How do knives get dull?

This thread reminded me of an old question. How do knives get dull?

I mean, obviously if you cut on a dinner plate instead of a cutting board, the china will dull the metal edge. But if a sharp knife is made of metal, and only touches vegetables which are considerably softer, how can it possibly be dulled by the process? Applies also to scissors with fabric and paper.

If I rub a piece of paper against a diamond (or even a knife across it) no matter how many times it’s never going to cut into the diamond, because the diamond is harder, right?

First of all, vegetables have measurable amounts of grit on them, so that is a clear source of material which will dull knives. Even without that, just because something is hard, doesn’t mean it’s infinitely hard - for example, if you hit a telephone pole with your car, you are going to deform the metal, even though it’s much harder that wood.
The edge of a blade gets microscopically deformed and eroded over time, even cutting soft materials. Also, them metal of the blade can be etched by acids in whatever it is cutting.

As I understand it, the sharp edge of a knife is very thin and can easily get warped or acquire breaks just by usage - but most knives also come into contact with other than soft materials.

A warped edge with a few dents can be straightened by using a honing steel. Larger defects require sharpening (which grinds away some of the metal to make a new edge).

I would think your cutting board more the fault of knife dulling than your vegetables.

Bread has iron it. Minerals are found in maple cutting boards. The knife edge will encounter things are are ‘harder’ than it on hardness scale.

There are basically two elements to a dull knife.

Superficial dulling results when the blade edge simply gets folded over. This is happening at a microscopic level, but it’s enough that the edge becomes much thicker than before the folding. This is why chefs will use a “sharpening steel” and why these come with just about every good knife set. Used just before cutting, it primarily works by unfolding the blade, making sure that you’re getting the best edge.

As others have pointed out, wood can bend metal. Heck, you can bend metal using your fingers in the right conditions. So this folding takes places normally, even with soft foods and good cutting boards.

At a more serious level, dulling can result from actual wear on the blade, rather than the superficial dulling caused by the folding of the edge. This requires a whetstone or some other more serious device to actually grind some metal off of the blade to restore a sharp edge. This kind of dulling is more likely to result from cutting bones, grit in the food, acids, glass or stone cutting surfaces, etc. You shouldn’t need to sharpen a knife very often. For household use, you can probably go several years between sharpening.

It’s relative - just because one material is harder or tougher than the other, it doesn’t mean that only the weaker one yields. If you rub a diamond with paper for long enough, noticeable abrasion will eventually occur.

With less extremely-different materials (such as steel vs potato), the wear can occur quicker.

This is speculation, something I’ve long wondered about: when you cut something with long molecular chains in it, like polymers or (I think) the proteins in meat and bread and the like, you will often break these chains. That is, sometimes the blade goes between molecules, but sometimes molecules break where the blade stresses them. Therefore, there would be reaction sites becoming available all the time around the edge, some of which would react with the metal or the oxygen in metal oxides at the surface. This would be a chemical etching mechanism that removes material from the sharpest edges and gradually erodes them round, wouldn’t it?

The interface between materials at the microscopic level is quite different that you might imagine it to be. The edge of a sharp knife is not “slicing” at the molecular level, it is squashing and tearing food apart through the relatively massive leverage of the applied PSI force being applied against the knife edge, and then it is slamming onto a relatively hard wood or plastic cutting surface. Also hardness is relative. The fine steel edge of a knife is actually quite plastic and subject to deformation and dulling at the molecular level.

Just think about shaving. Your face if you’re a male or your legs/armpits if you’re female. Even that peachfuzz they made me shave in boot camp was enough to dull a razor. For a blade to be so sharp it must be super thin. The smallest amout of resistance will bend it ever so slightly. Over time this will cause the blade to become dull.

I got introduced to the concept when I worked in a science lab. The guy that headed the lab showed me to the procedure for sharpening the blade for the microtome. We put the blade under a 500X microscope and the edge of it looked like the Himalayas. After the sharpening procedure (that took several hours) the the edge of the same blade looked like the Salt Flats. If you take a blade that sharp and cut yourself you will bleed as though you are a hemophiliac because it does no tissue damage and the blood won’t coagulate. Handle with care!

The point is that the dulling of a blade occurs at a level that is not obvious to the human eye. The edge gets irregular and just gradually deteriorates. The more irregular it is, the “duller” it is. That doesn’t mean that a dull blade won’t cut but it doesn’t cut cleanly and without resistance.

"You shouldn’t need to sharpen a knife very often. For household use, you can probably go several years between sharpening. "
I have seen statements like this often. I could never go years between sharpening. Maybe I am just more sensitive to how sharp the blade is cause I collect and make knives but I can tell a knife is duller even a week or two after I sharpen it. And yes I have heard chefs say they havent sharpened their fav knife in years, when I try it I find it quite dull. Could be they just get used to it I suppose. thoughts?

Depends on the hardness of the steel, maybe? I’ve got a second hand paring knife made out of what appears to be fairly soft steel, and it’s the only knife that I’ve never sharpened in over 15 years of use. Just using a honing steel makes it amazingly sharp.

I would be very surprised if blades were actually cutting molecules. Maybe it could separate large molecular complexes that are joined by electrostatic bonds, but the edge of a knife is huge compared to most molecules. It’s like saying that a shovel might cut grains of sand in half while you’re digging at the beach. Mostly, the grains are just going to part on either side of the blade.

When you say you sharpen that frequently, are you talking about using a sharpening steel, or are you using a whetstone or grinding wheel? You may be more the expert on this than I, but I’ve always heard a distinction between the way steels maintain an edge and the way sharpening rebuilds a lost edge.

Partly, it might be a matter of getting used to it. My wife is very particular about how she wants to have her really good knives sharpened… but she refuses to actually get it done. Our 12" chef’s knife has gone at least 12 years and is now so dull that it won’t cut a tomato’s skin. But if I use the point to get a hole in the skin started, it will cut pretty nicely through the rest of the tomato and the rest of the skin. For cutting meat, carrots, potatoes, etc. it really isn’t noticeable that it’s dull.

Don’t forget about deposits and microscopic corrosion dulling the edge!

Most shaving razors are dulled not so much by wear as they are by the oxides that form once the protective coating is rubbed off the blade (oil, laquer, wax, etc.). Even stainless steel forms oxides. As a matter of fact, I believe it’s the Chromium Oxide coating that protects the steel from oxidizing by physically blocking oxygen from reaching it. Also, minerals falling out of solution (calcium chloride) or reacting with other chemicals to form new compounds like Calcium Stearate (soap scum) may adhere to the edge as well. This is one reason why using a sharpening steel or leather strap works so well. These devices straighten the microscopic edge and clean off deposits and oxides.

That doesn’t seem quite right to me. I routinely see axle shafts and crankshafts which are hardened steel, much like knife blades, have grooves worn into them by the rubber seal that keeps the oil in your engine or differential. Both are well lubricated.

It’s been explained to me like this (In a cooking “knife skills” class) -

The edge of a well sharpened knife is very thin and ends up having little “teeth” to it no matter how hard the metal. Every time you use the knife you bend some of those irregularities one way and the other. Having those teeth randomly aligned makes the knife function less well and a steel - used after each job - keeps those “teeth” all lined up in the middle, more or less.

At some point though the edge gets rounded off enough that metal needs to be removed to get a new acute edge. That is sharpening. Some sharpeners look like regular steel but if you wipe the knife after using a regular daily use steel you won’t notice any grey; you do after you sharpen it - that’s the old metal that has been taken off.

As to how something softer can damage the edge of something harder - the pressure on a very sharp edge is intense. Remember Pressure = Force x Area. A very sharp knife is very narrow at its edge - a little area having significant force applied to it, equals lots of pressure on the edge.

Another set analogies: A metorite may be a hard rock of metal but as it goes throught he soft atmosphere the act of pushing lots of air molecules aside can burn it up; your belly is hard compared to water but a belly flop from the high dive is likely to damage your skin.

And nothing like a cite.

This depends on the molecules. For example, some materials like crosslinked polymers become one giant molecule. Urethane would be an example. Also, some materials have molecules that can be drawn out to be very long, like Silly Putty. I think foods would include many materials that contain some molecules that are extensive this way. I’m pretty sure egg whites congeal by crosslinking when you cook them, for example. And salt crystals are big molecules unto themselves - saying the formula for table salt is NaCl could be misleading, because the sodium and chlorine atoms aren’t paired, they’re in an assembly that gives a 1:1 proportion but could be as big as you have the patience to wait for when crystalizing the salt.

In the case of small simple molecules like sucrose or water or ammonia, no doubt the blade is going between them, or more insightfully they are travelling such that some that used to be neighbors wind up on opposite sides of the blade. But certainly, given the enormous size some chemically bonded systems or molecules attain, knives sometimes break molecular bonds.