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When you have made some stupid mistakes on a blackboard, you quickly want to erase it with a wet sponge before anyone sees them. So you clean the blackboard and within a minute the blackboard is clean and dry again!

I was wondering why the board is drying so quickly compared to other surfaces. Does it absorb the water or is it all due to "good" evaporation?

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  • [Already said] A blackboard is not porous, i.e. it actually never takes up much water from the sponge in the first place (and if you were to squeeze out more than a little, it would just run down to the bottom).
  • [Already said] Yet the surface is hydrophilic, i.e. the water that does stay on the board forms a very thin film instead of droplets (as you'd get on a plastic or freshly wiped glass surface), and together with the slightly rough texture this makes for a large surface area to only a very small volume of water. This surface is where evaporation takes place; the larger, the better.
  • The board is mounted vertically. That's the ideal configuration for convection: water vapour has a lower density than air, so close to the surface (which, because of the second point, quickly evaporates a lot of water into the air directly next to it) the air rises up, and because the entire surface is aligned in the same direction and air can efficiently stream along the surface from below (turbulence helps further), there's a steady supply of unsaturated air into which more water can evaporate unhindered.
  • [Already said] The bulk of the board is usually metallic, i.e. it has good thermal conductivity. To the touch (which emits heat into the board), one perceices it therefore as cold, but to the evaporating water (which requires heat) the same property has a warming effect. That keeps the evaporation speed high, both directly and through preventing the reduced temperature from mitigating the convection-causing density reduction.
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    $\begingroup$ Plus the fact that it's black and not shiny makes it (theoretically) absorb more heat from the light that falls on it, causing it to get warmer, which accelerates the evaporation. (Not sure how big an effect this is, but it won't hurt.) $\endgroup$ – Mr Lister Apr 9 '17 at 8:10
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    $\begingroup$ @MrLister no, it won't hurt (though the black colour also optimises heat radiation away from the board). But I'm pretty sure radiation is actually more or less completely insignificant, unless the board is directly exposed to sunlight. $\endgroup$ – leftaroundabout Apr 9 '17 at 12:01
  • $\begingroup$ @leftaroundabout But most blackboards are in well-lit rooms, right? Schools tend do have big windows and expose classrooms to sunlight pretty much. $\endgroup$ – Mołot Apr 10 '17 at 9:01
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    $\begingroup$ @Mołot yeah, but the point is that the rapid cooling is also observed when the board is not directly sunlit, so this can't be an important contribution. $\endgroup$ – leftaroundabout Apr 10 '17 at 10:16
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Although flat the blackboard has a texture so that when a damp cloth is rubbed across it the water adheres to the board and isolated droplets are not formed.
So you have a fairly uniform thin film of water across the board which is ideal for evaporation.

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I think the blackboards absorb very little if any water and, being smooth and vertical, there is just not that much water there after they've been wiped.

Also, the blackboards I'm most familiar with are made of painted steel. Probably their good thermal conductivity helps keep them at room temperature and not cool much as the water evaporates.

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    $\begingroup$ A blackboard isn't smooth at all. That is why the chalk transfers to the board. The added surface area for the surface probably helps. A saturated blackboard can indeed take a long time to evaporate. $\endgroup$ – Captain Giraffe Apr 8 '17 at 19:36
  • $\begingroup$ Im not sure about the physics involved, but you can buy "blackboard paint" in a spray can that behaves as described ,as far as I can see, on all surfaces. whereas any other kind of black spray paint doe not. $\endgroup$ – Philip Roe Apr 9 '17 at 15:44

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