[BenjiWiebe]

You're supposed to only grind wet, since the glass dust is a hazard. Grinding wet keeps it from being airborne.

Everywhere that I've read about grinding mirrors (and I've done a lot of reading) always says to grind wet. Any YouTube video you watch on the subject will also show them grinding wet.

When grinding a mirror you put water and grit on, grind until the grit is used up, wipe it off, and repeat. This is called a "wet".