaking lead sheeting is probably about as pure as you will find.

I think the old rule of thumb for muzzle loader projectiles is if you can easily put a significant scratch in the lead with your thumb nail, it is pure enough for your ML projectiles.

Here's a pic of 50cal balls I recovered from Whitetail I shot with a 30% wheel weight/lead mix.

Oddly, yesterday I was sifting through .62 caliber round balls in preparation for the early muzzleloader season here in PA, and look what rolled out of the pile. A 335-grain round ball with an air pocket clearly visible after the sprue was cut. While it’s probably true that an air pocket like this doesn’t have much influence over accuracy within fifty yards, it surely does not help accuracy. And when taking a muzzleloader poke at 100-125 yards, it for sure is hurting your chances of even making “minute-of-meat.”
Keeping lead hot (750-790 degrees F), stirred, fluxed, keeping the mold lubricated, and pouring from a ladle right from the pot will greatly diminish the possibility of getting air pockets. That said, this round ball was poured in a process that I believed met all those criteria. And then when I began sorting balls yesterday, I noticed a progression from heavily wrinkled balls to slightly wrinkled to perfect surface. Although I had thought all the clearly wrinkled / imperfect round balls had been put back into the pot during that pouring session, clearly a bunch got past me. It happens when I’m pushed for time.

That's for sure interesting and I have seen that same thing myself. Like said earlier post I pour mostly large dive weight's and sinker.. See holes like that by sprue more when running hotter than should be. The lead shrinks back into mold as mass cools.
I notice wrinkles in thing more when the mold isn't warm up enough. Then the crystal look as everything gets to hot. Dive weight's, sinker, jigs. That just not as important as something a guys trying to get consitant flight with. I guess need take little time take some pictures. Give everyone idea what kinda set up I'm using. I have a 54 cal muzzle loader both ball and conical mold for that. I have only ever shot sabots never used my own cast stuff yet. Really need to do that now too.

Mac

Mac, the bigger the lead object being poured into a mold, the greater the possibility of problems. Lead cools fast. As molten lead is poured into any mold, it begins to cool rapidly. The longer the pour, the greater the cooling and the greater the difference between what is already in the mold and what is being added. So big jigs, sinkers etc for sure. We frequently see air pockets right at the surface. Round balls for muzzleloaders can be better controlled. Some of the variation is due to the individual mold. Every mold I have responds differently to the same casting conditions. Some new molds in perfect condition nonetheless develop flashing quickly. Unless I want to fool with the hinge pin etc, which I normally don’t, I just keep cleaning the mold face and focusing on a tight alignment of the two halves. I also hunt with a .54 flintlock, and casting those balls is a much more consistently successful effort than for the .62. Anyhow, good chatting with you. Good luck and keep asking questions. Lot of really good people here, happy to help each other.

I have the same Lee production bottom flow pot. I let my son use it to make sinkers, he used whatever lead. He had left some in the pot when done. I caught on when I was about to load some self cast ingots in. Used welding gloves to tip the whole pot to drain unknown lead mix from the bottom, before casting my muzzy balls. I knew what I was melting but not what my son was melting.

I have some plumbers lead I use for muzzle loading. Been shooting wheel weight lead for shot gun balls. Have not shot any animals with them. I have killed deer with wheel weight 357 bullets. Both just dropped. I THINK those wheel weight balls will pass through a deer broadside at a range I would shoot. I think it will work just fine. Shot this 2X10 at about 20 yards.





The little hole is from a 22 at about 10 feet. Gave my grandaughter a 22 cricket. I wanted to make sure she knew it wasn’t a toy