Stropping
	Finest abrasives.
	Microbevels front and back.
	Use a jig.
	Copyright (c) 2002-09, Brent Beach

Overview

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.

The problem then is to reconcile the clearly observed dulling action of stropping with the reports of superior sharpness.

Context

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 down to 0.5 microns, then stropping is a mistake. If you stop honing at a rougher grit, then stropping may help. This result applies even to the Chromium Oxide honing compound advertised as half micron.

The Test Procedure

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.

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.
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 "primarily Chromium Oxide admixed with other fine abrasives (0.5 micron size)" according to the catalog. Checking around the net, I have read that a particular manufacturer produces 6 different green rouges, with chromium oxide content from 5 to 90%.
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.
White Crayon Next, a Hock iron using a white crayon. The white crayon usually contains aluminum oxide as the abrasive.
Tripoli 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.
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.

Effect on Edge Durability

Any one of these stropping compounds does a good job of scratching up the edge. On a knife, or a turning tool used with a slicing action, this is ideal. 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.

Discussion

First, my testing shows that stropping can only dull a well sharpened blade.

Second, many people find that stropping 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.

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

The blue line represents the ideal resharpened result - a new lower bevel parallel to the original. People who strop do not use a jig and most use leather which yields, so this result is impossible.

The two pinkish lines represent other possible (more likely) new profiles. The stropper has removed a little metal near the edge. He has changed the shape of the lower wear bevel. It is not the ideal shape (blue line), but it may represent a slight improvement over the worn shape.

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.

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. 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 - you ground a new edge on a coarse abrasive wheel perhaps. So, rather than bringing a sharp edge to the strop, you brought a thick edge - imagine this drawing with the tip chopped off. Then stropping might remove a bit of the bluntness. It would not be at the desired bevel angle, but at a larger bevel angle, but it would remove some bluntness.

The conclusion? If stropping improves sharpness then your honing failed you.

Resolution of this conundrum will require testing in the field. Have microscope, will travel.

Conclusions

1. If you are using 3M microfinishing abrasives you should not strop using any of these compounds on a maple board.
2. If stropping is improving the sharpness of your blades, then your previous honing step is producing a bad profile. You should investigate other honing techniques - in particular, the techniques discussed in these pages.

Qualifications

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.
2. It is possible that the same compounds on some other surface will produce different results. People swear by leather strops. Leather is bound to be softer than hard maple, so must result in more rounding of the edge. I cannot imagine any way that there would be an improvement.
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.
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 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.

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