Which sharpening stone to choose: 5 tests

When sharpening knives, it is important to start with a stone of the correct grain size. Which grain to choose depends on the condition of the sharpening knife. The duller the knife, the coarser the sharpening stone and therefore the smaller the grain size. The grain size is gradually increased during sharpening to make the knife increasingly sharp. So, which of the stones should I begin with? And how can I know whether my knife is sharp enough? Below are some simple tests to assess the sharpness of a knife. Regarding the particle size, we use the  Japanese JIS standard.


Test 1: visual test

The first test is simply to observe the condition of the knife. After years of use the blade may be dented or chipped. These defects are disastrous for the cutting properties of the knife!

No visible damage to the blade? We go directly to the second test!

If the blade is chipped, start sharpening with a sharpening  stone of ± 0 to 220 grit  (extra coarse to coarse). This grain size eliminates chipping from the blade at high speed. And can also reform the cutting edge if necessary. As soon as the chips have disappeared, the second test can be carried out.


Test 2: light reflection test

With the edge of the knife pointing up, place it in front of a light source. We observe a reflection of light on the edge of the blade where it is blunt. No reflection on the wire? We then go to the third test! If we observe a reflection on the wire, we sharpen on a  stone with a grain size of ± 220 to 600  (coarse to medium). Be careful to sharpen the knife at the right angle! We sharpen until there is no more reflection of light on the wire to then move on to the next test.


Test 3: ballpoint pen test

The third test is done with a ballpoint pen. A ballpoint pen is placed with the tip on the table at an angle of ± 30 degrees from the vertical. Slide the blade of the knife held vertically over the pen. Without exerting any pressure, only the weight of the knife rests on the pen. The knife slides on the blade? This means that the knife is dull or that it is not sharpened at the right angle! It must be sharpened at an angle on a stone with a grain size of 1000 to 2000, means medium to fine, then the test must be repeated until the knife ‘bite’ the pen. Does the knife bite the pen at the start of the test? That means he’s pretty sharp! We can therefore go directly to the fourth test!

This Sakai Takayuki bunka hangs on the pen which means it’s pretty sharp!


Test 4: paper sheet test

One of the most common methods for determining knife sharpness is the paper test. The test is short and to the point, and you should always have a piece of paper on hand. Hold the sheet of paper (A4 for example) with one hand in front of you. An attempt is made to cut the paper in its entire length from the edge. If the knife crosses the sheet smoothly, we go directly to the fifth step! Does the knife get stuck along the way, misfit, or even fail to cut through the sheet? We will then choose a sharpening stone of grain ± 2000 to 3000  (fine grain). The test is repeated until the knife slides through the paper, without resistance.

Does the knife slide through the paper? This means that its sharpness is sufficient for any pocket knife, bushcraft knife or any knife with a rather thick blade. It is obviously always possible to give it a higher edge, however as such a knife is generally used for heavier work, it will lose this edge quickly. A kitchen knife is mainly used for light work and can therefore retain its sharpness for longer. How to get a superior edge? See you in the fifth test!


Test 5: Tomato test

Due to tomato’s relatively tough skin and rather tender flesh, it gives a good indication of the sharpness of the knives. Try to cut a slice of a tomato on a cutting board without pressure. 

Does the knife slide over the tomato? We will then choose a sharpening stone of grain ± 3000 to 5000 (extra fine). The knife cuts nice slices in the tomato without flattening it? This knife is sharp enough for normal kitchen use.


Here’s More!

Sharpening from a grain size of ± 5000 (ultra fine) removes virtually no material from the blade. It’s basically smoothing out any irregularities of the blade. This means that the knife offers even less resistance while slicing. Do not repeat the paper sheet test after this step or as little as possible! This can cause the knife edge to lose its sharpness quite quickly. Other tests exist to test the blades from this grind / polish grain size:

  • A well-known test is the shaving of the hair on the arms or legs. We pass the knife at a very low angle (almost flat) over the arm or leg and try to shave the hair, being very CAREFUL. If the shave is a little rough, it means that you can continue to sharpen with a larger grain size.
  • The nail test is comparable to the ballpoint pen test. Also be very careful with this test! Place the knife blade on the thumb and hold the nail at an oblique angle. If the knife hooks into the nail without slipping, it means that it is relatively sharp
  • Another easy test is the falling tomato test. Keep the blade knife high on the table and let a tomato fall on the cutting edge from about 30 cm high. If the tomato is cut in half without resistance, it means that the knife has an optimal edge! We can have fun dropping the tomato from a height lower and lower to find the limit of the ultimate cutting edge!


Polishing – Sharpening

Looking for a polished and reflective blade like a mirror? With a stone with a grain size of ± 8000 , you can create a mirror edge. Another possibility for obtaining a mirror edge after sharpening on a whetstone is to sharpen  the knife.

You should now normally be able to determine which whetstone to start sharpening with. Want to know more about the sharpening itself? Check out How to sharpen on a Coticule waterstone!

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