Sharp Knives and Tomatoes

[Craig James]

So here’s a silly thread for a rainy (in the UK) Saturday afternoon, on a topic that has niggled me for ages.

What the hell is it with people slicing tomatoes on YouTube? At the point that my knives need sharpening they’ll still slice a tomato...

A more serious point on this as well. People always claim that a less refined edge is better for cutting a tomato as it doesn’t ‘slide’ off the skin. Well my knives in the kitchen sharpened to 6000 JIS slice a tomato skin just fine, better than fine in fact. Just to note I’ve also just dropped a tomato from a couple of inches above the blade on a newly sharpened knife and one that needed sharpening. Surprise they both cleanly slice said tomato.

yes - I’m sitting here in lockdown and I’m bored, hence why I’m talking about tomatoes.

have a good weekend!

 

[Obsessed with Edges]

Tomatoes are actually pretty easy.

Either of two things can make it so: (1) a toothy finish, which makes it easy on thicker/wider-angled edges; OR, (2) very thin geometry at & behind the edge, which almost renders moot the edge finish itself. As the edge becomes thinner in geometry, either a toothy finish or a polished finish can work very well, as the cross-sectional geometry makes the bigger difference there.

So, in a nutshell, one can take a thick-edged, factory-finished blade and add some toothy bite, to a complete apex, to make it work in a hurry. Or, one can take some time to thin the grind at & behind the edge, after which simply apexing the edge cleanly at most any finish will handle tomatoes like it's a walk in the park.

The one thing I've noticed, that can halt tomato-slicing in it's tracks, is an incomplete apex on the edge. That initial 'bite' needed, to keep the edge from sliding across the tomato's skin, is what makes most of it work.

 

[bemymonkey]

I had trouble with tomatoes for a while when I was still learning to sharpen freehand properly - I was getting a very smooth apex that cut even super flimsy newsprint very nicely but wouldn't push-cut through a tomato or bell pepper skin. That issue cleared up as I learned to refine my edge properly as I progressed through my grits instead of dulling the apex I'd built with each progressively higher grit.

I'd assume a lot of the people amazed by a thinly ground knife slicing sideways through a tomato stuck to a cutting board are stuck in the same rut I was in.

 

[maximus83]

Ask yourself why are so many 'tomato knives' sold with a serrated edge? There is one answer.
https://www.knifecenter.com/kc_new/store_store.html?usrsearch=tomato+knife

You can literally take a 120-grit coarse stone, and using extremely light strokes to finish up the apex, you will have no problem easily slicing tomatos. Even with that coarse of a grit. Because if you look at that edge under high magnification, it looks like a saw blade. Of course in practice, you don't need such a coarse edge to have a good toothy edge that will slice. For kitchen knives, I like to use a toothy edge finished on a SiC or AlOx stone (examples: Norton India, Baryonyx Arctic Fox) in the 300-400 grit range. Sometimes I add a micro-bevel too at a higher grit, but that is not necessary to get a good slicing toothy edge. And you can have a 'toothy' edge finished in much higher grits, too.

 

[Craig James]

I prefer my kitchen knives significantly finer as the majority (bar those I use for breaking down meat - which I finish at a lower grit) are used predominantly for push cutting and slicing.

my point really is that a refined edge does just as good a job as a toothy edge, actually better as it is less likely to saw and rip the product be it tomato or a different veggie. Granted you can only take this so far - a dull bread knife per your point is still going to be infinitely better than a butter knife

 

[WhitleyStu]

I usually ask new customers what kind of edge they want. If I get a blank look from them I explain the difference between a razor edge and a toothy edge. I use the analogy that you wouldn't try to cut a 2x4 with a razor blade and wouldn't cut a tomato with a saw. Now there are exceptions to this analogy. Something as soft as a tomato would do better with a slightly toothy edge, but something more firm such as an apple or orange will cut very nice with a very sharp edge.

 

[maximus83]

I prefer my kitchen knives significantly finer as the majority (bar those I use for breaking down meat - which I finish at a lower grit) are used predominantly for push cutting and slicing.

my point really is that a refined edge does just as good a job as a toothy edge, actually better as it is less likely to saw and rip the product be it tomato or a different veggie. Granted you can only take this so far - a dull bread knife per your point is still going to be infinitely better than a butter knife

As with most things sharpening, there's probably several approaches that work. Just an interesting but maybe relevant point that struck me: if you applied Cliff Stamp's approach, in a sense you could have the best of both worlds. As I understand it, Cliff's approach would look something like this:

1. Put a super acute secondary edge bevel on your blade (Cliff would go as low as 5 to 8 dps, I wouldn't go that low myself, but whatever you're good with, let's say 10 or 12 dps). Refine and finish that bevel so that it's more polished. Goal: be nice and thin behind the apex, and also refine that secondary edge, so you can glide thru whatever you're cutting.
2. Put a toothy apex edge (aka: micro) bevel on your blade that's a few dps higher. If you went 10 dps on the refined secondary bevel, say you go 15 dps on the micro. Keep this one relatively more coarse than the refined secondary for good slicing aggression.

Why? Well if you buy into Cliff's studies (for example, here), he finds a few really interesting points:
* In general, having a micro-bevel at a higher angle than the secondary, increases the strength of the edge. And as a side benefit, drastically eases maintenance because you only have to resharpen or strop that tiny micro-bevel, you don't have to resharpen the whole wider secondary bevel every time).
* In general, "edge retention increases as the micro-bevel angle decreases." So you still want a LOW micro-bevel angle for best results, but one that is still higher than your secondary bevel angle.
* Having this setup that combines a refined secondary bevel with a coarser/toothy micro-bevel, should give you simplified maintenance, better overall edge retention, and really good cutting performance. Again, if you accept his data.

I have not tried this exact approach myself, but I do respect Cliff and think this would be an interesting experiment to try. Just to see if one could combine the benefits of thinning your blade stock quite a bit in the secondary, refining that secondary, and then cutting a coarse micro-bevel. Curious to hear if anyone here has tried Cliff's approach and what the pros and cons were.