A blunt knife only fulfills its purpose to a limited extent, because the proverbial sharpness stands and falls with the central characteristic of the millennia-old tool. And while everyone probably has a clear idea of what sharpness is, a technically sound definition is hard to come by.
A recognized industry standard for sharpness only exists with simultaneous evaluation of the property of edge retention (DIN EN ISO 8442-5 cutting ability and edge retention). Again and again, a different, subjective perception of sharpness can be determined by different assessors. All too often the sentence “That’s not very sharp!” was the introduction to the appearance of larger amounts of blood. In all the important treatises on the subject of steel, types of cut, sharpening, leather and whet, the sharpness that is the goal of all these arguments and efforts is rarely dealt with. A uniform conception of the term is assumed. In order to understand all further topics, the questions to be clarified first are: What is sharpness? On what factors does sharpness depend? How can the sharpness of a cutting edge be checked? What is sharpness supposed to do?
In the following, the phenomenon of sharpness is considered separately from the topic of edge retention – the ability of a cutting edge to retain its sharpness over a certain period of time. Complicated interactions between steel quality, grinding angle, abrasive grain and material to be cut are relevant for edge retention. However, this is only about a definition and assessment of sharpness as a condition at a specific point in time. What is sharpness? Sharpness is the ability of a cutting edge to cut material mechanically and in a controlled manner. A molecular connection is always separated. Non-mechanical and material cutting would be, for example, cutting with a flame cutter, a plasma cutter, laser or water jet cutting.
Uncontrolled separation, on the other hand, corresponds to tearing. In order to try to use a technical measure of how sharp a knife can get at all, in principle only the concept of the demolition stump is suitable. The demolition stump is the distance measured transversely to the cutting edge in the microscopic range – namely in the scanning electron microscopic range – from which a cutting edge can no longer be sharpened. We think of a cutting edge as an edge, in cross-section as a point. Under the scanning electron microscope, however, a cutting edge looks more like a rock ridge or, in cross-section, like a stump. The narrowness of this stump is limited by the fineness of the steel structure. Steel has a crystalline lattice structure consisting of nests of carbides. The cutting edge cannot be narrower than such a carbide nest. With a fine steel structure and careful grinding, the width of the stump can be less than a micron (also known as a micrometer and also written as μm or My). A micron is one millionth part of a meter. For comparison, an average human hair has a thickness of about 60 μm, a fine dust particle has a maximum of 10 μm.
On what factors does sharpness depend? The sharpness of a cutting edge depends on three factors: Angle: The sharpening angle is specified differently by knife and abrasive manufacturers, either as an absolute angle between the two sides of the cutting edge (e.g. as 40°) or as half of it, the angle between the abrasive and the center of the blade (20° in this example). The general rule for this influencing factor is: the flatter the angle, the sharper the cutting edge. Grit: The grit, or the grit for short, describes the diameter of the individual abrasive grit of the abrasive – also referred to as abrasive materials in manufacturing technology – with which the blade was sharpened. The general rule here is: the finer the grain, the sharper the cutting edge. A final leathering of the cutting edge with a grinding paste (which contains an extremely fine, rolling grain and removes minimal wear again) or untreated leather (which smoothes the cutting edge again without removing it) can further increase the sharpness. Uniformity: The uniformity factor relates to both angle and grit. The angle must be maintained evenly over the entire length of the cutting edge to ensure sharpness. The abrasive must have a homogeneous structure for sharpness. Based on the manufacturer’s empirical values, mixtures of grain sizes can also achieve a very good result. What is important, however, is an even quality and distribution of the abrasive grains, which must also be free of foreign inclusions.
How can the sharpness of a cutting edge be checked? A reliable and low-wear test to determine the sharpness of a knife is to place the cutting edge loosely on the thumbnail with its own weight. Different properties can be tested using two different procedures that build on one another: In the basic test, the blade is placed across the thumb nail held at an angle. If the blade grips immediately and does not slip, it is sharpened. If it slips, the knife should be set a little steeper again. When it then grips, the blade is fine and even, but sharpened at a relatively steep angle. The actual cutting edge had no contact with the horn of the nail, but slipped off at the transition edge from the cutting edge to the side of the blade. The blade is only dull if it slips at every angle. The extended test is often seen by experienced professional grinders to check grit and evenness of the cut. Here, the cutting edge is pulled vertically over the tip of the thumbnail without pressure. In this way, grain and small nicks can be felt immediately. However, some experience and a sufficiently strong thumb nail that is not cut too short are prerequisites for an injury-free performance! Both are largely non-wearing methods for testing sharpness. However, they are not suitable for straight razors, as the very fine cutting edge of the razor can be damaged by the horn of the thumbnail with all caution.
Although the above methods for determining sharpness can be carried out easily and without tools, it is surprising that people perceive sharpness so differently and often use less suitable methods for assessing it. The attempt to check the sharpness with the tip of the thumb is common. But sharpness can hardly be felt, especially with a fine-grained and cleanly ground blade without burrs. A coarser grain or a burr, on the other hand, can be felt and is then often mistakenly regarded as sharper. Another prominent example is the paper cutting test. Here it can happen that the blade has been ground cleanly with a fine grain, but has a too steep or large grinding angle. The cutting edge then acts like a wedge in hard paper and prevents it from penetrating the cuttings. With the same blade, however, the hair from the forearm can possibly be shaved in one stroke. The cutting test in paper should also be avoided because the cutting edge is particularly badly affected by paper.
What is sharpness supposed to do? The demands on a cutting edge can be diverse, but essentially the work with a cutting edge can be divided into two categories: With pressure cutting, the blade is pushed lengthwise through the material to be cut with little offset. This type of cut is done, for example, with razor knives or leather-working knives (such as saddlery or skinning knives). For this purpose, the blades must be ground at a flat angle and with a very fine grain. The print cut usually requires the highest sharpness according to the factors mentioned above. With a pull cut, on the other hand, as the name suggests, the blade is pulled through the material to be cut. Typical examples of this type of cut are cutting bread or cutting a thick rope. A cutting edge honed with a coarser grain, which has a certain sawing effect, is advantageous here. Blades that are only intended for these two extreme examples are of course usually provided with a special serrated edge. Which angle and which grit is optimal depends not only on the use and the respective material to be cut, but also on personal preferences. In terms of grit, a chef’s knife can be brought to a very fine sharpness with little practice, for example on a stone with a grit of 8,000 JIS and then leather. In a light crust of bread or a ripe tomato, though, some might prefer the sharpness of a quick trigger on a 400 stone. With the knowledge of the physical relationships, hopefully everyone can find their own optimum. Have fun with your sharp knives!
