Crossout- respectfully- I don't think you are looking at this the right way. Even if you 'prove' to yourself that high alumina media is safe, what then? Will you ballmill black powder in your shed or basement, because it is safe? I'm just asking. I ballmilled BP in my shed, 2X750 gram loads. I thought I was being pretty safe. But when I made a 100 gram maroon with the mill dust, my attitude changed! I then realized that if my mill went BOOM for any reason at all, it would have destroyed the shed.
All black powder milling should be done with the assumption that the mill will explode. An unexpected mill explosion could ruin your life or kill somebody. Think of it this way: black powder plants and fireworks factories explode fairly often. These are people that know more than we do (at least more than I do)and they still have explosions. That's why there is only one black powder factory left in the whole US.
Lead is known to be non-sparking, and it is not hard. Would it be safe to mill with lead with no barricade? I certainly wouldn't do it. A bit of steel in the charcoal, rocks in the saltpetre or sulfur, dirt, there are just too many things to consider to say that ballmilling black powder is safe. Sure, it's safe. Right up until the second that it isn't.
I have been working on alternative methods for making black powder. It's certainly not everybody's cup of tea, but I use very small 304 SS media to mill my charcoal to a degree of fineness that is impossible with any other media. Yes this is a bold statement. I have milled charcoal and black powder with ceramic (high alumina), brass, lead, and stainless steel.
My tests have shown me that I can make screen-mixed black powder that rivals most milled powder. Also, in apples to apples comparisons, my home powder beat Goex. When I say 'apples to apples', I mean that my powder was pressed into pucks of a density of +1.8g/cc. It was then corned and graded to size. When used to launch baseballs, 10 grams of my powder (+-.01 gram) gave a flight time of 11 seconds. 10 grams of Goex gave 10 seconds. My 2FA sized grains tied with Goex 1Fg. 1Fg is the coarsest I can get up here.
I'm in the early stages of my testing at this point. I've already tested my powder in end burners, nozzleless rockets, and now with baseballs. Next will be with meal-coated rice hulls. I'm working on reducing the 'Yeah buts'. I also plan to use the same media in a modified Lortone mill to see how close I get to the more than acceptable performance I already enjoy.
Just another way to skin the cat. Don't flame me, BP millers! Almost all of you will never have a mill explosion