Here is a blackboard tip that eliminates yucky wet cloths and sponges while keeping blackboards looking like new. I take a clean small towel and put a little bit of lemon oil on it. I place the towel in a Ziplock bag and leave it overnight so that the lemon oil soaks into the towel. After I erase the blackboard, I clean it with the towel and it removes all the chalk dust without leaving any streaks. The blackboards look like new. I add a couple of drops of lemon oil from time to time but there should not be so much that it wets the board.