I got my Xtool S1 recently and this is one of the first successful engravement on slate I have done so far. The rock slate wasn't sprayed with any chemical. but the next engravement will be just so the difference can be palpable.
Clean the slate with dry cloth. make sure not to touch the surface with your hand.
For settings, see image above. wash with tap water and air dry or dry with paper towel.