Finally got a ball mill, Tips? Page 2 Tools and Tooling
That's the only time you need to worry about sparks with milling. You say you have lead media. There's hard lead media and soft lead spheres. Hardened lead wears very slowly when milling complete BP. I wouldn't worry about potassium nitrate being gray from lead contamination, myself. That's assuming you are using it for BPtype formulas.