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How to tell if your knife is sharp?

How to tell if your knife is sharp?

Before anyone who owns or wants to buy a knife asks the question "How to tell if my knife is sharp?", they should find out what the sharpness of their knife actually is and what it depends on. Then they can proceed to various methods of testing the sharpness of the knife.

From the user's point of view, it is more correct to consider a knife sharp when the blade is easily able to cut through a particular material. Otherwise, the knife, or to be more correct, the edge of the knife can be considered blunt and the knife needs to be sharpened. Apart from that, you need to know and understand whether the blade of your knife is designed to cut a particular material or not.

To check the sharpness of your knife, it is necessary to understand how knives with a blunt blade become sharp and what the sharpness itself is in technical terms. It should also be understood that there are different opinions and techniques in professional communities on how to achieve sharpness, so sometimes beginners and even sophisticated knife users get confused about the terms.

For this reason, it is essential to know and understand what makes a knife sharp, how to test its sharpness and which test is the best, or at least acceptable, for your knife.

Looking ahead, one may recall stories from legends and that one of the possible benchmarks for sharpness were once the swords of the famous medieval Japanese blacksmith Masamune of the late Kamakura period or the later Muromachi period blacksmith Muramasa. All fans of blades, in one way or another, should be aware of the complexity of the Japanese sword-making process and their quality which ensures durability and sharpness.

On the other hand, a lot has changed nowadays, and some things in production have become radically different. Knives of almost every kind and type are mass-produced, which also raises the question of quality and sharpness.

One of such tests, which is quite simple to perform, is the "tomato" test. If the kitchen knife is able to cut a tomato cleanly, without piercing, then it can be considered a really sharp knife.

Now we have come to the point where we can talk specifically about the concept of sharpness itself and the methods of checking the sharpness of modern knives and which one might be suitable for your knife in certain circumstances.

  1. Technical notion, factors and sharpness criteria
  2. Sharpness testing techniques
  • Visual inspection
  • Paper test
  • Nail test
  • Tomato test
  • HHT test (Hanging Hair Test)
  • Edge-On-Up BESS test

 

  1. Technical notion, factors and sharpness criteria

In technical terms, sharpness is ensured by the convergence of the secondary bevels planes, which in turn form the cutting edge of the blade. Depending on how precisely this convergence of the two planes is formed during sharpening, the sharper the blade will be. In sharpening communities, this convergence of planes is known as the sharpening angle.

There are several parameters that can be considered important sharpness factors:

  • Grit size of the abrasive stone - the smaller the grit, the cleaner the surface of the blade
  • Grain structure and hardness of the blade material - the finer and harder the material, the sharper the blade can be, but the edge may be more brittle and such a knife should be used only for its intended purpose
  • Blade geometry - the smaller the sharpening angle, the less resistance the blade will encounter when cutting
  • Last but not least, the experience and skill of the sharpener

As a reader, you should know that among the sharpness criteria, knives can be broadly organised into 4 main groups:

  • blunt knives
  • knives with acceptable sharpness
  • sharp knives
  • razor sharp knives

Unfortunately, knives used in the household and in the kitchen can be regarded as blunt or, at best, of acceptable sharpness.

 

  1. Sharpness testing techniques

The visual inspection consists of detecting glare on the cutting edge. Typically, the cutting edge forms a very fine edge that does not reflect glare. This edge is much finer than a human hair and in some cases can reflect light, but it is simply impossible to distinguish this with the naked eye. It is sufficient to look at the cutting edge of the knife at an angle to the light source to detect glare. On a blunt blade, the glare will definitely be visible. These glare spots usually represent wear or dulling on the cutting edge, also referred to as rolling. These wear spots can be located in individual parts of the cutting edge or along the entire length of the blade. If this is the case, the blade should be resharpened or, in the case of minor damage honed.

The paper test can be done on different types of paper depending on what is available or if you want a better test result. The most common types of paper that can be used are office paper for printers, newsprint, tissue, toilet paper, checkout paper and even cigarette paper.

The purpose of this test is to verify a clean cut over the entire length of the cutting edge. Therefore, if the cutting edge is rolled or chipped, the blade will tear the paper. A blade in good condition can cut even, thin strips, both lengthwise and crosswise across the sheet. When cutting, the blade can be held either perpendicular or at a slight angle to the plane of the paper sheet. In general, the thinner and softer the paper and the cleaner the cut, the sharper the tested blade is.

Sometimes newsprint is folded into a semi-circle and the blade of the knife is drawn across the maximum convex surface of the paper. The aim is to position the blade and the cutting edge at the same level with the convex part of the paper. If the cutting edge is very sharp, the blade will pick up and cut off a layer of paper.

Picture 1. How to fold a piece of paper. 

Picture 2. How to cut (The cutting edge should bite into the surface itself).

How to tell if your knife is sharp? 2

There is also a test where a strip of paper is folded lengthwise in the shape of a letter "V" (Picture 3) and placed on a table. After that, they try to cut it with a top-down motion. A blade with a high initial sharpness will do the job without any problems.

How to tell if your knife is sharp? 3

If the blade copes with this kind of task and leaves a clean cut, then you can say that such a knife is really sharp.

In addition to such paper tests, sometimes you can roll the paper up into a tube and try to chop it. If the knife is not sharp enough or not properly sharpened, it will not be able to handle this test.

Another method is a nail test. According to some sources, this test is performed by placing the cutting edge of the blade perpendicular to the thumb and if the knife 'grabs' the nail while running along its plane, then it is sharp. If the blade slips, it means that there are blunt spots on its edge.

Another opinion is that the cutting edge can be checked by running the nail perpendicularly along the cutting edge itself. In some cases, there is a difference of opinion and the nail can be run both lengthwise and crosswise.

To have a test along the direction of nail growth, the nail must be trimmed, but not polished. Then, after placing the cutting edge on the corner of the nail, it is possible with some experience to feel whether or not the cutting edge is clinging to the nail. It should also be noted that flaws of the cutting edge with this type of test can be detected at the deburring or sharpening stage. If the blade has no flaws, the nail will glide smoothly over the edge.

At the finishing stage, however, it is best to perform this test across the line of the nail growth. This is a similar process to that of the paper test. If the blade starts to tear the material when checking on paper, the nail will slip at the rolled spot, and in the chipped spot the nail will get caught. When tested crosswise in the absence of defects, the cutting edge will bite into the nail.

When testing with this method, as well as with the fingertip, the results are rather subjective and one has to be very careful. You also need to have a certain amount of experience in this area.

Shave testing is generally used as one method of checking the sharpness of a razor. However, this method is quite applicable to blades of various kinds of knives. You should be aware that this test is mostly suitable for blades that are sharpened at a sufficiently sharp angle. The larger the angle of sharpening, the more difficult it is to perform such a test without experience. However, this does not mean that the blade is poorly sharpened. This may be the case with some types of knives intended for use in the outdoors. Therefore, the results of this test may be considered rather subjective and not always safe.

The tomato test is suitable for most knives, except for those with a deliberately large sharpening angle. This test can be considered most suitable for kitchen knives as well as thin fillet knives. To carry out the test, simply put the tomato on a cutting board and try to cut a thin slice without pressure. If you manage to do this, your knife can be considered sharp. As a rule, the cut is made vertically in relation to the surface of the table.

There is, however, a more sophisticated method that requires an even sharper blade than usual. To do this, cut a tomato crosswise and place one half on a cutting board. Then make a horizontal cut with the knife, so that the tomato itself does not move and you can cut as thin a slice as possible. If you manage to make this cut, then your knife is considered to be very sharp.

The HHT test (Hanging Hair Test) was developed a relatively long time ago and is used primarily to test the sharpness of dangerous razors, as objects that have arguably one of the thinnest cutting edges.

Among the many other questions relating to this test, one question arises: how practical is this test for checking the sharpness of the blades of various knife types, as achieving such sharpness on a knife is an almost impossible task and on the other hand such a degree of sharpness is simply unnecessary. With certain types of knives with very thin cutting edges, this test can be carried out if desired.

In the usual concept of this test, all types of slices can be considered as HHT and are on average carried out according to some sources at a distance of 10 mm from the holding point, according to other sources at a distance of 15-20 mm from the holding point. Among all the known methods it is worth highlighting 5 main ones.

And so let us look at 5 types of this test individually: HHT-0, HHT-1, HHT-2, HHT-3, HHT-4 and finally, HHT-5.

ННТ-0 (Shave). With this type of test, it is possible to confirm that any part of the cutting edge of a knife or razor allows you to shave hairs directly at the holding point. Many experts do not consider this test to be a complete HHT. This fact simply confirms that the blade is capable of shaving. As mentioned above, all others should be done between 10mm and 20mm from the holding point.

ННТ-1 (Violin). This name is derived from the faint ringing sound that some razors make when the hair touches the edge. This is due to insufficient sharpness in relation to the hair. The edge does not touch the hair scales and this can be felt in your fingertips.

ННТ-2 (Split). This test is quite simple - when pulled perpendicularly along the cutting edge, the hair splits or cuts lengthwise.

ННТ-3 (Catch and Pop). With smooth lowering and minimal pulling over the cutting edge, the edge cuts through the hair and it bounces back for some distance. 

ННТ-4 (Pop). The hair is cut off directly as it touches the cutting edge, but still bounces back..

ННТ-5 (Silent Slicer). The name of the test speaks for itself. The hair gets cut when it touches the cutting edge, falls silently and does not bounce back.

Moving somewhat away from the notion of sharpness, one of the main points of this test is that the hair cut itself does not tell you whether or not the dangerous razor will shave comfortably. It is also worth noting that much depends on the hair used, as each type of test has additional aspects, and the test as a whole deserves separate consideration..

For this reason, it is up to each person to decide for themselves whether or not it is appropriate to perform this test. This test can also be regarded as one that requires a certain amount of experience and theoretical background, so it may not be suitable for a wide audience.

Edge-On-Up BESS test

Of all the tests described above, the Edge-On-Up test with the BESS (Brubacher Edge Sharpness Scale) is perhaps the most technologically advanced and accurate, as the sharpness of your knife and thus the sharpening level is measured in numbers.

How does it work? There is a thin synthetic wire in the holder of the device which must be cut in one movement by positioning your knife according to the instructions. A scale located underneath the wire holder measures the force transmitted at the moment of cutting and this data is the result of the test, which is expressed in grams and checked against a table.

The BESS measurement unit represents the maximum load of the cutting edge of the knife when cutting certified material. Thus, the lower the applied force, the lower the value in grams and the sharper the cutting edge. The advantage of this device is the ability to evaluate the sharpness of an individual blade or to carry out a comparative analysis of sharpness when resharpening.

A further advantage over the following test is that the BESS test requires only the device itself and certified material to be cut and does not require any special knowledge or physical effort to perform. It is also the best because it is extremely fast.

The rope test

In order to test a knife on a rope, you must first understand what is being tested and what the test is based on. The rope is used to test the cutting edge of the knife, and with that, its sharpness. The basis for a good cut and for checking the durability of the cutting edge are probably 3 basic aspects:

  • the knife itself - the ergonomics of the handle, the type and size of the blade are taken into account;
  • blade steel - material and its thermal treatment;
  • sharpening profile - whether the sharpening angle is suitable for the task at hand, and the quality of the sharpening.

Different specialists may have slightly different methods in these tests, but they mostly come down to a number of conditions or combinations of conditions:

  1. Rope of a certain diameter wrapped with masking tape;
  2. The cutting cycle, i.e. the number of continuous cuts;
  3. Sharpening at a specific angle;
  4. The cut is most often determined with a single stroke and depends on the diameter of the chosen rope;
  5. Control by paper test;
  6. Counting the number of cuts before the blade stops cutting the rope;
  7. Sharpening with a specific sharpening system;
  8. Use of different abrasives and sharpening techniques;
  9. An underlay can be an end grain cutting board or just a wooden board.

There are, of course, a number of advantages of this testing technique, but it also hides certain difficulties. First of all, this test is very labour-intensive and takes a long time to accumulate a certain amount of comparative data. Apart from the knife itself and the sharpening, the results are influenced by the physical strength of the person doing the test and the quality of the rope. In other words, this test is designed mainly for knife makers and sharpeners.

All tests can be roughly divided according to the level of difficulty, financial cost and accuracy of the results. None of the tests can be called universal, simple or affordable, so everyone is free to choose which test is the best for your knife and for which task the knife is sharp enough.

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