sharp/dull blade drawing Stropping/Lapping small map
Finest abrasives.
Microbevels front and back.
Use a jig.
Copyright (c) 2002-15, Brent Beach


1. Overview

Sheet abrasives use grit particles bound to a non-compressible backing. Stropping uses grit particles on, or perhaps pressed into, a compressible surface (sometimes a non-compressible surface soft enough to allow the grits to penetrate). In this section the stropping surface is wood or leather and the grit particles are a variety of natural and synthetic minerals in the form of stropping compounds. The compound usually is in the form of a hard, waxy bar that contains one or more minerals in a variety of grit sizes.

There are a number of reasons to be wary of stropping.

In spite of these good reasons to avoid stropping, people often report better results after stropping their tools. They use a variety of stropping compounds and stropping surfaces. The Lee Valley Green Compound on leather is a popular choice.

I decided to look at the surfaces left by stropping to get an idea of why stropping was helping. To my surprise, in all cases stropping left a worse edge than I was getting after the 0.5 micron 3M abrasive sheet.

What do I mean by a worse edge? The quality of an edge is determined by the geometry of the bevels at the edge and the quality of th steel in those bevels. A worse edge either has a geometry different from that intended, or has lower steel quality, or both.

In these tests, you will see that both of these criteria are worsened by stropping. The steel at the edge has been damaged by stropping. As well, stropping with a soft medium can change the shape of the tool at the edge, moving you away from the desired geometry.

The problem then is to reconcile the clearly observed dulling action of stropping with the reports of superior perceived sharpness. At the end of this section, I suggest a number of situations in which stropping may help.

2. Stropping Compounds

Lapping or stropping differs from abrasive sheets because the abrasive grits are not bound to the backing. Initially the abrasive sits on top of the lapping surface. During use, the abrasive particles are driven into the lapping surface. Bigger grits are driven farther in than smaller grits, potentially making for a more even effective grit size.

2.1 Tooth

Tooth is a term used in knife sharpening which refers to leaving a certain amount of roughness on the blade near the cutting edge. The scratches act as microscopic saw teeth which assist in the cutting action.

You may have landed on this page looking for advice about stropping your knife or carving chisel. This page is not for you! These tools often require a little tooth to work well. In that case you would not hone the knife with the 0.5 micron abrasive, since the scratches it leaves are too small. You would stop at the 5 micron abrasive and still be able to skip the stropping step.

Plane irons do not need tooth in the same way that knives do. In fact, the larger the scratches at the edge the faster the iron wears. This page is actually concerned with stropping of plane irons after a final honing step using a 0.5 micron abrasive. This abrasive is off the USA grit scale and finer than 15,000 grit in the Japanese scale.

This page shows that if you want the smoothest possible edge and if you hone on sheet abrasives down to 0.5 microns, then stropping with any of the stropping compounds listed here is a mistake. If you stop honing at a larger grit, then stropping may help. This result applies even if you strop using the Lee Valley Chrome Oxide honing compound advertised as half micron.

2.2 Stropping Compounds

There are many stropping compounds other than the few I tested here. For example, finely graded diamond grits may perform well on a strop. This testing is not an exhaustive test of stropping compounds. Rather it is an examination of one popular compound, followed by tests of several other compounds I happened to have in my shop.

2.2.1 Lee Valley Veritas Honing Compound

One fact to keep in mind with any stropping compound: it is not the finest grit in the compound that determines the edge quality, it is the coarsest. The fact that the Lee Valley green stick contains some 0.5 micron Chrome Oxide is not important since the stick also contains a lot of larger Aluminum oxide grits.

Update Jan 2010: The advertising for the Lee Valley green stick has changed since last I looked. They now say: "The average size of scratch pattern it leaves behind is 0.5 microns". Do not be confused. Since only the tip of a grit scratches, a 0.5 micron abrasive could not produce 0.5 micron scratches. There are people who sell Chrome Oxide paste and loose grits that are actually 0.5 micron graded, but that does not appear to include Lee Valley.

Update April 2012: Confirmation of my 2002 test results has actually been sitting out there on the net since Nov 2008. Ben325e tracked down the maker of the Lee Valley green crayons and discovered that the bars contain 50% to 60% Calcined Alumina with mesh size -300.

What is calcined alumina? From Allied Quarry:

mineralpercentagemax %
Fe2O3.0112%.02% max
Na2O.4%.6% max
CaO .03%.02% max

More important for us is the typical sizing. They give

+100 mesh0-8%149retained on 100 mesh screen
-100 mesh70-95%149passes through 100 mesh screen
-325 mesh5-12%44passes through 325 mesh screen

The screened calcined alumina could contain up to 8% 100 grit particles. As well, at least 88% of the particles are retained on 325 grit screens - 88% of the calcined alumina is over 44 microns.

Combining the percentage calcined alumina and the percentage of that that is over 44 microns, at least .5 * 88 = 44% of the actual abrasive is 44 micron or larger.

Calling this a 0.5 micron abrasive suggests that someone along the line between manufacturer and retailer does not understand the product.

2.2.2 Solvol Autosol

Another popular abrasive with the stropping crowd is Solvol Autosol. It apparently uses a natural abrasive called infusorial earth. Other sources suggest it uses alum earth.

Source 1: Infusorial earth is a naturally occurring sediment - formed from the shells of small algae called infusorians, the shells of which are composed almost entirely of silicon dioxide.

Source 2: Alum earth is a naturally occurring mineral. The term Alum earth usually refers to Iron Pyrite - FeS2. Some use the name to refer to an aluminum mineral. The most common oxide of aluminum is is corundum - Al2O3. Aluminum makes up about 8% by weight of the Earth's solid surface..

Size 1: As it naturally occurs, infusorial earth particles vary in size from 3 micrometres to more than 1 millimeter, but typically 10 to 200 micrometres. It is likely that some sizing is done by the manufacturer - reducing the range of sizes. I have no information on that.

Size 2: Without knowing the source and the production methods it is hard to guess what particle sizes occur in autosol.

Hardness 1: Silicon dioxide has Mohs hardness of 7, which is about the same hardness as tool steel.

Hardness 2: Aluminum oxide has Mohs hardness 9.

Hardness 3: Iron pyrite has a Mohs hardness of 6. This is not hard enough for use with tool steels.

Hardness 4: Silicon carbide as Mohs hardness 9.5.

Generally, to be effective, an abrasive should be much harder than the specimen. Silicon dioxide and iron pyrite are not hard enough.

3. The Test Procedure

Starting with sharpened blades, I tested 4 stropping compounds. If you are unfamiliar with my sharpening system, you can see it at my jig page. All images are taken using an Intel QX3 microscope.

In these tests I used a piece of smooth hard maple as the strop. In all cases, the stropping was done with the iron in the jig to ensure the blade was stropped at the same angle as the final microbevel. To set a baseline for how the maple alone scratched the honed bevel, I began with no stropping compound on the maple.

In each test the stropping compound was applied to smooth hard maple, then the iron was stropped using only a pulling motion at the same angle as was used for the 0.5 micron abrasive. In all cases the image on the left is before stropping, that on the right after. The stropping compounds included the green crayon from Lee Valley and 3 stropping crayons from Delta.

4. The Results

Bare Wood Strop

To start, an iron stropped on bare wood. The is an old laminated Stanley iron. The area below the blue line is the 15 micron area. Between the yellow and blue is the 5 micron area. Above the yellow, the 0.5 micron bevel. Bare wood scratched up the iron a little.

side by side images, bare wood strop
Green Crayon

The next iron is a different old Stanley laminated iron (both are V logo irons). The stropping compound was Lee Valley green. Again, the three different areas of the iron are shown on the left. The image on the right has both of the fine bevels well scratched.

This buffing compound is advertised as "primarily Chromium Oxide admixed with other fine abrasives (0.5 micron size)" according to the catalogue. Checking around the net, I have read that a particular manufacturer produces 6 different green rouges, with chromium oxide content from 5 to 90%.

side by side images, green crayon wood strop
Green Crayon

Thinking I have blundered, perhaps not having started with a sharpened iron (I had just taken 6 irons out of the pile sharpened a couple of days ago), I took a fresh image of the Smoothcut iron, before and after the green crayon. In this case, I have marked the area on the right image where part of the 5 micron bevel has not been scratched.

See the Lee Valley green crayon discussion for more on the actual grit content.

side by side images, green crayon wood strop
White Crayon

Next, a Hock iron using a white crayon. Again, the picture on the left the iron before stropping, the one on the right after.

The white crayon usually contains aluminum oxide as the abrasive.

side by side images, white crayon wood strop

Then a second Hock with Tripoli.

Tripoli is a naturally occurring mineral which is a crystalline silica. I am not sure if or how they grade it. I suspect considerable variability. [It turns out that tripoli is the "silicious coats of perished diatoms." A diatom is a small plankton, usually unicellular, with a cell wall made of silica (hydrated silicon dioxide -- SiO2). (Silica is the abrasive usually found in toothpaste.)]

side by side images, tripoli wood strop
Red Rouge

Finally, a Lee Valley A2 blade on red rouge.

Red rouge typically uses Ferric Oxide as the abrasive. Again, I have been unable to find any grading information for red rouge. I suspect considerable variability in grit size within the bar and between bars.

side by side images, red rouge wood strop

5. Effect on Edge Durability

Any one of these stropping compounds does a good job of scratching up the edge. On a knife, or a carving tool used with a slicing action, this may be important. On a plane blade, how does stropping affect durability?

Here is the Smoothcut mentioned above, put through the usual test. The last time the Smoothcut did quite a bit better than this - getting a wear bevel of only 7 pixels (0.0005") compared to 9 pixels this time (0.007").

Using a stropping compound has reduced the Smoothcut to the performance of the Parplus blade.

four images, wear after 50, 100, 150 passes

This result is one small confirmation of the legion of tests done by Metallographers on the effects of various grit sizes on the crystal structure of steel. The larger grits deform the crystal structure well below the bottom of the deepest scratch, weakening the metal at the edge. The metal wears away more quickly. The tool dulls faster.

Even if you manage to strop in a way that does not change the shape of the bevels at the edge, you are weakening the steel as compared to the edge you get using my jig and sequence of abrasives.

6. Other Compounds

It has been argued that this test - hand lapping - is an unfair test of these compounds. The argument is that these compounds are intended to be used with a power stropping wheel. Such wheels are made of felt, leather, or cotton and usually are found on electrical grinders turning at 1,800 rpm. I find no such restriction on the Lee Valley green crayon advertising. However, the other crayons may well be sold primarily for powered strop use only.

There are stropping compounds that contain only finer grits. Tormek makes a stropping compound that contains Aluminum Oxide grits from 1 to 3 microns. This is also intended to be used on a powered strop, although the Tormek strop spins a lot more slowly than most powered strops, turning at 90 rpm. Hand America make bars and pastes that contain only Chrome Oxide grits in the 0.5 micron range (I have not verified this).

Because these purer compounds contain only very fine abrasives they can only be effective when used with micro bevels. They remove metal so slowly that if they were used on the entire bevel it would take a very long time to remove the scratches left by the previous grit.

John D. Verhoeven used pure Chrome Oxide abrasive on a powered wheel and got good results. He used micro bevels. You could perhaps spend enough time on this type of powered strop to make a difference even if honing the full bevel. I am thinking of doing some testing on this.

Many people report good success with a commercial chrome polish sold as Solvol Autosol. Some searching suggests that autosol uses "alum earth", a natural mineral which might be made up of either some oxide of aluminum or iron pyrite - depending on the definition. The manufacturer does not specify clearly anywhere what the abrasive is or the sizes of the particles. While it may be useful as a polish - its intended use - it is doubtful that is a good stropping compound (for the same reasons that the Lee Valley Green Compound is not a good stropping compound).

7. Other surfaces

This test used on stropping compounds on hard maple - a fine grained hardwood with few inclusions that might scratch the tool.

7.1 Leather

clean leather strop Leather is another popular surface. Everyone has a mental image of a barber stropping a straight (cut-throat) razor on a long leather strop before each shave. Many swear by their leather strops, with and without stropping compound.

It turns out that leather by itself, with no stropping compound, has no significant ability to remove metal. Testing of clean leather as a strop, with subsequent examination of the stropped tool using a Scanning Electron Microscope (SEM) by John D. Verhoeven showed the abrasive grooves on faces and the bur size along the edge were not significantly modified. In the picture, the left is the edge bevel before stropping on a clean leather strop, the right image is after.

Stropping using leather with a stropping compound can be very effective, even with a stropping compound that contains only 0.5 micron grits (see previous section).

With light pressure on a powered leather hone the tool will only slightly dent the leather surface. The faster the leather moves past the tool the more likely the tool is to ride on the leather rather than sink in. Hand stropping on leather, even hard leather, is almost certain to have the tool sink into the leather and dub the edge.

After creating a flat bevel on a hard abrasive surface, it makes little sense to take that bevel to a soft surface that really can only make the bevel less flat.

As well, any leather surface will become contaminated with filings. As the leather loads up with filings it is no longer a neutral base for your abrasive.

7.2 Medium Density Fibreboard - MDF

MDF is another popular stropping surface. If has the good properties of being very flat and very stable, as well as often showing up in the offcut bin. Since there is really nothing else you can do with an MDF offcut, using it as a lapping surface seems to have only pluses.

However, think about how fast MDF dulls power tools - much faster than ordinary wood dulls power tools. MDF is all by itself a powerful abrasive. What would a bare piece of MDF do to your tools? I am going to test this soon, I hope.

MDF, like all surfaces used for stropping, ends up with particles of abrasive and metal filings embedded in its surface. As well, before these get pushed into the surface, they roll around between the tool and actually wear down the MDF. Your surface will soon go out of flat. Can you tell when this has happened?

Finally, metallographers - who make a science of polishing metal - always flush the abrasive surface continually during use to ensure that filings and broken grit are moved away from the working area. If you use baby oil on 3M abrasives, the cutting fluid lifts the grits and filings. The blade sweeps them out of the working area. You cannot do anything similar with MDF because the oil would soak into the MDF causing it to expand.

7.3 Steel plates

Soft steel can also be used as the stropping surface. How soft? Well the steel used in old saw blades is soft enough. It is also flat, assuming is has itself been polished with a very fine abrasive. I have seen reports of people using fine diamond powder or paste on small pieces of saw blade steel.

Once the diamond grits are embedded into the steel, you can use baby oil with this surface - you gain the ability to keep the stropping surface clean while stropping.

A well polished stropping plate made of steel like this and used with a cutting fluid like baby oil could be comparable to the 3M Chrome Oxide abrasive sheet. More testing opportunities!

8. Stropping Videos

From time to time, visitors to my website email me telling me about a stropping video they have seen that shows that stropping works. I was sent one such link in the summer of 2012 - just after I had reread many of the Sherlock Holmes stories. The writer asked if a particular video would change my opinion of stropping.

This is the link to the stropping video.

So, where does Sherlock Holmes come into this? There is a phrase that comes up in a few of the stories. In particular, in The Sign of the Four, Holmes says "You know my methods. Apply them, and it will be instructive to compare results."

The problem with the Holmes stories is that Holmes never clearly outlines his methods. Each example appears to be a different type of conclusion drawn from a different observation. In fact, I believe that Doyle works backwards from the conclusion to the method of detecting that conclusion. He picks a distinctive person, imagines traits that person would have, imagines ways to show those traits, presents those ways and asks for you to reverse his thinking.

My methods, as outlined in the header of each page: finest abrasives, microbevels front and back, use a jig.

Before reading on though, stop and think about the three principles and about this video. Does the approach in the video satisfy my principles? Would it then change my opinion of stropping?

I am writing this section with reference to this video, but it applies to all the stropping videos I have seen.

8.1. Use a Jig

This is boring for you, but I have to begin here. The presenter does not use a jig. That means we know nothing about the angles he is putting on his knives. The variability in edge geometry that must result from this factor alone is far greater than the variability from any other factor - the type of strop, the particular stropping compound. You cannot replicate (repeat faithfully) what he is doing, so you cannot prove or disprove his technique. The result is magic thinking. It is not science. It is not something you can hope to do yourself with any reliability. You are asking for heartache.

In particular, this makes his paper cutting test invalid. We don't know if the knife is sharper because it has a different geometry or for some other reason. Since it sounds different when cutting, I think it has a smaller included angle at the edge, so cuts more readily. Perhaps he has stropped it less often - has blunted the final included angle less often. Who knows? But, since he freehands, we don't know so we cannot draw any conclusions from the results of this test.

8.2. Use the best abrasives

I discuss the Lee Valley green compound in several places in these pages. The most complete discussion is just a little farther up this page. While Lee Valley calls this a 0.5 micron abrasive, in fact it should be called at least 44 micron (88% of the abrasive is over 44 microns).

The maker of this compound (and several others) says that this compound is approximately 12,000 grit. The maker says their green compound is 6,000 grit and their black is 3,000 grit. They do not specify the grit system, but they presumably are using something like the Japanese grit system. Perhaps they are using the Shapton grit system. If so, then the rough translation to microns would be white at about 1 micron, green at about 2, black around 4.

However, like the Lee Valley green compound which calls itself 0.5 micron (about 15,000 grit), these compounds cannot be predominately of the grit size they claim. How do I know that? In fact, in two places in the video the claim is clearly disproven. First, when the presenter lightly strops the knife on the white compound black filings quickly appear. A true 1-micron abrasive cannot do this. Any filings produced would be very small (fractions of a micron) and would not be apparent so quickly. Second, the scratch patterns that show up in his microscope images are not those left by 1 micron grits. Any such scratches are too small to be discriminated by a microscope that uses visible light.

The conclusion - the compound uses a very coarse abrasive. I would not be surprised if it is comparable to the Lee Valley green discussed earlier. It could be that they combine a bit of 0.25 micron aluminum oxide grit with a whole lot of the calcined alumina that the Lee Valley bar uses. Perhaps a slightly finer version.

In any case, it is clear that the abrasive they are using on the final edge is not a fine grit. Large grits deform the crystal structure of the metal at the edge, weakening it. The edge will wear faster because of this.

8.3. Use Microbevels

Fine abrasives cut slowly. No matter what any abrasive manufacture says, this is just the reality of the physics of abrasion. This has been demonstrated in countless metallography labs over the last 150 years. Any manufacturer who claims that he has a superfine abrasive that cuts like a coarse abrasive is - not to put too fine a point on it - a liar.

You want to finish with as fine an abrasive as possible. These very fine abrasives can remove metal and shape the edge while producing little or no subsurface deformation. Again, a fact that has been demonstrated in countless metallography labs since at least 1864. The news of this fact has been known to many since those days, but is slow to reach the tool sharpening folks. Definitive demonstrations have been coming out of metallography labs for 30 years. The facts are well known in many engineering circles.

So, we want to use fine abrasives in order to retain sound metal at the edge. Fine abrasives cut slowly. Only by using precise angles and by starting the finer abrasive at a slightly larger angle, can you remove sufficient metal to make a difference.

Since the presenter of this video freehands, that is out of the question.

8.4. Use Appropriate Geometry

What? A fourth principle? Yes, I think I must add a fourth principle - use appropriate geometry. This fourth principle is far more important in knives than in plane irons or chisels, so discussing it here makes sense.

In this video the presenter talks about convex edges and V edges. He has a preference for convex edges when dressing deer, since the convex edge does not catch on the bone. [When he says edge, he doesn't mean edge. He means tool profile. We usually call the various parts of the profile the bevels.]

Tool profile is a crucial factor in knife performance. Before discussing convex and V edges, let's backup a bit and think about profiles and how they affect tool use.

With planes and chisels, the part of the tool away from a very narrow section near the edge never makes working contact with the wood. Our only concern is that the primary angle be small enough to allow us to use our desired honing angles. Using a primary angle about 4 degrees less than are smallest honing angle does this.

With axes there is one case where the primary angle is a factor. With a splitting axe, the tool just back of the edge must be thick enough to split the wood. If it is too thin, the axe sinks into the wood but does not split it. Friction between the wood and the axe slows forward motion. If the axe profile is too thin, the axe stops before the wood splits. It also makes it very hard to remove the axe. In this case, the wood does make working contact with the tool back of the honed edge, so that geometry is important. For a splitting axe, that angle must be large.

With knives, the situation is much more complex. The correct primary and honed angles depend on the particular application. A knife for chopping vegetables can have very fine primary and very fine honed angles. If you are careful with your vegetable knife - you do not drag the edge sideways across the cutting board - a very sharp knife will last for a long time and will provide fast slicing action. Use the same knife to cut a chicken bone and you will lose the edge rapidly. A thick knife - too large a primary angle - does a poor job of slicing vegetables because the thick section forces the vegetable to split ahead of the edge.

We are getting a bit far afield here, but one other typical knife problem is worth mentioning - cutting soft cheese. As you slice down through the cheese, the cheese slides up the blade, clinging to both sides. The friction between the cheese and the knife can be significant. It can make cutting cheese very difficult. The solution to this problem is not found in changing the primary or honed angles. Since the cheese is soft, small angles for both work well. The solution is putting a surface on the knife to which the cheese cannot cling. Laser etching solves the problem. Using a relatively coarse abrasive on the primary bevel (away from the edge) also solves the problem.

For dressing meat, a knife with a thin blade (small primary angle) but a relatively blunt edge (large honing angle) will cut meat but not catch on the bone. This is the profile the presenter wants.

You get this profile - thin blade, relatively blunt honed bevel - not by trying for a convex edge. You get the correct profile not by using a soft abrasive surface and deliberately rounding the bevel (making it convex). You get this profile by using jigs and hard flat abrasives to produce the correct primary and honed angles.

In the video, he has one knife that cuts paper much better than all the rest. I suspect this is a newer knife that he has not freehand honed very often. The honed bevel therefore has a finer included angle at the edge (has not been dulled by repeated freehand honing).

[In adding a fourth principle, I am aware of this second occurrence of the number 4 - in the Doyle title and in the number of principles. Is it just a coincidence? Is there a deeper significance?]

8.5. Summary

In answer to the reader's question - Did the video change my mind about stropping? The answer is no. The video is magic thinking. Magic thinking will not produce consistently good edges.

In answer to my question - or perhaps, to Sherlock's challenge - You know my methods. Apply them, and it will be instructive to compare results. - Did you come to the same conclusion as I did?

9. When to Strop

There are two very different user experiences with stropping.

First, my testing shows that

if you sharpen using the abrasives and jigs I use, to an edge with the desired geometry and condition
then stropping with standard stropping compounds will have a negative effect on edge condition and may also have a negative effect on edge geometry. It cannot improve either.

Second, many people find that stropping with these compounds improves the usefulness of their blades. There are many anecdotal reports of much better tool experience after stropping. Many of these reports are related to carving chisels, but some apply to bench chisels and plane blades. Proponents of stropping are absolutely convinced of the benefits of stropping.

How then to reconcile my results with the popular experience with stropping? There are two slightly different cases:

  1. - restoring a slightly dull tool, and
  2. - as the last step in sharpening.
drawing of stropped edge

9.1. Restoring a dull tool

First, a worn tool that is restored by honing.

In the usual side profile drawing of a worn blade at the right - drawn to scale from measurements taken during a test - the black outer line is the original sharp profile, red the worn profile. Recall that his diagram represents the tool profile within 0.003" from the original edge.

The blue line represents the ideal resharpened result - a new lower bevel parallel to the original. This result is not achievable with a strop that uses a very fine abrasive - fine abrasives remove metal too slowly. If you were to use a coarse stropping compound (the Lee Valley compound) you would be compromising the condition of the steel - resulting in faster subsequent wear. If you strop freehand or use a soft stropping material like leather, you cannot achieve this result.

The two pinkish lines represent other possible (more likely) new profiles. In the first, slightly flatter case, you have raised the back of the blade slightly (are stropping at a slightly larger angle) so may slightly reduce the size of the bulge in the wear bevel while only slightly increasing the included angle (from the original). You could increase the stropping angle even more and create a flat facet that reaches the edge, but you have increased the included angle and reduced the clearance angle. Even though the bulge in the lower wear bevel is gone, this blade may now not have enough clearance to cut into the wood.

With a worn edge, you have two options. Repeated stropping which produces a slightly different worn edge, or the three step honing which produces a sharp edge.

People who argue that they just spend a few seconds bring the edge back are dodging reality. One strop proponent reported giving his bench chisel a quick touch up before each tenon. A well sharpened tool should do dozens of tenons without any attention. This person is spending a lot more time sharpening and worse. is always using a dull tool.

9.2. Last step in sharpening

The second case often cited is stropping of a freshly honed tool. That is, as the last step of sharpening people strop the tool. If you use the abrasives and the jig I do, honing produces exactly the edge profile you want and steel condition you want. Further abrasion, particularly freehand abrasion, can only begin the wear process! You would notice no benefit. In fact, you would notice reduced edge sharpness and durability.

But, say you do notice improved sharpness and edge durability! The only possibility is that you did not start with the ideal edge.

Perhaps grinding has produced a thick edge - your final abrasive was coarse. The thickness of the metal at the edge is a function of the depth of the scratches you are producing. Put another way, the thickness of the edge is a function of the heights of the scratch walls. Stropping, even with a coarse stropping compound like the Lee Valley green crayon may reduce the heights of those valley walls, making the edge thinner.

A second possibility is related to freehand honing. People who freehand do not get flat bevels, they get faceted bevels. A couple of careful pulling passes on as strop could remove some of those facets, making the surface of the tool near the edge more regular and hence more usable.

9.3. Wire Edge

If you use coarse abrasives at the edge, you may have produced a burr or wire edge. Your subsequent honing steps may not have removed that wire edge. In this case, your stropping step may remove that wire edge.

wire edge

The wire edge has a thickness - if you were really careful you could measure it. The wire edge meets the tool at a surface of that thickness. Rather than the edge being the line of zero thickness at the intersection of two planes, the edge is a surface with width equal to the thickness of the wire edge.

Stropping can remove a wire edge by flexing the wire edge until it breaks off. The resulting edge has a face the thickness of the wire edge, which is not ideal, but at least the wire edge is gone.

Stropping might also actually abrade the wire edge away by rounding the edge until there is no metal to hold the wire edge.

While stropping has improved this edge, the edge was far from ideal before the stropping and far from ideal after the stropping.

9.4. Conclusion

The conclusion?

If stropping improves sharpness, then your grinding/honing steps have failed to produce a good edge.

10. Summary

In the overview, I presented 3 reasons to be wary of the claims made for stropping:

Compressible surface

If the strop does not make contact with the edge, why are you stropping? You are only smoothing the bevels back of the edge.

If the stop does make contact with the edge, a compressible strop surface will dub the tool.

Grit Size

It is clear from the micrographs on this page, and on pages linked form this page, that stropping compounds produce larger scratches than a 0.5 micron or even 5 micron sheet abrasives. Metallography has shown that scratch size and sub-surface damage are directly linked. Sub-surface damage at the edge means a softening of the metal at the edge and reduced edge life.

Slurry Effect

When you use a strop you will quickly see swarf collecting on the lap. This swarf is made up of metal filings and broken abrasive grits. Metallography has shown that this swarf is responsible for increased sub-surface damage. That is, swarf degrades the quality of the steel. You get a softer tool that wears more quickly.

11. Conclusions

What conclusions should you draw from these tests?

  1. If you are using 3M microfinishing abrasives you should not strop using any of these compounds on a maple board.
  2. If you are using some other abrasive or blade holding method, and if stropping is improving the sharpness of your blades, then your previous grinding/honing steps are producing a bad profile. You should investigate other honing techniques - in particular, the techniques discussed in these pages.

11 Further testing

My testing is limited to a certain number of stropping compounds on a particular strop. It is possible that other combinations might produce different results. No combination will produce a better edge profile than 3M microfinishing abrasives on glass with a jig.

Some possibly better (than the test) combinations include:

  1. It may be that some other manufacturer's Chromium Oxide compound does not contain the range of grits found in the bar I bought. Lee Valley's current green crayons may not even be the same. In fact, you can buy a Chrome Oxide paste that is 99.9% chrome oxide.
  2. It is possible that the same compounds on some other surface will produce different results. I have argued above that leather and MDF are not suitable. Metallographers use diamonds on linen. I have never heard of anyone hand stropping on linen, but it could work. Diamonds on steel or cast iron might also work.
  3. It is possible that the same compounds on powered strops (either discs, felt wheels, or leather belts) might work better. I have read, but not verified, that high speed grinding makes the grit appear smaller -- for example, the scratches from a 60 grit wheel might look like those from a 180 grit bench stone. I plan to test this option.
  4. If you use other sheet abrasives, even 3M wet-and-dry abrasives sold in lots of stores, often auto stores, your bevels may be so bad (the front and back bevels near the edge have deeper scratches than those produced by the honing compounds) that these compounds can help.
  5. If you don't use back bevels and you are getting a fine wire edge that is not being removed during honing, stropping on a bare leather surface may help.
  6. If you don't use back bevels, then stropping on the front and back of the blade may improve the quality of the back face - smooth the back wear bevel a bit. This would only apply if you stropped using a jig on a hard surface.
  7. Verhoeven's testing was limited to a electron microscope examination of the surface of the tool. He did not actually compare the resulting edge in use. We know from Metallography that the crystal structure of metal can be damaged below the surface even if the surface looks smooth. Verhoeven is a metallographer. I am not sure why he was satisfied with looking at superficial edge smoothness when he must be aware of the issues with sharpening methods using large grits at the edge.

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