Just to be on the safe side, I usually "burnish" mine by placing a piece of white paper (like copy paper) over the area I have chalked. Then I rub all over the area that I chalked (rubbing my fingers over the white paper). In doing so, the pressure sets the chalk and the white paper absorbs any residue that may come off.
More often than not, the white paper comes off clean, but there are times where it does have some chalk residue on it, so I continue to do it.
Hope this helps (and makes sense),
Carol