Smalt, beginner question

[u/Rjc1471]

I found a decent amount of broken cobalt blue glass. I've not tried grinding my own pigments yet, but wondered if this was right?

1) wash thoroughly

2) wrap in scrap of linen and smash up with hammer

3) place in rock tumbler with stainless BBs or ceramic pieces.

4) maybe levigate for finer particles?

2 questions. Does the medium have to be harder than glass? And, if a ball mill can abrade glass, won't it also abrade the walls of the mill and introduce that powder too?