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First is to pay attention to the angle. Unless the knife is badly worn and needs some serious grinding, you should use a slightly steeper angle than the original edge.

  1. Can you please indicate on my picture where "the original edge" is? What does "original edge" mean?

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  1. How do I calculate the "original edge" of WÜSTHOF 4596-7/20 Classic Ikon 8-Inch Cook's Knife? Do I need a protractor?

  2. At what angle ought the knife meet my Glass Stone? Please specify a number.

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    Go to the scrapyard a pick up some trash metal and learn to get a sharp edge. Then use an old knife and restore the edge. Once you know all this, you might get your expensive knife sharp again.
    – Johannes_B
    Jun 18 '20 at 5:08
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The "sharpening angle" of a knife is the angle between the sharpening surface and the blade. The 'original edge' is the angle at which the knife has been sharpened previously. Sharpening at close to/slightly below this angle will make sharpening the blade easiest, as you don't have to work hard to change the angle (which requires moving more material from the knife).

I could not find any info on the factury angle for the knife you mention, but most Western-style chef's knives are sharpened at an angle of about 20 degrees. Note that this is a "double-bevel" knife, so the angle will be about 20 degrees on both sides of the blade. See for example this page for some graphics and more info.

The original angle is not something you calculate. Either you find reliable information from the manufacturer, or you have to somehow approach this angle by trial and error. A 'trick' that I've seen some knife sharpeners do is to 'drag' the blade over the sharpening stone, slowly decreasing the angle until there is a change in how much friction you feel.

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I'm afraid you are asking the wrong questions here. But let's try answering.

  1. "The original edge" in this case is a slightly shortened formulation for "the angle the knife's edge had when it was originally produced".

  2. You don't calculate the original edge. In principle, you could try finding a data sheet for the knife or calling the producer to learn the exact number, but that number is quite useless when sharpening.

  3. I can't provide you with an exact number, and while I have read the writings of knife afficionados who care about that number, it seems that the exact number is a matter of personal preference for them. Anyway, knowledge of this number is also quite useless when sharpening.

To answer the question in the title: you don't deduce the angle. Deducing is a cognitive process. Sharpening is a motor skill. A person who can sharpen knives will lay the knife to the stone and recognize the correct angle by look and feel.

A person who is just now learning to sharpen knives will, of course, not be able to recognize whether the angle is correct. They - you - will just have to hold it at a roughly okayish angle and try sharpening. At this stage, you won't be able to keep the same angle for the duration of one stroke anyway. And the commenter you are citing really found the right way to describe how to recognize the "roughly okayish" angle - you have to aim to be at a slightly steeper angle than the angle of your original edge.

Take that comment literally and apply you while practicing. To start, take your knife, hold it at the stone in a position to sharpen, at an arbitrary angle, and now slowly change the angle in both directions while using your eyes and the physical feedback of the knife in your hand to decide when it is closest to the original angle, then make it the tiniest bit steeper than you can. Then try sharpening at that angle. You can repeat this adjustment process as frequently as you need, even for every stroke, until you notice that you can match the angle without needing it.

You can shorten your total learning time by finding a teacher who is sitting physically close to you, observes you, and can recognize when you are holding the knife at an improper angle. But a protractor cannot do that job.

Nowhere in the practice process is knowledge of a number helpful, unless you have some rainman-rare savant ability which lets you exactly visualize angles to sub-degree resolution when told their number, and hold something with your hands at the angles specified by that number. If you have that, you already know the original angle

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  • Thanks. Do you mind please indicating on my picture where "the original edge" is?
    – user24882
    Jun 17 '20 at 21:02
  • I'm curious as to why you recommend a slightly steeper angle. I have always assumed an angle 'close enough' (be it steeper or shallower) to the original edge would be the goal. And just as a note, knowing the approximate angle can be helpful even for non-savants. Knowing you should be sharpening at around 15-20 degrees rather than ~5 or ~30 is a start!
    – LSchoon
    Jun 17 '20 at 21:02
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    @Accounting If you look closely at the cutting edge of the knife, you'll see that there is a line where the big flat side of the knife changes to the actual cutting edge. It is the angle of this latter part of the blade that we're looking for. This part is actually marked 'The Edge' in your image.
    – LSchoon
    Jun 17 '20 at 21:03
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    @Accounting in your picture, the edge is marked - the label says "the edge", in the lower left corner. You cannot see its angle from that perspective and at that zoom. "The original edge" is not something you can photograph, it means the state in which the knife was when it was produced.
    – rumtscho
    Jun 17 '20 at 21:10
  • @LSchoon I have seen all possible opinions on how steep the new angle should be, from slightly steeper, to slightly shallower, to trying to give it a rounded profile, to deciding to create your new personally preferred profile from scratch... I went with "slightly steeper" because it is already part of the original quotation and because it is a good choice for a beginner. It doesn't have to be the only good choice. I think that applying a number in the way you describe is a hindrance, not an advantage - when you are engaging your attention in trying to visually match the observed angle ...
    – rumtscho
    Jun 17 '20 at 21:17

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