Testing Sharpness with the Thumb

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Noesis

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On occasion, I sharpen knives for local restaurants. Every time I finish a knife, there is always some arrogant redneck who will grab a knife I just finished, rub his thumb laterally over the edge, and act suspicious. Mind you, these workers are not the chefs themselves. Some of them are totally removed from the food preparation of the restaurant. If they voice any disapproval, I usually grab a piece of paper and cleanly slice off a strip to show them that the knife is sharp. Some still act suspicious, because the edge doesn't "feel sharp" to them.

How accurate is the thumb testing method? These people are hurting my business and I need to be able to explain to them why they are not the experts they pretend to be. Should I recommend the thumb-nail method instead?

I am beginning to think that in the world of rednecks, this ability to "feel" whether a knife is sharp is one of those basic abilities that is judged essential for a man to have, like being able to tell when a quarterback should have called a pass instead of a run, or being able to operate a tractor. Every one I encounter seems to think he has it mastered.
 
Very coarse edges catch the skin strongly and this is the type of edge a butchers steel produces and thus it is what many are used to in kitchens and thus it is what they expect. It is also actually sharper for a lot of slicing than a very high polish, but not what you would want for dicing/chopping. Find out what they are usually cutting, make sure your blades are optimally sharpened for those materials and then be willing to demonstrate and be confident in encouraging them to do so.

-Cliff
 
IMHO running the thumb laterally over the edge doesn't tell you anything about sharpness at all. It is a useful method to detect a burr if you run it more from spine towards edge laterally, but I prefer to use the pads of my fingers. Running the thumb across will produce a sensation when you have a burr, a rough edge or a coarse edge. Completely meaningless.

Running the thumb or better the fingers along the edge (obviously with very little pressure and only milimeters at a time) a sharp edge will grab the skin and will try and slice you. I use that method a lot. It will also tell a coarse edge from a polished edge. Even more sensitive is doing the same with wetted thumb or fingers. This is, btw., also the technique Murray Carter suggests and that is recommended in the Barbers Manual from 1950 for the test of a stropped straight razor. In an intermediate edge, I will also run a fingernail over the edge but only to check for nicks and rough edges.

I concure with Cliff. Take pride in your skill and do some stunts for them carving news paper, shave some armhair, cut some cigarette paper etc.
 
I'v found that anytime someone doubts the sharpness of one of my knives, i just shave about a square inch of hair off of my arm- this usually convinces them.
 
Brad Greulich said:
I'v found that anytime someone doubts the sharpness of one of my knives, i just shave about a square inch of hair off of my arm- this usually convinces them.
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I did this at the first few restaurants I went to, and now I look like I'm getting ready for a swimming competition. My legs are next, I guess.:D
 
If you're sharpening at the restaurant... what about having them bring out something to slice or chop? Seeing it work on what they use it for... ought to be convincing enough. Slicing paper may not be as convincing.

For the 'thumb tester'... after he finishes his 'test'... tell him "OK for the next test, put your thumb on this cutting board......" :)
 
Tell them that knives don't cut laterally--only longitudinally--and have them check the sharpness in that direction (carry band-aids).:D
Greg
 
While thumb testing is a pretty crude test, I think it has its place, especially in the real world of ordinary people (non-enthusiasts) who let their knives get quite dull before sharpening, and then don't need or expect extraordinary sharpness just a half-way decent working edge.

The skill of the "thumb tester" is a factor, as is how much sharpening they do. I can tell with my thumb a blade that will push cut paper and shave reasonably well. Whether the ordinary guy I described in the first paragraph can, I have my doubts.....
 
HoB said:
IMHO running the thumb laterally over the edge doesn't tell you anything about sharpness at all. It is a useful method to detect a burr if you run it more from spine towards edge laterally, but I prefer to use the pads of my fingers.
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Ditto. Fingers are a lot more sensitive than the thumb. I make a point to check both sides of an edge, too, since a burr may hang to just one side.

I don't think it's the most accurate method, certainly, but the edges that catch my skin and try to dig in on even light pressure are the same edges that shave thin slivers off newsprint or fillet magazine pages, while a dull edge will slide over the skin smoothly; so, there's something to it!
 
Dog of War said:
The skill of the "thumb tester" is a factor, as is how much sharpening they do. I can tell with my thumb a blade that will push cut paper and shave reasonably well. Whether the ordinary guy I described in the first paragraph can, I have my doubts.....
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Good point. It only really works if you know how a truly sharp knife feels, which most non-knuts don't.
 
The only way to test sharpness with your thumb is to use the nail. Place the edge lightly on the nail (perpendicular) and try to move the blade sideways. A sharp edge will dig in and resist the side to side movement. If the edge digs in with just the weight of the knife, it is shaving sharp. You could use a Bic pen in place of your nail. Once I get to this stage, I go on to the finer stones with light pressure to polish the edge.

I can't feel a burr with the pad of my thumb but I can by dragging my nail downwards. It seems like each of my thumb pads have different sensitivity. If I scrape downwards with a fingernail I can feel a wire edge. A few swipes on the stone at a slightly increased angle usually takes care of that.
 
Noesis said:
On occasion, I sharpen knives for local restaurants. Every time I finish a knife, there is always some arrogant redneck who will grab a knife I just finished, rub his thumb laterally over the edge, and act suspicious.
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Knife sharpening is one of those things that will always attract a jerk if one is in the vicinity. I'd simply tell them "the knife is sharp, you just don't know how to test it" without even looking up from what I was doing.
 
The last guy that I saw do this "thumb test" had blood dripping from his digit before he had made his determination on whether or not the knife was sharp. :D


FWIW - My advice on kitchen knife sharpening is to thin all bevels and bring to a high polish. With rare exception, pro cooks/chefs will love to work with your edge when presented in this condition. A super high polish is good for both push and slice cutting for almost every task found in the kitchen.


--Dave--
 
A super high polish degrades extremely rapidly with use on a cutting board on the very soft stainless steel knives used in a kitchen and in a short period of time will fail to be able to slice a tomato readily, start to slip on meats, trimming fats, etc. . Different knives of course do different things, some are almost focused choppers or push cutter like paring knives and some do almost all slicing like bread knives or carving blades so they are not all going to be optimal with the same finish.

-Cliff
 
Thanks Cliff, I'll mention this to the professional chefs I service, they likely don't know what they want. :p


--Dave--
 
On a more serious note...

Any edge degrades quickly when it's on a cheap stainless knife. The difference between the polished edge and the toothier edge is the level of performance provided while the edge is still serviceable.

When sharpening professionally, the idea is to provide the best possible edge that will perform at the highest level for the longest period of time. It's a comprimise for sure, but when all factors are weighed, the polished edge wins everytime.


--Dave--
 
Most are likely to just repeat known mantras which are little more than myths. Furi-Tech has done extensive research in this area, specializing in sharpening knives for kitchen cutlery by noting the effect of wear on kitchen cutting boards both under a microscope as well as having the sharpness tested by CATRA. The work just replicated what Swaim did over a dozen years ago who did work as a professional cutting fish, not that it matters of course, someone in the food fishery (non-commerical) would notice the same thing if they did the same work.

As noted it depends on the knife. Many Japanese kitchen knives are generally recommended to be sharpened at a more coarse finish, 1000 grit is common which is fairly coarse for a japanese stone. In contrast most meat cutters prefer very fine finishes and tend to use smooth steels frequently. At the extreme end people who work on breads will prefer a much more coarse finish. I leave them at 80/100 grit AO for those who work for the crustier breads. The Spyderco mediums work fine for normal breads but have a much lower edge retention than the really coarse belt finish.

Of course it is hardly the case that a chef's has intrinsically more information about food preperation than an ordinary mother preparing meals for her family or has an intrinsic greater standard of cutting ability and sharpness. This is more dependent of the nature of the individual than the job. The most demanding person I know in regard to discriminating sharpness and cutting ability for food has a severe physical handicap and thus can exert very little force when cutting.

-Cliff
 
D_R_Sharpening said:
Any edge degrades quickly when it's on a cheap stainless knife.
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Relative to a more optimal grade of steel of course, however with a suitable level of finish the edge retention can be very long even on the low end steels and easily exceed a high end steel with a less optimal finish. To be specific, AUS-4A would readily outslice S30V with the finishes at 90 silicon carbide vs mirror respectively. By this I mean the S30V blade would start slipping on softer foods far faster. You can even take a mild steel bar and slice foods for a very long time if the edge is thin and left very coarse. Essentially the amount of wear and deformation which needs to take place to reach the same extent of blunting is proportional to the size of the micro-serrations.

The difference between the polished edge and the toothier edge is the level of performance provided while the edge is still serviceable.
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The difference is in both the initial cutting ability and the edge retention, the edges are both sharper and last longer. Mike and Joe both showed this in detail about a dozen years ago. Mike was a professional who cut fish, Joe wasn't, that of course makes no difference. You can read the same in Lee's book on sharpening who covers kitchen knives specifically. Many makers known for high cutting ability and edge retention, who make knives specifically for foods are similar in respect to edge finishing grit, Dozier, Boye, Wilson, etc. .

Not every kitchen knife is of course a dedicated slicer and not everyone cooks the same foods or even cuts in the same manner. It also depends on the type of steel in the knife and how it was hardened. D2 doesn't even respond well to high polishes at low angles and will just break apart while M2 excells there. Thus M2 makes an excellent meat knife or small paring blade while D2 works well as a utility slicer staying much sharper cutting through stacks of rhubarb.

-Cliff
 
Cliff Stamp said:
Many Japanese kitchen knives are generally recommended to be sharpened at a more coarse finish, 1000 grit is common which is fairly coarse for a japanese stone.

-Cliff
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I was going to respond to all your obsurdity but thought better after considering your above statement. I figure that anyone who would make such a statement knows not what they speak of. I have little time to waste on educating you.


--Dave--
 
I've been testing edges with my fingers for close to 50 years. I wouldn't dream of using my thumb (that is the sign of an amateur). I use the pads of the index, middle and ring finger of my left hand. By spreading the fingers a bit I can do a pretty good survey of a three inch strip of edge at a time. What I am feeling is the drag on my finger prints. It gives me a pretty good read on how well an edge will do at slicing (draw cutting), particularly how it will do on things like leather or flesh.

If a smooth edge does not feel sharp using this test it either has not been honed all the way to a thin edge (it needs more honing) or the edge is not honed at a sufficiently low angle. If your edges don't feel sharp to the touch they may indeed not be honed optimally. This is commonly due to honing at something absurd like 20 degrees per side (40 degree included angle). There is no reason to put that obtuse an edge on a kitchen knife. I would do most of my honing at 10 degrees per side with a few light finishing strokes at 15 degrees or less.

Another way to get a good slicing edge that feels sharp to the touch is to leave a rougher finish on the edge. With a rough edge finish you can get aggressive slicing even if your honing angle is rather obtuse. This provides a microserration pattern on the edge. I used to do slash testing with my fighting knives. I found that thick blades that were sharpened to somewhat high honing angles did not feel sharp or slash well. If I went back and did minor refinishing of the edge using a file it both felt sharper and slashed more effectively.

In conclusion I would take another look at any kitchen knife that doesn't feel sharp. It may really be telling you something.
 
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