Posted by: Part-Time Audiophile | June 18, 2009

Knife Edge Finishing

So you have your fancy knife. You’ve been using it to great effect. Vegetables fear you. You steel your knives regularly so you can be prepared to do battle in Kitchen Stadium at any moment. But … something has changed. You steel, you’re careful, you have great cutting boards … but things just are not the way they used to be. What has happened? Your knives are beginning to dull. What to do?

Strop it!

I’ve already posted a set of videos on knife sharpening with my hero, Dave. He progresses through a nice set of Japanese water stones to get to a keen edge. But to finish, he uses a strop.

Why?

Well, think of it this way. When you use a hone of any kind, it will dig into the metal to remove some (even if it’s a tiny, tiny amount) to create or recreate the edge. That digging will look like grooves or scratches in the metal. Those grooves will generate friction as food passes across them, friction increases drag, drag will increase the amount of force required to move the knife through something, which makes it feel dull. Got that? Okay, well, if we reduce drag, we reduce the amount of force required, right? Right. Hence, we use a progression of grits in our abrasives.

Generally, we start off pretty rough. On average, and depending on the amount of damage that needs to be addressed, you’ll start somewhere in the middle of the road to touch up your edge, say, with a 1,000 grit stone. If the damage is severe, or you just want to limit the amount of time you’re planning to spend doing this (a 1,000 grit stone may take a while to get you where you want, like, say, an hour — this is steel, remember), you might want to start with a rougher stone, say a 500 or even lower. Anyway, wherever you start, you’re scratching that knife with that size grit. To remove those scratches, you need a finer grit. So, a 2,000 grit stone will follow a 1,000 grit stone, essentially halving the size of the scratches on the blade. Likewise a 4,000 grit will follow a 2,000. After 4,000 you’re really no longer sharpening your knife, you’re polishing it. Yes, you’re removing steel, but unless you’re able to hold that angle precisely over every stroke (this is completely beyond my ability, apparently, but YMMV), you’re probably not adding much to the edge. Anyway, moving to an 8,000 grit stone will put a really nice polish on that blade and make the scratches on it pretty much invisible. Done, right?

Yes, if you’re like most folks, you are finished at this point. An 8,000 grit stone isn’t called a “finishing stone” for nothing. But … you don’t have to stop there. Remember that bit about grooves and friction? An 8k stone has grit elements that are a couple microns across. Too small to see without a loup, but there nonetheless. And far short of the ideal — frictionless — or at least something approaching it, that is, a polish “like glass”.

To get that, you need a finer grit. Manufacturers make stones with 10k, 12k, 16k, and 30k grits — these stones are pretty much smooth to the touch. The actual grit size of an 16k stone is about a micron — a 30k, therefore, is about half of that. And polishing with either will (or, really, anything over 8k) will create a mirror finished edge. The downside is that these stones are not cheap, and the very highest grit stones can be two to five times more expensive than the step below them. A very popular 10k stone, made by Naniwa, is over $250.

Enter strops.

A strop is just a strip of leather. Nothing scary there. Barbers have been using strops to “fine tune” straight edge razors for decades before Gillette & Schick started selling safety and cartridge razors to us, the public happy to spend way more money for inferior (but easier!) products. Using a strop on a kitchen knife works pretty much the same way. Things get interesting when you add an abrasive to the strop.

By “charging” a strop (loading it with an abrasive), you can create a setup that can polish your edges quite nicely. Chromium oxide is an inexpensive but very popular abrasive and creates a beautiful edge. It’s abrasives are about .5 microns, which makes it equivalent to a 30,000 grit stone! Even more extreme is a diamond spray with particles half that size, which in effect creates a 60,000 grit surface for polishing.

Ok, so no. No one needs that level of polish on their knives. And no, that edge cannot remain that sharp for long. But its still pretty cool, no? 🙂

There are some great times to strop. After you’ve run your knives across the stones, you strop to finish. You can also strop instead of sharpening — assuming the edge is in reasonable shape — just a quick couple of passes on a charged strop may be all you need to get that fearsome edge back in shape. In this case, it’s like steeling, but way better for your edge. In fact, the delicate but wickedly sharp single-beveled Japanese sushi knives should never touch a steel (or ceramic or glass) hone at all — you should strop instead. Treat your traditional kitchen knives the same way. Steel before use, strop once or twice a week, and you should be able to put off trips to the stones even longer.

The way this works is you lay your knife onto the strop spine-first, then rotate your blade down till it “just bites” the leather. Then, pull it backwards away from the edge while maintaining that angle and sweeping the blade to the side to catch the entire length in a single stroke.

Yeah, I know, easier said than done! Luckily, Dave Martell has an explanation and some photos on his site.

Looking for strops and abrasives? Well, Dave has some nice stones on his store. He used to resell Hand American strops and abrasives, but that relationship seems to have been discontinued. Some alternatives can be found on JWW.

For those video junkies, here’s one on stropping. Personally, I recommend Dave’s techniques — maintain angle, use little or no pressure, &c, but anyway, it’s something. More interestingly, it’s something not to do:

Last note about strops (for now, anyway): thinner is better. You want that strop to give as little “give” to your stropping as possible. What that dude in the video is doing is dulling his carefully honed edge.

Why? Well, thick, soft leather — like belt leather — will deform to the edge of the steel. That is, the steel is harder than the leather and with pressure (making it “sing”) will actually make the leather wrap around the edge a bit. Which is putting pressure on the edge. Which dulls it. It will realign an edge, yes, but the more the leather wraps around the edge, the more of the edge it takes away at the same time. What you want is a mounted strop — that is, not a straight razor “hanging” strop. Leather mounted on a wooden paddle strop is a much better solution. Better still, try a strop magnetically mounted to a steel plate.

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