# What makes a knife sharp?



## joejoe (30 Aug 2008)

What makes a knife sharp, I know how to sharpen a knife and all that, but what actual makes the knife sharp? Hope I am making sense to a least some of you.

Joejoe


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## Caveat (31 Aug 2008)

Do you mean what constitutes "sharp" or do you mean what exactly is the physical process that renders something "sharp" ?

Or something else?

(What an amusingly unusual question BTW )


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## ClubMan (31 Aug 2008)

joejoe said:


> but what actual makes the knife sharp?


Sharpening it does.


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## Brendan Burgess (31 Aug 2008)

any help here:

_JoeJoe: There's a hole in my bucket, dear Liza, dear Liza, There's a hole in the bucket, dear Liza, a hole.

__Liza: Well fix it dear JoeJoe, dear JoeJoe, dear __JoeJoe__, well fix it dear JoeJoe, dear __JoeJoe__, fix it__

JoeJoe__: With what shall I fix it, dear Liza, dear Liza, with what shall I fix it, dear Liza, with what?

__Liza: With straw, dear __JoeJoe__, dear __JoeJoe__, dear __JoeJoe__, with straw, dear __JoeJoe__, dear __JoeJoe__, with straw._ 

This goes on in the same format with different lines:

_But the straw is too long.__
Then cut it.__
With what shall I cut it?
__With a knife_ 

_But the knife is too dull.
__Well sharpen it__
With what shall I sharpen it?__
With a stone_ _

But the stone is too dry__
Then wet it__
With what shall I wet it?__
With the water_ 

_But where shall I get it?__
In the well__
With what shall I fetch it?__
With the bucket

But there is a hole in the bucket dear Liza...
_


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## Remix (31 Aug 2008)

Looks like a team of researchers at UCD are on the leading edge of this topic.

They state quite bluntly that


But they make a good stab of coming up with a new definition.


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## joejoe (31 Aug 2008)

Remix said:


> Looks like a team of researchers at UCD are on the leading edge of this topic.
> 
> They state quite bluntly that
> 
> ...


 
more like the cutting edge.

Joejoe


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## joejoe (31 Aug 2008)

ClubMan said:


> Sharpening it does.


 
another pointless post by Clubman.

Joejoe


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## joejoe (31 Aug 2008)

uiop said:


> Maybe you can view it in terms of the equation below:
> 
> Pressure (Pascals) = Force per unit area (Newtons per metres squared)
> 
> ...


 
Ok that make sense, so how do gauge how sharp my kitchen knife is?

I find is amazing that a knife with so little pressure / effort can cut.

Joejoe


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## Niallman (31 Aug 2008)

I read in a cook book before that a rule of thumb is if you draw your blade across a tomato without pushing down on it and it cuts the skin then you have a suitably sharp knife. The key is to sharpen it little and often to maintain a good edge. I always give my knives a quick sharpen before each use.


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## joejoe (1 Sep 2008)

uiop said:


> It depends how you want to define 'sharpness' . You can define it according to the P=F/A equation. The 'sharper' it is, the greater the pressure at the point of application.


 
so how narrow is the edge of a very sharp knife?

Joejoe


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## DrMoriarty (1 Sep 2008)

Very narrow.


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## ClubMan (1 Sep 2008)

joejoe said:


> another pointless post by Clubman.
> 
> Joejoe


It answered the question as originally posed as far as I can see.

Glad you found _Brendan's_ nursery rhyme answer less pointless than my answer to your question.  

I don't know why I'm bothering but these links which took about ten seconds to find with  might be of interest:

[broken link removed]
[broken link removed]


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## MOB (1 Sep 2008)

I find a relatively cheap serrated-blade  knife (about the size of a standard bread knife) is the best for all-round kitchen work.  It cannot be sharpened, and has a limited life span of maybe 6 years in constant use, but it cuts beautifully, and passes the 'tomato test' effortlessly.


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## Purple (1 Sep 2008)

joejoe said:


> What makes a knife sharp, I know how to sharpen a knife and all that, but what actual makes the knife sharp? Hope I am making sense to a least some of you.
> 
> Joejoe


Buy  and let us know if it's any good.


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## bankrupt (1 Sep 2008)

Purple said:


> Buy  and let us know if it's any good.



I didn't think it was much good when I flipped through it at the book shop.

I could never get the hang of sharpening and tried many different tools (steel, wheels, stones etc), until I came across this:  http://www.amazon.com/Furi-Ozitech-Diamond-Fingers-Sharpener/dp/B000F8SIOW which works absolutely perfectly.  My knives are now like razors and sharpening is as simple as drawing the blade through this a few times before use.


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## Carpenter (2 Sep 2008)

This link gives a useful primer on sharpening theory (courtesy of Ron Hock, blade maker)

[broken link removed]

It's worth remembering that stainless steel blades (as most kitchen knives tend to be) are not the best material for taking and holding an edge- sharpenability and toughness suffers because of the metallurgical composition which renders them "rustless" (high carbon content aids sharpenability whereas chrome content (in stainless) is required for corrosion resisitance.  So there!


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## joejoe (2 Nov 2008)

Carpenter said:


> This link gives a useful primer on sharpening theory (courtesy of Ron Hock, blade maker)
> 
> [broken link removed]
> 
> It's worth remembering that stainless steel blades (as most kitchen knives tend to be) are not the best material for taking and holding an edge- sharpenability and toughness suffers because of the metallurgical composition which renders them "rustless" (high carbon content aids sharpenability whereas chrome content (in stainless) is required for corrosion resisitance.  So there!



Thats very interesting, cheers Carpenter.

Joejoe


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## Lollix (2 Nov 2008)

What makes a knife sharp? If you remove the blunt, what's left is the sharp. 
Simple, ask anyone in Limerick


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## mathepac (2 Nov 2008)

Lollix said:


> ...
> Simple, ask anyone in Limerick


As in "C'mere I wan'cha"?


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## Sarah W (2 Nov 2008)

When it slices through your thumb - requiring 5 stitches and a permanent loss of feeling - instead of the frozen cheese you were aiming at, it's sharp. Trust me on this one.


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