Can anyone tell me how you know if your lead melt is contaminated?

I just picked up some lee casting gear and 12Kg of what i was told were lead ingots from flashing scrap.

First melt was fine and poured a couple of hundred pretty good rounds.
Today on the second attempt the bullets started coming out with a bright blue tint in places and hints of a blue crystalline substance in them. Also the melt developed a huge quantity of bright golden dross.

I was using a wooden stick to stir it so the charcoal from that should have served as flux.

I'm casting for muzzle load so i want the lead as pure as possible.

I'm hoping someone can explain what's going on?