A dull knife is only partially useful, because the proverbial sharpness is the central characteristic of this millennia-old tool. And although almost everyone probably has a clear idea of what sharpness means, a technically sound definition is still hard to find.
A recognized industry standard for sharpness exists only when the edge retention property is simultaneously assessed (DIN EN ISO 8442-5 Cutting ability and edge retention). Furthermore, differing, subjective perceptions of sharpness are frequently observed among various assessors. All too often, the phrase "That's not very sharp!" has been the prelude to the appearance of copious amounts of blood. In all the important discussions on steel, grinding methods, sharpening, leathering, and honing, sharpness—the very goal of all these debates and efforts—is rarely addressed. A common understanding of the term is assumed. Therefore, to understand all further topics, the following questions must first be clarified: What is sharpness? On which factors does sharpness depend? How can the sharpness of an edge be tested? What is the purpose of sharpness?
The following discussion considers the phenomenon of sharpness independently of the topic of edge retention – the ability of a cutting edge to maintain its sharpness over a specific period. Edge retention involves complex interactions between steel quality, grinding angle, abrasive grit, and the material being cut. Here, however, we focus solely on defining and assessing sharpness as a state at a specific point in time. What is sharpness? Sharpness is the ability of a cutting edge to mechanically and controllably separate material. What is separated is always a molecular structure. Non-mechanical and non-material cutting methods include, for example, cutting with an oxy-fuel cutter, a plasma cutter, laser cutting, or waterjet cutting.
Uncontrolled cutting, on the other hand, is equivalent to tearing. To attempt a technical measure of how sharp a knife can actually become, the only suitable concept is that of the "cut-off stub." The cut-off stub is the distance measured perpendicular to the cutting edge in the microscopic range—specifically, in the scanning electron microscope range—beyond which the edge can no longer be sharpened. We imagine a cutting edge as a point, or in cross-section, a tip. Under a scanning electron microscope, however, a cutting edge looks more like a rocky ridge, or in cross-section, like a stub. The limit of the stub's narrowness is determined by the fineness of the steel's microstructure. Steel has a crystalline lattice structure consisting of carbide nests. The cutting edge cannot be narrower than such a carbide nest. With a fine steel microstructure and careful grinding, the width of the cut-off stub can be less than one micron (also called a micrometer and written μm or µm). A micron is one millionth of a meter. For comparison, an average human hair has a thickness of about 60 μm, a fine dust particle a maximum of 10 μm.
What factors determine sharpness? The sharpness of an edge depends on three factors: Angle: The sharpening angle is specified differently by knife and sharpening tool manufacturers, either as the absolute angle between the two sides of the edge (for example, 40°) or as half of that angle, the angle between the abrasive and the center of the blade (in this example, 20°). Generally speaking, the shallower the angle, the sharper the edge. Grit: The grit, or simply the grain, refers to the diameter of the individual abrasive grain of the abrasive material – also known as an abrasive substance in manufacturing – used to sharpen the blade. Here, the rule is: the finer the grain, the sharper the edge. A final strop on the edge with a honing compound (which contains an extremely fine, rolling grain and removes a minimal amount of material) or with untreated leather (which smooths the edge without removing material) can further increase the sharpness. Uniformity: The uniformity factor relates to both the angle and the grit. The angle must be maintained consistently along the entire cutting edge to ensure sharpness. The abrasive must have a homogeneous structure for optimal sharpness. Based on the manufacturer's experience, mixtures of grit sizes can also achieve very good results. However, consistent quality and distribution of the abrasive grains are crucial, and they must be free of foreign inclusions.
How can you test the sharpness of a knife edge? A reliable and low-wear test for determining a knife's sharpness is to gently rest the edge on your thumbnail using its own weight. Two different, progressively challenging tests can be used to check various properties: In the basic test, place the blade across your thumbnail, which is held at an angle. If the blade grips immediately and doesn't slip, it's sharp. If it slips, try applying the knife at a slightly steeper angle. If it then grips, the blade is finely and evenly ground, but at a relatively steep angle. The actual cutting edge didn't make contact with the nail but slid off at the transition between the edge and the side of the blade. Only if the blade slips off at every angle is it dull. The extended test is often seen performed by experienced professional sharpeners to check the grit and evenness of the grind. In this test, the edge is drawn vertically across the tip of the thumbnail without applying any pressure. This method allows you to immediately feel the grain and any small nicks. However, some experience and a sufficiently strong thumbnail that isn't cut too short are prerequisites for safe execution! Both methods are largely wear-free for testing sharpness. However, they are not suitable for straight razors, as the very fine edge of the razor can be damaged by the horn of the thumbnail, no matter how careful you are.
Although the methods mentioned above for determining sharpness are simple and require no tools, it is surprising that people perceive sharpness so differently and often use less suitable methods for evaluation. A common approach is to test sharpness with the tip of the thumb. However, sharpness is hardly perceptible by touch, especially with a fine-grit, cleanly burr-free blade. A coarser grit or a burr, on the other hand, can be felt and is then often mistakenly perceived as sharper. Another prominent example is the paper-cutting test. Here, it can happen that the blade, although cleanly ground with a fine grit, has too steep or too large a bevel angle. The edge then acts like a wedge in the hard paper and prevents penetration. With the same blade, however, one could perhaps shave the hair on the forearm in one stroke. The paper-cutting test should also be avoided because paper is particularly damaging to the cutting edge.
What should sharpness achieve? The requirements for a cutting edge can be varied, but essentially, the work performed with a cutting edge can be divided into two categories: In a push cut, the blade is pushed lengthwise through the material being cut with minimal offset. This type of cut is performed, for example, with straight razors or leatherworking knives (such as saddlers' or furriers' knives). For this, the blades must be ground at a shallow angle and with a very fine grit. Push cuts generally require the highest level of sharpness, according to the factors mentioned above. In a pull cut, as the name suggests, the cutting edge is moved through the material being cut with a pulling motion. Typical examples of this type of cut are slicing bread or cutting a thick rope. Here, a cutting edge with a coarser grit, exhibiting a certain sawing effect, is advantageous. Blades designed exclusively for these two extreme examples are, of course, usually equipped with a special serrated edge. However, the optimal angle and grit depend not only on the application and the material being cut, but also on personal preference. Regarding grit, a chef's knife, for example, can be brought to a very fine edge with a little practice on an 8000 JIS grit stone followed by stropping. However, some might prefer the sharpness of a quick honing on a 400-grit stone when cutting through a light bread crust or a ripe tomato. With an understanding of the underlying physics, everyone can hopefully find their own optimal setting. Enjoy your sharp knives!