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Razor Edged: How sharp is sharp?

Razor Edged: How sharp is sharp?

photo by Chris Stickley

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All practitioners of the art of test cutting want a blade with a keen edge;  let’s face it, a dull blade isn’t going to cut well.  “Razor sharp” is the phrase most often bruited about, but just what does that mean?  How do you measure sharpness and, once you understand what that is, how can you maintain your blade to that level of keenness?

In this first article on the topic of sharpness, we’re going to explore some of the basic concepts of sharpness, metal formation and edge maintenance.  Future articles will deal with specific types of steel, forging processes and edging techniques as practice around the world.   Note I’m not entering in to the long-running katana/Damascus steel vs European sword debate.  Personally, I’m a firm believer in a blade being no better than the smith who forged it and the warrior who used it.  It’s undeniable that individual swordsmiths in different parts of the world did, at various times, discover advanced techniques in metalworking and create miraculous works of art as weapons;   for example: Carbon nanotubes in an ancient Damascus sabre.  This isn’t to then say that every Damascus blade had the same qualities. No single country can take credit for having created the “ultimate sword” technology.

Sharpness Theory

To explore what sharpness is, we’re going to be borrowing from the modern realm of knife making and sharping.  Generally speaking, there are several qualities which govern how well an edge can cut:

  • Blade  thickness
  • Blade shape
  • Edge angle
  • Edge thickness
  • Edge smoothness

 

Lets look at each of these individually:

common blade crossectionsBlade thickness: Simply put, the width of the steel a blade is made from will greatly influence the possible thickness of the final edge.  Even with the best possible blade geometry (the shape of the blade in cross section:  often wedge-shaped for single-edge swords, or diamond shaped for double-edged swords as seen to the right, though more complicated geometries certainly exist), a thinner blade will generally be able to be brought to a finer edge thickness than a wide blade

Blade shape: This is the shape of the cutting edge:  straight, curved, or — rarely seen in swords — serrated.  Curved blades generally assist when making slicing cuts

Edge angle: Once the steel of a blade narrows to where the edge will be, a final edge is put on using various stones (natural or synthetic).  The quality of the steel and the blade geometry determine how fine the edge can be.  Edge angle is determined by measuring from center of the blade to the flat cut by the stones.  Knives, and most swords, have an edge angle of between 25 and 30 degrees.

Edge angle

Edge thickness: The thickness or width of the final edge.  It’s generally accepted that any thickness of a few one-thousands of an inch (1/1000 of an inch = 30 microns) can be classed as “sharp”.  Some materials and angles might even allow for a single 1/1000 of an inch (30 microns), but wear will reduce such an edge very, very quickly.  A good relative gauge of sharpness is the thickness of a sheet of paper (about 3/1000 of an inch thickness).  We’ve all had experience with paper cuts;  that thickness or less can well be considered the definition of sharp.

Edge smoothness: When sharpening a blade edge, the maker is limited by the size of the grit in the polishing material.  Simply put, while it’s theoretically possible to have an edge of 1/10,000 of an inch (.03 microns) thick, if the grit in your polishing material is 30 microns (1/1000 of an inch), it will be carving grooves of that width in your final edge.  Thus, super-fine edge smoothness requires careful polishing with the finest stones available.   Note that steel naturally forms a “burr” (a thin ridge of metal pulled to the very edge of the blade) during sharpening;  burrs create wear and prevent easy cutting.  You can test for a burr simply by running the edge of a piece of paper down the blade.  Where a burr exists, the paper will “catch”.

A fine example of what happens to the edge of a blade during sharpening is shown in the following video, in which a knife maker — using modern sharpening equipment — examines the edge of a knife at different times during the sharpening process.

 

 

Measuring sharpness

While today there are very expensive technological devices for measuring sharpness, very few weapon makers use these.  A surprising number of references fall back on old subjective tests to determine the relative sharpness of a blade.  In other words, test cutting.

These tests can include:  cutting paper, cutting a hair or shaving the hair from your arm, cutting rope, cutting silk, and any number of other tests which will be familiar to practitioners of the cutting arts from old manuals, legends and myths.

 

Dulling an edge

Many people think that a dulled edge comes from the edge of a blade being chipped and nicked, or that the fine edge is worn away, leaving a wider edge thickness.  This does happen to a certain extent, but in the vast majority of cases what happens is the fine edge of the metal folds over in one direction or another, in effect creating an even greater edge thickness than expected and thus more resistance when cutting. This folded edge requires the practitioner use ever more force to perform the same cuts, which causes even more folding, and so on.

A simple test for a burr or folded edge is to hold the blade up to the light and look down the edge;  gently move the blade back and forth under the light.  If you see individual flashes up and down the edge, those are folds reflecting the light. You can also use the paper-test mentioned above, but the light test will reveal even very fine folding  the paper may not catch on.

Does this mean the sword must be re-stoned?  Actually no:  the process of using stones to polish and sharpen a blade actually wears away material which subtly alters the shape (geometry) of the sword.  This is why so many people suggest that only a professional polisher/ sharpener be allowed to sharpen an antique blade.  Changing the weapon’s geometry may mean the entire blade angle will need to be redone to match.

A simpler and much easier technique to bring a blade back to sharpness is to use steeling or stropping techniques.  Steeling is most often seen being used by chefs who use specially made sharpening steels.  These do not wear the steel of the edge away, but actually straighten and re-align the folded edges back to a fine edge thickness.  Not a practice usually recommended with swords, as one needs to be able to run the sharpening steel down the entire length of the edge which maintaining the same angle and pressure.   In steeling a blade, you run the edge down the steel, with the edge facing the direction of travel.  Practiced knife steelers say that knives should be re-edged after every use, and that only two or three passes with the sharpening steel is required to re-align the edge.

 

 

Another, much easier to use technique is to use a section of leather to strop an edge;  this is the same technique used by old-fashioned barbers to re-edge straight razors.  Plain leather (tanned side up) can be used, or polishing compounds of varying grit sizes can be rubbed in to the leather to assist with the stropping.   There are a couple of major differences in stropping over steeling;  first, in stropping the edge is held down on the leather and the blade pulled away from the edge.  Secondly, it does take a bit more time to strop a sword-edge than when using sharpening steel;  stroppers say it can take 10 – 2o passes down the leather to realign the edge and remove any burr.  On the other hand, with the proper set-up you can easily make long stropping passes down the length of the blade, allowing better control of angle and pressure.

An excellent introduction to stropping for tameshigiri practitioners:

 

 

In the next few articles we’ll examine how the metal of the blade influences its cutting ability, and some of the historical elements of blade manufacture.

 

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