|MadSci Network: Physics|
Ink chemically bonds to the surface of the paper. This chemical bond isn't easily broken, that's why it's difficult to erase ink without destroying the surface of the paper.
When paper is made, a "sizing" is put on the surface. The sizing is a chemical treatment of paper which gives it resistance to the penetration of liquids (particularly water) or vapors. (See http://www.paperonweb.com/dict11.htm.). The sizing is used to control how ink flows and attaches to the paper.
Graphite (the "lead" in a pencil) attaches via a mechanical bond to the paper surface. That is why a mechanical action, such as rubbing with an eraser, removes pencil marks.
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