Slate top for a ceramic workshop.

 I am setting up a creamic workshop for my wife to do her own 
pottery.  I would like to build a wedging table and I have seen one 
in a professional studio that had a slate top.  It seems to be a real 
practical way to go. I need to know once I get the slate how to 
polish and seal it so the clay won't stick and it will be easy to 
clean.  Thank you for your help.  Is there a certain type of slate I 
should use in this project.