How does Stropping Work?

[lp149]

Hi, new to sharpening here.

I just can't see the logic in stropping a knife. I know it is done using light strokes utilizing very little pressure.

Is the burr really that fragile that only light strokes are required to remove it? If so, the burr and cutting edge would deform and bend quite easily after one cut through a tough piece of tough meat, no?

What if I scrubbed the burr using the rough side of my sponge, wouldn't that remove it in the same manner as stropping and/or polishing?



Cheers

 

[Blade Lab]

A burr is distinct from the edge apex. Assuming that you've minimized the burr on the stone, it consists of tiny metal shavings that adhere to the edge, or it can be a thin wire or foil that's loosely attached to the edge. If you don't get rid of the burr, your knife won't perform very well.

As a rule, you want to load your strop with a compound made of aluminum oxide, chromium oxide, diamonds, or CBN -- all of which are harder than most knife steels. Some very hard steels require diamond or CBN. So it's the compound and not the strop substrate that's actually doing the majority of the burr removal. Leather contains trace amounts of silicon carbide, so it will do *some* honing on its own.

The burr is more fragile than the edge bevel itself, so if it isn't too big it can be removed with light strokes. If you've left a really massive burr it may be impossible to remove it by stropping, in which case you'll have to go back to the stone to make it smaller.

It's certainly possible to dull a clean edge on a strop if you apply too much pressure or come in at too high an angle. Since leather is pliable, applying too much pressure, even at the correct angle, causes the edge apex to sink into the leather, which will round the apex and dull the knife.

 

[chalby]

It's not only for removing remnants of a burr. It is also good for realigning an edge after use, where the edge has very minor damage/misalignment.

It depends on the steel whether you would need or want to use a compound. Simple steels (ie. low volume of carbides) will respond well to a good leather alone, whereas the higher carbide steels will need compound to help abrade the harder parts of the steel matrix. There are different materials that people use for strops as well from denim to wood etc.

Generally though, the evidence I have seen suggests stropping to be very effective for edge maintenance/refinement. Methods such as drawing the edge on wood, cork or other materials, or as you suggest, scrubbing with a scourer or sponge tend to tear the burr off leaving a less than perfect apex. It probably doesn't affect sharpness so much as longevity of the working sharpness. At least that is the impression I have got from the reading I have done.

You could easily experiment though. Try sharpening the knife, then use it for a time until it is starting to noticeably become slightly dull, then strop it and you will notice that crisp sharpness come back. (At least if you have a decent stropping technique.) Or sharpen and de-burr on the stone and compare cutting receipt paper or something similar before and after stropping.

 

[Blade Lab]

"It's not only for removing remnants of a burr. It is also good for realigning an edge after use, where the edge has very minor damage/misalignment."
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I think the steeling is a better option for realigning edges, because you do want to apply some pressure to straighten an edge.

 

[Obsessed with Edges]

How well stropping works is directly proportional to how fine an edge already is before stropping. If the apex of the edge is too thick, round or blunt, stropping won't make an improvement. That's usually a good test for determining if enough sharpening work has been done on the stones, before stropping. If the strop isn't effective at all, it's a strong clue more work is needed on the stone. And if stropping is done with fairly aggressive compounds on such a thick, round or blunt edge, the edge would more likely become even more rounded or blunt.

On the other hand, if the edge is thinned out adequately and the remaining burrs or their remnants are also as thin as they can be, then stropping on the simplest of bare materials like leather, paper, linen/denim or wood can make an immediate, noticeable improvement in sharpness.

And some steels at their typical heat-treated hardness won't necessarily burr as much as others, or hold burrs as tenaciously. So a steel that's not prone to forming or holding burrs much at all won't respond in the same ways to stropping - especially with only bare materials like leather, paper, etc. But those steels can still be refined to greater sharpness by honing them on a loaded strop using an abrasive polishing compound appropriate to the steel. In that capacity, so-called 'stropping' is more akin to enhanced sharpening and polishing by abrasion, as done with stones. But 'stropping' in its original sense is basically just about cleaning away the thin, weakened bits of steel hanging at the apex, and also sometimes about aligning a very fine, thin edge - as with a barber's straight razor for shaving.

Edited to add:
Some scrubbing sponges with a rougher scrubbing side actually use a plastic material infused with very fine abrasive particles, like aluminum oxide. Those are capable of dulling a sharp edge, if the pad contacts the edge during cleaning of kitchen knives & such. 3M's line of Scotch-Brite scourers, with green or blue rough scrubbers on one side of the sponge, are examples of those. They come with warnings not to use them on coated cookware, for example, because the abrasive in the pads is capable of stripping the coating off such items.

 

[chalby]

"It's not only for removing remnants of a burr. It is also good for realigning an edge after use, where the edge has very minor damage/misalignment."
--------
I think the steeling is a better option for realigning edges, because you do want to apply some pressure to straighten an edge.

It depends on the edge, but personally I don't use very much pressure at all when steeling either.

Steels like for example 12C27 can be brought back to very sharp with a few passes on a strop after losing a fine edge in use. Again harder or high carbide steels will probably need some sort of compound to quickly bring back a sharp edge. Sure, steeling will also work in that situation, although the harder steels don't tend to respond so well to steeling.

As mentioned above, there is a point where stropping or steeling will stop being effective and a touch up on stones or whatever will be necessary.

 

[PMQ]

If you go straight from apexing to the strop, you'll find that the burr scratched the strop quite badly, I've tested it myself. It's possible to strop this way, but not recommened.

That's why it's common practive to strop on the stones a couple strokes to weaken the burr. Imagine bending a paper clip back and forward, you're gradually weakening the paper clip, and then it takes very little force to break it into two. The same applies here, you're flipping the burr side to side to weaken it. So when you strop on leather, be burr can quickly fall off.

 

[garry3]

Hi, new to sharpening here.

I just can't see the logic in stropping a knife. I know it is done using light strokes utilizing very little pressure.

Is the burr really that fragile that only light strokes are required to remove it? If so, the burr and cutting edge would deform and bend quite easily after one cut through a tough piece of tough meat, no?

What if I scrubbed the burr using the rough side of my sponge, wouldn't that remove it in the same manner as stropping and/or polishing?



Cheers

Yes, you can cut through material to remove a burr or use probably a hundred other different methods.

Many times I don't bother stropping at all, especially on the toothier edges I like on some knifes. Right off the stone can work just fine too, it just depends on what level of refinement is required.

This video has very good information on burrs and may answer some of your questions....