Why is it that when you erase a chalk board, the area where the chalk used to be becomes the cleanest? By that I mean that when you erase a chalk drawing, the board gets smeared with chalk dust, but the area where the drawing used to be has less dust on it than the rest of the board.

For example: In the first picture below I draw a simple chalk smiley face. Here the face is noticeable because it is the area with the most chalk. For the second picture, I erase it. You can still make out the picture, but notice that you recognize it because it is now the area with the least chalk.

I would expect that if chalk was stuck to a certain region of a chalk board, then after erasing it, some chalk residue would remain, but instead it seems like the opposite happens. I don't have a good answer for this problem.

After Before

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    $\begingroup$ I wonder if grease of any sort is used as a binding agent in blackboard chalk? I'd guess that blackboard chalk is not simply a cylinder cut from the natural rock. Indeed I've just recalled that blackboard chalk is barium carbonate (so the dust particles are relatively heavy and drop quickly to the ground rather than pervading the air in the room). I don't think that barium carbonate occurs naturally as a rock. $\endgroup$ – Philip Wood Jun 28 at 22:14
  • $\begingroup$ From what I could find, most chalk today is made from calcium carbonate or calcium sulfate. Like you said Philip, they seem to be forming it from some mixture rather than cutting it directly out of a rock. I can't find a definitive answer for what they mix it with, but it seems like there are probably trace amounts of clay in the finished product. $\endgroup$ – Shep Bryan Jun 28 at 22:53
  • $\begingroup$ Then it has not been cleaned properly - clean means no swipe marks or dust trails... $\endgroup$ – user207455 Jun 29 at 6:00
  • $\begingroup$ I'm not sure there actually is less chalk dust where the smiley face was drawn. I am thinking if this could be an illusion caused by the strokes/stripes that are left behind. Just as we see stripes on the clean part of the board, because the dry sponge does not do a proper job when wiping, we might except similar stripes where the smiley face was. Possibly, the smiley face is just as dirty as the rest of the board, but dirty in a "different direction", which makes it look darker or lighter and thus seem cleaner. $\endgroup$ – Steeven Oct 17 at 10:39
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    $\begingroup$ Strange that it does not work on a green board. That might indicate that it has something to do with the chalk/board $\endgroup$ – Shep Bryan Oct 23 at 17:52

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