The topic of edge retention is one that has been swirling around on various knife sharpening forums for many many years, it is a topic that I don't will ever end because there are no finite solutions, no magic formula exists to increase the life of an edge. Please remember, these are my opinions only, I'm basing what I say on years of experience, a decade of trying to figure it all out and I have finally reached a place where I no longer lie awake at night thinking about it, believe me, I've done that.
The reality is that as a knife sharpener, I have three duties that I should fulfil, tasks that will have my customers come to me for a sharpening service as few times as possible, to widen the gap between a full sharpening, to increase edge retention. Now a knife owner has many more responsibilities than I do and there are also some things that we can't control. The steel the knife is made of, I can't change that and yet it is the key factor to consider, the quality, the hardness of the steel that the knife is made of greatly influences edge retention. (I will talk about my responsibilities later in this article)
Here is the confusing parts, parts that I find confusing that is. We often hear the terms "polished edge" and "toothy edge" but what does this have to do with how long a knife stays sharp, I don't fully understand the science behind it but I will do my best to explain.
On one hand, we have folks, smart people who are convinced that a toothy edge on a knife will last longer than a polished edge. I don't know the answer but I don't think it has anything to do with the actual toothy edge (1,000 grit edge for example) as opposed to a 6,000 grit edge. Why would an edge that is not fully refined, a toothy edge, stay sharp longer and is it actually sharper or does it just "feel" sharper.?
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| This is not a toothy edge, it's just a picture of a knife I worked on. We can't see a toothy edge with our naked eye. |
Some people say a toothy edge lasts longer, some don't agree and will polish the edge with no fear of it impacting edge retention, in fact they do it to enhance edge retention.
Also, are we talking about all knives or just the average, "soft" knives, knives with a hardness of 54-56, which encompasses a whole whack of knives. And hard knives, 60-67, especially all those dream knives that fall in the range of 63-64, what difference does it make if they have an 8,000 grit edge, which many do, they still have great edge retention.?
Here are my thoughts on the matter, this is the way I solved the problem for myself. I have come to realize that a sharpener is almost powerless in his or her efforts to completely solve the edge retention issue, to make it go away because as I said there are only three things that I can do to promote good ER (Edge Retention) and yes I will mention them later on.
I do not understand why a toothy edge retains its edge longer and I am not saying it does. What I do believe is that a soft knife, if over refined, in other words, if I sharpened it up to 6,000 grit for example can experience a poor ER. This is because the area behind the edge, the secondary bevel, the area that supports the primary edge is reduced in width over time, with repeated sharpening which in effect is weakening the primary edge. There was a study by a group of engineers that determined this.
Here is the problem that I have with all of this:
Let's say we had 20 identical knives and sharpened 10 of them to 5,000 grit and 10 to 1,000 grit. Then, we cut the identical food products, or something until the edge failed. We did this with both sets of knives and then, maybe, we could see results that would lead us to believe that the 1k edge did indeed last longer.
This is all fine in a laboratory type of environment but once a knife leaves my hand, who knows how it will be treated. where it will be stored, how it will be washed and what type of cutting board will it be hitting on day to day basis. How will the knife be maintained, what type of Steel does the owner use, if any and is the owner have skill with a Steel or he is just slapping the knife against it and removing the established edge? (I remember bringing a freshly sharpened knife into a chef, it was extremely sharp and the first thing he did was use it open a heavy plastic wrapped cut of meat, that 30 seconds took the edge off.)
(Don't worry, I will provide what I think is the best edge in terms of sharpness and edge retention.)
Now from a physics point of view, a polished edge will not only slice better but will have improved edge retention. According to the author Chad Ward in his awesome book An Edge in the Kitchen:
He writes about Force equalling pressure over area and an edge 1,000 of an inch thick and one pound of pressure concentrates 1,000 pounds of pressure per square inch at the edge. On the other hand, a toothy edge is like a fine row of needles with each point coming under pressure, so these individual points are on their own and wear down and roll over. You cannot argue the science here. I am just telling you what he wrote, I didn't write it but I have to agree with it.
We are talking about kitchen knives here cutting food, not an edge designed to cut rope and cardboard.
Here are the three things that I can do as a sharpener to help with edge retention.
Number 3:
Sharpen the knife at an angle that is appropriate for the given knife. I sharpen hard knives between 10 and 15 deg per side and I sharpen soft knives between 15-20 degrees.
Number 2:
Finish the knife at the right grit, although the overall impact on edge retention is minimal in my opinion, when you consider the guy cutting a plastic bag open, whether I finished the knife at 1,000 or 2,000 grit is insignificant in terms of edge retention when you consider that the knife could be used on the wrong cutting board or chucked in a drawer. I finish soft knives anywhere from 1,000 to 3, 000 grit and hard knives can go up to 8,000.
NUMBER 1
The most important thing that I can do is EDUCATE people, tell them what not to do with their knives and manage their expectations when it comes to how long the knife will stay sharp.
In my opinion, the best way to keep a knife sharp is to ensure that it is thin behind the edge and the edge is clean when finished. This thinning reduces friction, a knife edge is under constant siege, it is subjected to a lot of pressure every day as it slices through a huge variety of products and then comes in contact with the cutting board.
All that I can do is sharpen the knife to the absolute best of my ability keeping the rule number 2 and 3 in mind and then help folks understand that the edge is a microscopically thin strip of metal and it will fail eventually, so treat it with respect.
Peter
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