My final ball milling jar could have passed for a commercial version. However, the project turned out to be a bust.
I disagree with Harry Davis who, in the Potters alternative, says that he sees no advantage of well matured porcelain over well matured stoneware.
I made my jar out of cone 6's stoneware and fired it to cone 7, just to make sure it was well matured. With some balls made out of the same material I ran it for about 10 hours to grind off any roughness. After rinsing it out thoroughly, I did a one hour test milling with just water and balls. My one gallon jar produced 4.81 g of dried, powdered stoneware. This is about 10 times more than would be acceptable.
Maybe remaking it with porcelain would have been better but I think that stuff is still not as hard as the porcelain they use to manufacture these jars.
I just bought a 5 L porcelain milling jar with balls.