We know that Japanese knives and now some European knives come from the factory with acute angles of 15-18 degrees per side. Most of the European knives we buy such as the majority of the Henckels lineup come at 20 deg and some brands of knives can have an angle as high as 25 deg per side.
So what is the big deal, what's the problem with that you may ask:
There is no real problem, the reason that many knives you buy come with a 20 deg angle is that they (makers) know that the steel in the knife is not hard enough to support a more acute angle. The edge of a knife is very thin, that is what makes it sharp, if the edge of that knife is composed of soft steel, it only makes sense that it will not hold up to much abuse.
What is soft steel and how the heck can steel be soft in the first place?
The steel in knife is measured on scale of hardness, the Rockwell Hardness Scale, if you see a number like HRC 56 on your knife that is a medium hardness and very common. Now even one digit higher or lower on that scale represents a significant difference, so a less expensive knife have a hardness of 54 which is relatively soft, even if it is steel. Now Japanese hand made knives, the full carbon knives can have a hardness of 62-64 which is a very significant and the steel is extremely hard.
So if you have a "hard" knife it can be sharpened at very acute angles and stay sharp for a longer period of time, there is no way to say how long...it's just longer and it makes sense.
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| Japanese Nakiri - Full Carbon knife, very hard , 12 deg per side. |
So can we sharpen a soft knife at 15 degrees per side?
Certainly, we can sharpen it at 7 deg and it will have an incredibly sharp edge. HOWEVER, that edge will fail very quickly, on day one even, the first time it cuts anything.
Basically, unless you have a knife that is 60-65 on the scale of hardness, you don't want to go below 15 deg per side, it just won't hold it's edge. Also, keep in mind that there is price to pay for these very hard knives, they are brittle and can chip easily. If you dropped a 500 dollar Japanese carbon knife it could chip, the tip could break off or it could snap in half. If you drop a 50 dollar Henckels, it will probably just make a mark on the floor or nothing at all, unless you bent the tip.
Acute is good if the steel can handle it, otherwise, keep it at 19 degrees.
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| Henckels at 19 degrees |
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| Wusthof at 10 degrees - Edge Life Expectancy - 15 minutes |
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