I've had this blackboard, made with real slate, since 1975. I had it on 20th Street in NYC; I moved with it to Third Avenue, and one time while I was away my friend Mickey Lee Donaldson wrote me a message on it. That was thirty years ago, and I haven't erased it. One time, here in Baltimore, when it was leaning against the wall, one of my dogs rubbed against it with his tail, but either I was lucky, or the chalk marks were so old that they hardly smeared at all.
Can chalk marks, meant to be erased over and over and over, get old and permanent? Can shadows become permanent? Can reflections? Write in the dust, and slowly the marks fill up with new dust. Write in the dirt, and quickly wind or rain will wash your words away.