Razor Sharp in a Nutshell

Introduction

A lot of people use the term razor sharp without being particularly conscious about what it is or what it can do.  So I decided to do a little research and condense down to the fewest possible words what I found. To guide my investigation I formulated several questions that I wanted to be sure I got answers to:

• How sharp an edge is razor sharp?
• What would I have to do to get a knife razor sharp?
• How can I verify I have created a razor sharp edge?

Razor sharpness implies microscopic structures not directly visible to the human eye, so I looked for scholarly research accompanied by close-up images.   My main reference sources are at the bottom and are well worth reading for anyone who wants to probe deeper. Note that razors are as sharp as they are because they are usually used in a push-cut rather than a slicing action (eg to part whiskers when shaving). But razor edges tend to be delicate, as in surgical scalpels that only cut a few inches of material before being thrown away. So something slightly less than razor sharpness is probably what you will have on working knives most of the time. Still, a near razor sharp knife is a pleasure to use.

How sharp an edge is razor sharp?

The most obvious place to look for razor sharp is razor blades. I found that even academic articles on sharpening used commercial razor blades as part of their baseline. Articles [1] and [2], document a two part study written by the same team. The first paper describes the derivation of a proposed standard metric for measuring blade sharpness, called Blade Sharpness Index, or BSI. The second article analyzes key aspects of knife geometry related to blade sharpness. The most essential of the authors’ findings were:

1. An edge radius of 1 micron (which, for comparison,  is 1/25 of .001 inches, or by my math, around 10,000 iron atoms) is considered sharp and a radius of 5 microns is already considered blunt (at least from a razor sharpness point of view). Article [3] measured commercial razor blade edge radius at about 0.4 microns typically
2. An infinitely sharp blade would have a BSI of zero
3. A new scalpel with a 1 micron edge radius will have a BSI of around .2
4. A BSI rating above .5 is considered blunt
5. Three attributes are important to determining a blade’s sharpness:
   • radius of the cutting edge. This is the most important factor
   • angle of the wedge just behind the edge. This is not as critical as the edge radius. However, angles above 25 degrees more rapidly reduce effective sharpness (ie increase BSI)
   • blade profile (ie with or without micro-bevel). They compared a scalpel blade having a single 25 degree bevel leading up to the edge with a single edge razor blade having a main 17 degree bevel and a final 32 degree micro bevel before the edge. The scalpel was sharper, indicating that the angle closest to the cutting edge determines the sharpness of the blade

What would I have to do to get a knife razor sharp?

In [3], the author compares several sharpening systems and methods as to their ability to obtain a clean, razor edge on the blade being sharpened. I slightly simplified his waterstone technique using the stones and tools I have (800 and 4000 grit waterstones and 6 micron diamond paste, versus his 1000 and 6000/8000 grit stones and 1 micron diamond paste, and was still able to achieve an edge that would pass my razor sharpness test (see below) when sharpening our Coho outdoor knife with its 25 degree edge angle. Here is how I did it:

• start with a knife that already has a good edge (no light reflects off of the edge)
• use a sharpening guide set to an angle appropriate for your knife (those with a practiced hand should be able to sharpen freehand without a guide). The angle should be barely steeper than the existing edge angle. This ensures that the new edge angle you create is consistent and extends right to the edge
• using an 800 grit waterstone make 10-15 “stroke cycles” across the stone using light pressure and a dragging action rather than pushing the edge into the stone. Here, a “stroke cycle” means to drag the blade across the stone on one side, then flip the blade over to its other side and drag back. Each stroke must involve the full length of the cutting edge (Note: in [3] the author’s sharpening stone technique uses 10 special 4-stroke cycles, which you may want to try if the above number isn’t working for you due to the toughness of your steel or your blade’s initial state of grind)
• repeat the above step on a 4000 grit waterstone, reducing to very light pressure during the strokes (the more delicate the edge becomes, the easier it will be to bend it over if too much pressure is applied)
• repeat the above on a smooth leather strop (smooth side of old leather belt will do, the back side of which is glued to a board) loaded, ideally, with 1 micron diamond paste.

How can I verify I have created a razor sharp edge?

Lacking an electron microscope, the best indicator of a razor sharp knife is what it can do. So again I turned to razor blades to establish a baseline. I tested using a new single edge utility razor blade .008″ thick and with approximately a 13 degree angle behind the edge (this is the type of blade used for removing stickers from window glass). I used a fresh new blade to make two types of test cuts (described below) using the .002″ thick pages from an old Uline catalogue (which I assume many people will have access to) and observed what I saw and felt as cuts were made. I’m not sure how blunt (in terms of microns) an edge has to become to not pass my razor sharp tests, but I observed that a fresh razor that had been used to cut through a 3/8 sisal rope just once would no longer pass test 2 below.

My two test set-ups were:

1. Hold a single page of the catalogue so that it hangs vertically and with steady, gentle pressure make a vertical push-cut down from the top edge about 1/2 an inch from where the page is being held
2. Hold the page at one of the narrow ends, raising it up high in front of you. Approach the page with the knife held in the other hand, gently making contact with the paper about 2/3 above the bottom end of the page. Drag the page over the knife and note whether the weight of the paper is enough to initiate a cut

Results with my test razor:

1. Yes, the blade almost without fail cuts down crisply and easily with push-cut action alone
2. Yes, even with a slow gentle movement of the razor against the paper, the blade usually initiated a cut. 

So if you’ve gotten your knife truly razor sharp, it should be able to pass at least test 1. I found it much harder to get a knife to pass test 2 unless, like a razor blade, it offers a single sharpened point on which all the paper’s weight will be focused. Also, knives with too great an angle behind the edge, even if the edge is very narrow, may have difficulty with these tests.

I’d love to hear how well you fared with the above sharpening challenge. Just leave a comment below. Your email will not be shown.

References

1. Part 1 of a pair of technical papers by the same team from Ireland. They offer a proposed standard way to calculate blade sharpness which they call “BSI” or “blade sharpness index.”
2. In Part 2 the above team look at a variety of factors influencing the effective sharpness of a blade and quantify which are most important.
3. Experiments on Knife Sharpening takes an academic yet practical look at a variety of sharpening strategies and offers many useful conclusions.