02 January 2008

First, a bit of history ...

There's been lots of talk recently about sharpening knives and other toys with edges ... what to use, how to do the deed, etc. ... but we haven't really touched on what sharpness *is*, and how an edge can [for some purposes] be *too sharp* ...

If yer eyes are already rolling back in their sockets, just move along to another blog or continue on and kwitcherbitchin.

Ahem ...

What is sharpness ?

Let's start with this:

Sharpness is the condition of an edged tool which allows it to be used for it's intended purpose.

Vague, huh ? To an extent, it needs to be vague ... because what is sharp to someone using a knife as a utility tool for camping isn't the same as sharp to someone using a chisel or plane to carve or shape wood ... or the same as sharp to a butcher ... or the same as sharp to a barber using a straight razor to give a customer a super smooth shave.

Some folks have referred to 'slicing' sharp and 'pushing' sharp, to distinguish between the two primary types of cutting methods used with edged tools.

Both of these cutting actions test a different kind of sharp on an edged tool ... slicing tests the bevel or grind angle of the edge and pushing tests bevel as well, but adds a focus on smoothness of the edge.

Here's where it becomes really fun: the kind of edge you want for one test is in many ways the opposite of the kind of edge you want for the other test.

Let me illustrate a bit:

Say you're getting ready to carve the Thanksgiving turkey [or any other turkey with whom you have a grudge]. You want an edge with a moderately fine bevel [say 19 to 21 degrees], which will force the slices apart as you're moving the cutting edge through them ... and you want the edge to have just a bit of 'tooth' in order to slice through tissue membranes and not just mash them.

Hold that thought while we look at the other extreme ...

If you're a woodworking fiend [and I mean that in the kindest possible way], you need to have chisels & planes which are sharp ... as defined by the original bevel angle, and smooth as possible so as not to tear out chips or chunks of delicate and EXPENSIVE wood that you're carving or planing down to the correct size for your project.

Now let's look at what would happen if we tried to use a woodworker's dream edge to carve a turkey. The slices would tear and fall apart.

Huh ? Why would that happen ... ?

Because the edge is so polished that it will actually create a bit of suction as it cuts through the meat and that, coupled with the mashing effect it has on membranes because it is 'toothless' will give you terrible results for slicing tissue.

The flip-side situation is equally bad, because the fine toothed edge that serves us well for slicing through tissue will rip the heck out of those very expensive wood fibers, pulling chips & chunks out and causing the woodworker to scream blue murder and begin looking for fresh pieces of wood to finish his/her project.

So how do we solve these problems ?

By asking what the intended purpose of the tool is before we begin sharpening ... and using the appropriate sharpening tools &/or methods to do the job ... and by knowing when to quit sharpening the tool.

Assuming the your edged tools are in reasonable shape [MacYoung, this is NOT a safe assumption for you to make. EVER!], you won't need power tools or extremely coarse hones to begin the job. For kitchen knives, utility camping / outdoor use you'll want to start around 500 grit and finish at about 2000 to 2400 grit.

If your knives need a lot of work, you might start at 200 grit ... but be careful, grits that coarse take off A LOT OF METAL VERY QUICKLY. Let me repeat that, just in case it didn't come through clearly the first time:

... you might start at 200 grit ... but be careful, grits that
coarse take off A LOT OF METAL VERY QUICKLY.

Another factor which may come in to play with modern knives is the hardness of the steel. This isn't as big a factor as some folks would like to make it, but it can have an effect on how quickly you'll get the results you want.

Typical abrasives [hones, sharpening stones, etc.] are made from things like silicon carbide, aluminum oxide and various ceramic or clay binders with these [or similar] abrasive materials mixed in and fused with them.

These are great for steels hardened to about 60 on the Rockwell scale, but will be less effective on steels much harder than about 65 or so.

For steels hardened to those rarified levels [you won't find many, and in my not-so-humble-opinion, they're not worth the trouble] you'll want a different type of abrasive material: industrial diamond or cubic boron nitride.

Don't worry about these for now, just be aware that the folks who want to sell sharpening gear to you will push these little darlings at you because they can ... and because they're EXPENSIVE.

Ok, back to the real world. Good quality silicon carbide [Norton Crystollon stones] or aluminum oxide [Norton India stones] are fine for most applications ... and if you want to get fancy, you can use waterstones [ceramic or clay binders with these same abrasives] in the appropriate grit ranges to do the work.

How to choose ?

How much sharpening do you do ?

How much fussing are you willing to do to get ready for sharpening ?

Norton [or similar] stones should be used with light mineral oil as a lubricant while sharpening ... and gently rinsed & lightly re-oiled after use before you put them away.

Waterstones need to be soaked in water before use, and use water as a lubricant during sharpening. They also required gentle rinsing before they are air dried and put away ... AND they will require periodic re-flattening of their surfaces in order to maintain a useful working area.

The reason for this is that the binding materials which hold the abrasives in these stones wears away during the sharpening process.

This wear is responsible for two of the better qualities of waterstones: they cut quickly, because fresh abrasive is constantly being exposed; and they polish the edge while they are sharpening, because the binder material which is wearing off during the sharpening process mixes with water and forms a paste which lubricates and also contains small amounts of the abrasives which are being released by the wearing of the binder.

Which one is right for you ?

If you just want sharp edges and don't want to spend too much time maintaining your hones, go for oilstones.

If you feel the need to become somewhat fanatical about your sharpening gear, try waterstones ...

Either way, you'll have the basic tools required to get edged things sharp.

But what about other tools ... ? Ceramic hones, diamond hones, strops, ScarySharp, etc. ... ?

Ceramic hones can be very good, and I've recently been told about some which sound excellent ... but they won't do a 'better' job than the oil or waterstones I've already talked about.

Some folks like them because they've been told they can be used dry. True enough, but they work better with water as a lubricant ... and they still need periodic gentle cleaning to get the ground off metal out of their surfaces, so it all balances out in the end.

Diamond hones are lovely, and I own and use several for specialized tasks [like fixing serrations] ... but unless you're working on this kind of stuff all the time or have knives / tools made out of the previously mentioned super hard steels, save your money for buying more knives.

Strops are lovely, and are an absolute necessity for putting the final polish on straight razors and some superfine woodworking tools ... but again, save your money unless you use these tools or plan to make a go of sharpening as a business [not something I would try, but you might be able to do it].

ScarySharp ... a great concept, but it's primarily good for woodworkers who have tools with rectilinear edge geometry and large edge bevel area.

It was designed for woodworking tools and not for knives, and when one attempts to apply it to knives ... well, it just seems to fall short.

At least for me ...

Ok, I've spewed long enough ... chew on this and I'll just wait for the questions [and comments] to start coming in.