While browsing around Bushcraft, survival and knife communities on-line, I often came across the description like "Shaving sharp!", "Scary sharp!" or "Crazy sharp!". All of these statements are some what correct and does describes the "Sharpness" yet it does not describe the sharpness in any measurable way. But still, I think it is totally fine as long as that statement is not attached to the knife's capability or try to state how good the knife is by it. The worst example of that I have seen was "What is the sharpest Knife out there?" or "This knife is the sharpest!".
What is sharpness?
sharpness, in my opinion, is basically "How well that edge can cut in to the object". In other words, "how thin the cutting edge is".
you may have read between the line of these description that the sharpness has nothing to do with the capability of the blade. Well... to be accurate, the quality of the the blade does have tiny part of affecting it but not as much as other factors.
Before continuing farther, please look at very short video clips below.
Sample 1
Sample 2
Sample 3
As you can see from these videos, all three blades can cut paper equally well to others. there are no visible differences of cutting ability between them.
Here are the sharpening method and specs of blades used in videos.
All the blades are sharpened with same equipment and method. The only difference is the cutting edge angle:
step 1 Fallkniven DC4 sharpener Diamond sharpening then ceramic sharpening
step 2 compound loaded strop.
Knives:
sample 1 YP Puukko
sample 2 Billnäs axe
sample 3 Blade made from Tuna can
Any surprise in here? Yes, the Tuna can blade can cut as good as the professionally made knife blade.
How is this possible? It is possible because the quality of the blade has nothing to do with the sharpness of the blade. It's all in "How it has been sharpen", how thin the cutting edge is.
Let's go back a bit and look at that statement, "the quality of the the blade does have tiny part of affecting....".
The "Tiny part" which affects the sharpness of blade is the hardness. Not a necessity, but certainly does have affect on "achieving" the good sharpness.
In theory, as long as one can make the edge very thin, then that edge is a sharp edge. But in practice, achieving that level of thinness is very difficult or in some case, near impossible.
But talking within the knife, made from some kind of metal or alloy, that factor is ignore-able or compensate-able by humanly possible skill of sharpening.
How do you measure it?
Well, to be honest, I am not sure how to measure the sharpness. Theoretically, it is "thinner the sharper" so to measure/compare the sharpness, we probably need to look the edge under microscope to measure the thickness of the very edge.
But that is not convenient method and not available for most of us, so for the moment, I guess we need to stick to "Scary sharp!! it cuts a paper!!!" :)
However, there are many ways to test it and here are some of my favorites.
- Observation test: Simply look at the edge through the reflection of light. This will reveal most of in-perfection of edge/sharpening. When the edge is good, you should not be able to see the ridge line shining!
- Paper cutting test: The most popular test which does not tell to compere one is better than others. But this test can tell generic sharpness by feel of blade cutting through, sound and condition of cut edge of paper. I use this method to check if any in-perfection edge left. if some part is not sharpened enough to cut, smooth cutting will stop on that spot of blade.
- Shaving wood test: This could actually tell how good the blade/edge is as quality of blade point of view, but I mostly use it to find the in-perfection of edge/sharpening. Shave the end grain of wood, if it is sharp enough for me, then the surface of end-grain reflects light. Also if there is any in-perfection on the edge, this will tell exactly where, by leaving not shining surface.
What is it good for?
At first, we are talking about a knife and a knife need to be sharp to do the job intended. If not, then that is not a knife any more. so, it is good to have it sharp.
Secondly, sharp edge will help you do the job faster and possibly safer too!
But we need to keep the usability in our mind as well. Thinner the sharper but thinner means less durable. I will talk about this topic some other time, because it involves edge geometry and other factors.
Why do we care?
or I should probably say "Why do I care" the sharpness?.
I care because there is bit of "My ego is bigger than yours" effect. It just is nice to hear from some one "man,,, your knife is sharp!" :)
OK, OK, talking serious. I think it tells the part of manufacturing quality. Let's say when you got two brand new out of box knives, and one is sharp and other is brunt. Which do you think is better made/quality? I think it tells the manufacturer's dedication and care in some level. In some case even the capability/skill of the maker. So, it is in some way important to check how sharp it is.
On the end, What should we do with sharpness?? Well, the sharpness of the blade is totally up to you! How shitty the condition or made the knife is, sharpening it will make that knife "Crazy sharp".
Even you have got super sharp knife out of the box, it will looses the sharpness soon or later. When that time comes, you need to sharpen it to get it usable again. So, don't get bothered with sharpness of a knife that much!
To be continued.
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