From Skylighters web site;
"Ceramic is hard, does not wear out quickly, but some types of ceramic do spark. Once again, many pyros have used it for all their mill runs, but that does not mean that the next time couldn't present a problem. Some people have said that if they bang pieces of their ceramic media together in the dark, they can see little sparks being generated. Others say that these are just little luminous 'light' sparks and they don't present a danger in milling.
Lead or brass is reliably the only really spark-free media and is considered to be the standard for milling comps like BP. But lead is softer than the other media and will wear down over time. If one mills a white powder like potassium nitrate with lead media, the powder will come out of the mill gray. That means there is some lead in the chemical, which will be in the smoke that results from it being burned. That presents a danger of lead poisoning if one breathes a lot of that smoke. But lead media has probably been and will continue to be used more than any other in amateur pyrotechnic milling.
Nothing is perfect. Everything presents some risk as well as benefit. We just have to weigh the plusses against the minuses and 'lay our money down.' Nothing in fireworks making is completely safe. But we can make wise decisions and then take steps to minimize the inherent risks. You're going to see in a minute that those of us who mill complete compositions assume that 'this time' the mill could explode. Even though it has never done so during the last 200 runs, we plan as if it will explode, and take the necessary precautions."
There will always be speculation about ceramic milling media, there is always the possibility that the cascade heap will fall too fast and a single particle of BP could get pinched just right causing the jar to burn. IT CAN HAPPEN. Lead is heavy, does a decent job but consider this, when was the last time you wore a respirator when emptying your jar? Not a mask, a respirator? Got black boogers after your were done? Where did that now powdered and oxidized lead go?
Without getting all of us in a he said-she said argument about milling media, there is absolutely no safe milling media, period. Keep in mind these words: "we plan as if it will explode, and take the necessary precautions.""