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Pencils create marks by physical abrasion, leaving behind a trail of solid core material that adheres to a sheet of paper or other surface. Wikipedia

Normal paper is mainly composed of cellulose fibers, which were "funded" by the plants. And this "solid core material" is supposedly composed of graphite and a type of binder clay. Paper Image source

"Adhering" is usually caused by intermolecular forces. (e.g.: when sugar adheres to wet surfaces because of Hydrogen bonding). The questions are:

  • Is the case of a pencil mark some kind of intermolecular force too?
  • If so, what is it?
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The graphite itself can adhere to paper fibers with London forces; it's not a strong adhesion because the marks can be erased easily. Some of the graphite is just mechanically stuck between the paper fibers; rubbing a mark from a soft pencil can easily blur it. Enough graphite is deposited that a line drawn by a pencil can conduct electricity.

The clay gives the lead structural integrity; an "H" pencil has a harder lead, and deposits less graphite; a "B" pencil is softer, and an "F" pencil can be sharpened to a finer point.

In my time as an art major, I made a lot of sketches using vine charcoal on newsprint. Vine charcoal has no binding clay, and the marks were easily smeared. On paper without much "bite", it was easy to lift the marks off completely with a piece of kneaded rubber, and if you wanted to keep a charcoal drawing (or a soft pencil drawing) you really had to spray it with a fixative.

So I'd say that it's possible that the binding clay also plays some role in binding the graphite or charcoal to the paper.

In the manufacture of graphite pencils, air-classified grades of Volclay sodium bentonite and micronized hectorite are regularly used to bind the graphite compound in pencil lead. --- American Colloid Company, Industrial and Household Applications

Bentonite and hectorite both have ionic surfaces that probably help bind it to OH groups on the surface of cellulose fibers.

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  • $\begingroup$ So the clay (or any other additives) doesn't have any effects on strengthening the van der waals forces? $\endgroup$ – It's Over Jan 21 '15 at 16:14
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    $\begingroup$ Without knowing anything about the structure of the clay it's hard to say. The fact that it's harder to erase marks made with a high-clay "H" pencil than with a low-clay "B" pencil suggests that it's possible that it affects the way the graphite binds to the paper. The clay probably has charged sites on its surface that can play some role in binding it to the paper, but I'm speculating. $\endgroup$ – Fred Senese Jan 21 '15 at 17:08

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