Old Slate Chalkboard

Old Slate Chalkboard
How do you restore the surface of an old, 4’ X3’ slate chalkboard? It used to be in a one room school house for almost 100 years and then in a storage barn for the past 25 years. It appears to be in good shape, except for dirt and debris on the surface.
I’m thinking of vacuuming off the dirt and dust, but what then? Hot soapy water, use a fine grit paper to sand it, or wipe it off with a damp cloth? I’d prefer not to use any harsh chemicals.
Once it is clean, what would be the best finish to still use the slate as a chalkboard in our game room?
We’re going to mount it on the wall in portrait orientation, so that young and old can reach it for drawing, keeping game scores, and whatever comes to mind.