On Monday I wrote about my paper cut.¹ Everyone I spoke to, who had read the entry, felt my pain. We’ve all done it.
Still, it was perplexing. How does this happen? It’s paper for crying out loud!
Tonight I got a well thought out answer from Thurston Chandler at Yale. I have incredible respect for Yale, for the work that goes on there, for their students and faculty. However, that all pales in comparison to having the name, “Thurston Chandler.” I grew up knowing no Thurstons and no Chandlers. This becomes a double treat!
to think the following makes sense. I am something of a knife collector and am pretty well versed in multiple sharpening techniques, so I do have a good idea of what the concept of ‘sharp’ actually means.
In the case of being cut by paper there are several factors as work which also play a role with other cutting tools ( or accidents ).
First is the concept of pressure, or force divided by the area over which it is applied. It’s no secret that a finishing nail is easier to drive than a railroad spike and that a sewing needle takes very little force to stick into things.
A cutting blade has an edge which is beveled to what might appear as an infinity thin edge. It’s pretty obvious that something so thin can part softer materials just like a splitting wedge works in wood. At the microscopic level no edge is actually acute, they can be thought of as having a (very thin ) flat (in truth rounded) side that actually does the cutting, so there isn’t anything all that odd about paper being ‘sharp’. However, the main body of the blade behind the edge has to be strong enough to hold the edge part in line as it bites into the substance ( or finger) being cut.
This is why it is much easier to get cut with writing paper than newspaper or magazine paper, even though those papers are thinner and thus the edge is ‘sharper’. Even stiff paper is only dangerous when it’s under the proper (unlucky) amount of tension to hold it stiffly enough that it stays perpendicular to the skin while sliding along exactly the same spot for a brief moment.
But there is another effect at work here that makes relatively thick paper a clerical work hazard. If you measure the thickness of that paper and then find a piece of thin steel with a very ‘dull’ safely rounded edge you will find it difficult to machine the sort of instant cut achieved with paper. Certainly the steel blade is strong enough that it is really far more dangerous if sufficient force is applied, but it simply isn’t going to bite into anything under trivial pressure the way paper or a properly sharpened lade can. The edge only has to be stronger than the substance you’re trying ( or not trying ) to cut, and the surface of our skin is soft and tender. The other requirement is that the actual cutting surfaces have to be harder than the substance being cut and here again we are softer than the wood that the paper is made of.
The real secret is what I’ll call ‘itty-bity serrations’. What we think of as serrations on a knife come in two types, smoothly scalloped (wavy) undulations of a cutting edge and the little interrupted square notches often found on the table knives we eat with. The wavy edges work well because they actually allow a longer cutting edge to be placed on a shorter blade ( imagine straightening out the curvy edge ) and they ensure that the cutting pressure is concentrated in smaller contact points along the blade than a straight edge would allow. This is why bread knives often look like this, you want to be able to cut under light pressure so as not to crush the bread.
But the other sort of serrations are quite different, as most table knives are virtually harmless and have no parts at all that are sharp in the conventional sense, yet they still can cut our food. These serrations function a little like dull saw teeth, the little square edges each catch on little bits of the meat you’re cutting so that the motion of the knife tears apart the food at the edge, making a cut, rather than sliding harmlessly back and forth on the surface.
With the paper, microscopically it actually has lots of little tiny wood fibers sticking out of the edge which look and feel soft and fuzzy when long. But when cut very short as a side effect of cutting out our letter size paper these fibers resemble sandpaper (which I could point out makes for agonizing paper cuts). And this abrasive very thin surface really can easily cut you with a combination of pressure concentrated along the edge and a sliding, cutting motion which draws this cutting edge with its multitudes of little wooden teeth over your unprotected skin.
Real knives are much the same, at the microscopic level (see image) the edge of a scarily sharp knife is quite rough and irregular.
Hopefully this helps answer your curiosity (:
See what I mean about Yale?
¹ – With a photo universally acclaimed as being vaguely pornographic – though it was a photo of the cut.