The title is provocative, but these 7 statements are too often made without reservation. As with most things, the reality is nuanced and depends on multiple factors. My intent with this article to demonstrate that knife burrs can take a variety of forms and explain why the 7 statements are at best only sometimes true. Hopefully, this article will provoke the reader into re-evaluating their understanding of knife burrs.

A sharpening burr is the metal that remains outside of the sharpening triangle. The burr forms from incomplete grinding of the bevel near the apex when the last few microns of the edge flex away from the abrasive surface. With sufficient grinding of one side of the bevel, the burr will grow and curl enough to be detected by lightly brushing your fingers off the edge of the opposite side. What we sense is the compression of skin at the fingertips as it catches the curled burr.

To avoid duplication and making this article unreasonably long, I won’t provide individual evidence for each of the “7 myths” but instead I will give a few examples that demonstrate the phenomenon of burr formation and attempted removal. I do plan to discuss these further in future articles.

When sharpening (or thinning) a dull knife, we will initially be grinding the shoulder of the bevel (away from the apex) and as we progressively remove material, the new bevel will eventually extend to the apex. It can be difficult to determine when this has occurred. Deliberately continuing past the point of reaching the apex and raising a burr is a simple and largely effective technique for determining when a new bevel has been ground all the way to the apex. In principle, repeating this procedure on the opposite side should indicate that sharpening is complete and we can move on to a burr removal step. Unfortunately, the burr is often just bent to the other side rather than having been removed and a new burr formed. Detecting a burr on the second side does not necessarily indicate the bevel setting step is complete. 

An example is shown below where a Victorinox paring knife is sharpened with a Shapton Pro 320 stone. While sharpening, I alternated sides occasionally and felt for a burr frequently. The first pair of images show the condition of the blade when I first detected a burr.

The knife was then sharpened on the second side until a burr could be detected by feel on the first side. As the images below show, the burr was only flipped, the bevel does not yet reach the apex on the second side. It can also be observed that 320 grit grinding lines are visible on the last 10 microns of the burr in the lower part of the image.

A cross-section through the burr from the first side, reveals a significant burr that was only flipped or bent back, not removed and new one formed. Even though I was trying to minimize the size of the burr, a relatively large and sturdy burr has been resulted.

From the SEM image of the above burr cross-section I would estimate that more than 10 microns of steel need to be removed from this second side to apex the knife and and as much again to raise a new burr. In the Abrasion Rate vs Grit article, I reported a removal rate with heavy pressure on the Shapton Pro 320 corresponding to a removal rate of 15 microns per 100 strokes from the testing coupon. This gives a rough idea of how many strokes should have been required to make a new burr on the second side. In the example shown above, I used “normal” not heavy pressure and I already felt the (flipped) burr within about 25 strokes. This problem could be avoided by performing a burr removal process prior to raising a burr on the second side.

A common approach to sharpening is to raise a burr on each side of the blade and then use light pressure at the same angle, alternating sides until no burr can be detected. This issue with this approach is that coarse hones require a minimum threshold of pressure to abrade steel and the light pressure will rarely remove the burr, but rather align and refine it. If we successfully hold the sharpening angle, only the burr will contact the stone and we will initially produce enormous pressure on the burr even with the lightest touch and abrasion can occur. However, this large lateral force will also bend the burr away until it aligns with the sharpening angle, preventing full removal of the burr at that sharpening angle.

In the example below, I sharpened the Victorinox with a DMT Fine diamond plate, changing sides frequently and stopping when a burr could just be felt. SEM imaging reveals a significant burr, tens of microns long, was formed.

Following the formation of the burr (and the above imaging) the knife was “lightly deburred” with gentle, edge-leading strokes on the second side (with the burr). After 20 strokes, no burr could be felt on either side.

SEM imaging of the second/deburred side shows that the burr has not been fully removed. A burr of this size and deflection cannot be felt with the fingertips.

The knife was then stropped, 10 laps on 1 micron diamond kangaroo tail strop, resulting in a highly refined and aligned burr.

The sharpening procedure shown above produced a blade which was “sticky” to the touch due to the exception ability of the thin burr to penetrate skin with minimal force. The blade was able to cut through zip ties almost effortlessly. 

(sometimes) You can feel a burr with your fingers

There is no doubt that a stiff burr that is folded to one side can be detected by brushing your fingertips off the edge. You can feel a burr as small as 1 micron if it is rolled sufficiently to one side. However, a large burr 10+ microns long and 1-2 microns thick cannot be felt if is standing up (or aligned). In my experience, I can feel the burr if it is deflected or curled about 10 degrees or more from normal. Additionally, a foil-burr that is significantly longer than it is thick may be difficult to feel as it is too flexible to give any resistance to our fingerprints. Yes, you can feel some burrs, but others are not tactilely detectible.

(sometimes) A burr can be removed by cutting into felt/wood or stropping on clean leather or fabric

I would classify sharpening burrs into 4 types:

1. Imaginary burrs – loose steel particles often magnetized by grinding that stick to the edge and are visible at low magnification or in bright light. These can be “wiped” away with a fabric strop or felt, or by cutting into wood or cork. Stropping on clean leather alone should remove this swarf.
2. Flexible/foil burrs that can be folded or rolled by cutting into wood or hard plastic, and sometimes torn away by stropping. These burrs may be so flexible that they can’t easily be felt by brushing your fingers off the edge. This type of burr can be observed to fold, bend, or tear off (when viewed with magnification) during cutting into hard plastic or the edge of a finger nail. Stropping on clean, smooth leather alone, without first rolling it, will not typically remove these burrs. Smooth leather at high angle or rough leather at the sharpening angle should remove them. Another option is to remove these by slightly increasing the sharpening angle – a large burr will often release as a long strip, or wire visible to the naked eye.
3. Thicker (1-2 micron or more) burrs of fatigued or brittle metal that can be partially broken away and partially rolled by cutting into wood and partially removed and partially stood-up by stropping. The resulting edge should have slicing aggression and catch fibres when cutting paper towel. Stropping on clean leather alone likely removes only the weakest regions of the burr. This type of burr will produce a non-liner edge, and is often mis-described as micro-serrated.
4. Thicker (1-2 micron or more) burrs of relatively undamaged and aligned metal that are unaffected by these procedures. The DMT-Fine images shown above and the dual-grit sharpening article are good examples.

The example below shows a foil-type burr and its conversion to a wire-burr. A foil burr can be identified by the aspect ratio of length to thickness being higher than about 10:1.

The next example shows a burr on a Victorinox paring knife sharpened with an Atoma 400 diamond plate. Sharpening was performed by frequently alternating sides until a burr could just be detected by my fingertips. Evan though I tried to minimize the size of the burr and stop when I could just detect it, the burr is more than 20 microns long. This burr has dimensions and length:width ratio somewhere between that of a foil (greater than 10:10) and a “sturdy” burr (around 3:1). It can perform well unless or until subjected to lateral force in which case it will roll.

A burr of this dimension is difficult to remove and certainly will not be removed by stropping or cutting into felt or a cork. I show that even cutting into the cross-grain of a maple block has little effect.

In the example above, 10 slices into the cross-grain of the maple block slightly defected the burr but did not remove any of it.

Some burrs can be removed easily, most cannot.

(sometimes) You can remove a burr with light (pressure) alternating strokes on the stone.

This depends on the type of stone and the angle of sharpening. If the angle is higher than the (sharpening) angle, the burr can be abraded and a micro-bevel formed. Nearer to the sharpening angle, a finer stone that releases abrasive (slurry or mud) could remove the burr. However, on a stone/hone that typically requires some pressure both to release grit and to grind, low pressure at the sharpening angle will not typically remove the burr.  The effect of low pressure, alternating strokes depends on the thickness/sturdiness of the burr. It is often possible to refine the burr without fully removing it. When performed well, this approach can refine the burr to produce a keen edge with a sharper angle than would be achieved if the burr were removed (for example, as the dual grit technique does).

An aligned burr is typically undetectable – being no longer able to feel the burr does not mean it was removed.

(sometimes) Edge-leading, alternating strokes will avoid a burr.

In general, as we sharpen a blade, at different points along the edge there is a competition between burr formation (incomplete grinding across the edge as the apex flexes away) and burr breaking (micro-chipping). Looking at the non-linearity of the edge, the high points are usually burrs and the low points are usually micro-chips. We can often identify a burr from this edge non-linearity. Edge-leading strokes generally increase the rate of burr breaking, and in many cases this process dominates and a burr is avoided. However, in some cases a burr will form even with edge-leading, alternating strokes – for example as observed with the DMT F (600) diamond plate. 

It is easier to avoid a burr than remove one. This direction of stroke usually reduces the probability of burr formation – in some cases it is effective, in others it is not. 

(sometimes) A burr is the result of steel being pushed/moved/drawn out past the edge.

It is true that metal can be pulled or drawn out of position in the direction of grinding during sharpening, but the scale at which this happens when hand sharpening is typically measured in nanometers, not microns. A sharpening burr (typically a few microns to a few tens of microns in size) results from incomplete grinding across the edge, which occurs when the apex/burr bends away (in the perpendicular direction) from the grinding media.

This is more of a semantic issue than a myth, but still important to understand.

(sometimes) A burred edge will fail quickly

A two micron thick burr that is standing up and has no significant microstructural damage is surprisingly strong, and in principle, could outperform a burr-free apex, as shown in the dual grit sharpening article. In the absence of microscopic damage, a 25 degree burr at the end of a 35 degree bevel is not weaker than a burr-free 25 degree bevel and will cut better than a burr-free 35 degree bevel. As always, the sharpening procedure should be matched to the cutting task.

(sometimes) Burrs are bad

I am confident that many people are deliberately but unwittingly forming burrs with their sharpening process, and they are happy with the results. Most so-called “toothy” edges formed by coarse stones are burrs, and there is no question that these edges can perform better in certain tasks than a refined, burr-free razor-type edge.

Final Thoughts

The performance of a cutting edge is largely determined by the last few microns of the edge, at a scale too small to visualize without electron microscopy. Burrs that are visible with modest optical magnification and bright light are typically (relatively) large with dimensions of ten microns or more. Burrs can be created in different forms and sizes and their presence can dramatically reduce or improve the cutting performance of the blade. Many so-called de-burring procedures are only effective for some types of burrs and may refine the burr rather than remove it.