A PRIMER ON KNIFE SHARPENING
by Steve Bottorff
color photographs by Carol Butz
I really admired my grandfather, who was a doctor, pharmacist, hunter and
gardener. I especially admired two of his nonprofessional skills - even
when he was in his nineties he could put a razor edge on a knife, then use
that knife to put a perfect point on a pencil.
I learned to sharpen with hand stones at Grandpa's knee, but I always
had trouble with certain knives. For years I searched for the ultimate
knife sharpening method. I realized that I might not have the skills
required so I was willing to use whatever gadgets and machines I could
find. Testing over 30 different knife sharpeners taught me what works and
what doesn't, and I decided to write this article and share the
information. BTW: I gave up on sharpening a pencil with a knife and
bought a pencil sharpener.
The instructions that come with sharpening equipment is often
inadequate. Some give no instruction at all. I note in this article when
equipment comes with good instructions.
There are two schools of knife sharpening - those who like a knife to
keep some roughness from the stone and those who believe that it should be
as smooth as possible. Both approaches have their benefits.
Blades with a rough edge can be aggressive cutters, especially when the
blade is thin. They have micro-serrations that act like a
microscopic saw. These micro-saws are very well suited for slicing
fibrous material, such as a rope. This edge is easy to produce because you
just stop sharpening after a medium stone (200 to 300 grit). Blades
sharpened this way do become dull faster as the points wear or bend, so
frequent touch-ups are needed.
Smooth edges are best for cutting with a straight push and are preferred
by barbers, surgeons and woodworkers. Research done by John Juranitch of
Razor Edge Systems (1) shows that butchers can cut more meat per shift and
tire less when using a smooth edge. Analysis with an electron microscope
(2) has confirmed that wood cutting ability is correlated to edge
smoothness. Sharpening a smooth edge requires more work, but the results
are worth it.
To be sure you are improving your sharpening; you need an objective way
to test the results. Tests evaluating sharpness range from cutting silk to
chopping trees. What you need is a test method that are useful in your
workshop as you are sharpening. A major knife maker tests
sharpness on nylon paint brushes.
Most people test an edge by rubbing their thumb lightly across the edge
and feeling how the edge grabs as it tries to cut into the thumb pad. To
keep your thumb calibrated, test a known sharp edge like a new razor blade
Shaving hair on your hand or arm is another common sharpness test.
Shaving sharpness can be achieved even on heavy hunting knives or an axe.
I own a hunting knife that will shave even though the edge angle is a
rather blunt 30 degrees. I use the term shaving sharp to describe this
degree of sharpness and razor sharp to describe even greater
sharpness. Razor sharpness is comparable to a razor blade and
will literally pop the hairs off your hand or arm. Razor sharpness is only
possible with both a polished edge and a small edge angle.
Testing by shaving can be misleading if the blade has a burr or wire
edge. Steel naturally forms a burr - a thin bendable projection on the
edge - during the sharpening process. A blade with a burr will shave but
will not stand up to hard use. To test for a burr, slide your fingertips
lightly from the side of the blade over the edge. You will feel the burr
drag against your fingers. Test from both sides, because burrs are usually
bent over one way or the other. As your sharpening improves you will be
looking for smaller and smaller burrs.
The glint along this edge means a dull blade.
Many good sharpeners, including my grandfather, have learned to see a
dull edge. Hold the blade in front of you with the edge in line with a
bright light. Move the blade around a bit. A dull edge will reflect a
glint. Nicks and burrs will also cause glints. When the blade is sharp
these glints will be gone.
I had a eureka moment a few days ago. I was standing in my kitchen and
say a single strand of a spiderweb in our back yard, about 30 ft away. Now
I know that a spider web of about a ten thousandths of an inch, so at 30
ft it is well beyond the resolution of the human eye, let alone through a
screen door. But as a light source reflecting the sun it is perfectly
visible. (It is the same with stars, they are too small to see them but we
can see the light.)
Then it occurred to me that this is how I see the old dull edge of
knives and scissors even without magnification. A bright light is the
secret, coupled with rolling the edge so you see every possible angle
between your bevels. Any glint of light as you roll and you have found the
dull spot. Eureka!
Another test for sharpness is to press the edge lightly on your
thumbnail at about a 30-degree angle. If it cuts into your nail it is
sharp. If it slips it is dull. The sharper the blade, the smaller you can
make the angle before it slips. Try this with a new razor blade to see how
a really sharp blade feels. The down side of thumbnail testing is that the
little cuts in your nail get dirty and look bad until the nail grows
out. For this reason some people do this test using a plastic pen or
No shop is complete without at least one bench stone, preferably two or
more of different grits. I recommend you buy the largest sharpening stones
you can afford. Stones for shop use should be as long as the longest knife
you plan to sharpen. Remember that Momma probably owns the really big
knives around the house, and you will be expected to sharpen her 8 or 10
inch butcher knives. Smaller stones are handy for field use. Large
tool suppliers such as MSC or McMaster-Carr and restaurant suppliers are
good sources for sharpening stones.
Natural sharpening stones include both stones found in nature and
reconstructed stones. The original Washita and Arkansas stones were
quarried natural stones, but now many stones sold by these names are
reconstructed. The abrasive material is novaculite, a mineral related to
flint and quartz containing mainly silicon dioxide. The relative hardness
of novaculite is 6.5 on Mohs scale, just a bit harder than file
steel. The original Japanese and Greek waterstones were also from
natural sources. Natural abrasives work well on carbon steel knives,
but they struggle with harder tool steels and tougher wear-resistant and
stainless steels. For modern steels I recommend stones made with
manufactured abrasives and industrial diamonds.
Aluminum oxide, which has a relative hardness of 9.2, is also bonded to
form reconstructed stones, including modern Japanese water stones (resin
bond) and India stones (vitrified bond). Originally this material
was from natural sources (emery and corundum), but manufactured abrasives
have dominated since the early 1900s.
Ceramic stones are made from alumina (aluminum oxide) or silicon carbide
in a ceramic bond. Silicon carbide has a hardness of 9.5 and will
sharpen anything except carbide tipped tool bits. Spyderco and
others offer ceramic stones in a wide variety of sizes and grits.
Industrial diamonds are made into hones by bonding them to steel and are
therefore also called diamond files. Diamond has a relative hardness of
10. Two very different types of diamonds are used in diamond hones.
Monocrystalline diamond hones last longer because the diamonds do not
fracture readily. Polycrystalline diamond is less expensive.
Diamond hones are made by DMT, Eze-Lap and others. DMT uses
monocrystalline diamonds. EdgeCraft's unique answer to bench stones
is the Chef'sChoice 400 series diamond file system. It consists of
rather thin diamond hones that fit on a magnetic holder. It is a
very good value. EdgeCraft has a good pamphlet on sharpening which
you can request from the address at the end of this article.
An inexpensive alternative to stones is silicon carbide sandpaper. A
piece of silicon carbide (also called wet or dry) sandpaper glued to a
wooden block will work as well as a stone. Wet or dry sandpaper on
plate glass is popular with woodworkers for sharpening plane irons and
chisels, and for flattening the sole of planes. This method is
called Scary Sharp by those who promote it.
You will also need a guide to control the sharpening angle. Guides are
available for knives, chisels and plane irons. The drawback of most guides
is that they waste about 3 inches of stone, so you would need a longer
stone. If you mount your stone flush with your work surface, you can
utilize the full stone length.
The Razor Edge Guide
The Razor Edge guide clamps on the blade with four Allen screws and I
find it inconvenient to use. Also I managed to grind away some of this
guide when I tried it on diamond hones.
Unknown, Buck HoneMaster and Razor Edge guides.
ROD-GUIDED STONE SYSTEMS
The Lansky rod-guided sharpening system has been the
industry standard for years, with good reasons.
Rod-guided systems have a rod on each stone
that slides through a hole in the guide. This controls the angle and also
prevents scratching the blade with the stone. Since the guide slides on
the rod and not on the stone, a smaller stone is needed. Rod-guided
systems sell in the $30 to $50 range, depending on the number and type of
stones. A variety of stones are available, including ones for serrated
blades. They will sharpen up to a 4 inch blade before you have to move the
guide to a new position.
Lansky, GATCO and DMT rod guided systems.
Rod-guided systems are available from Lansky, GATCO, DMT and others. The
Lansky has an aluminum guide that goes from 13 to 25 degrees in 4 steps;
each angle is 3 to 5 degrees lower than indicated. The GATCO guide
is aluminum and reinforced plastic and goes from 17 to 34 degrees in 6
steps, each step is about 6 degrees greater than indicated. I prefer
the GATCO to the Lansky because of the GATCO's larger stones and selection
of angles. The DMT Aligner guide is all plastic, and goes from 12 to
35 degrees in 7 steps, which are not marked. With DMT diamond hones
the Aligner would be the pick of the litter for this size of system.
The EdgePro Apex Sharpening System
The class act in rod-guided systems are the EdgePro Sharpening Systems.
Ben Dale, the owner of EdgePro, has spared no expense in his pursuit of
excellence in hand sharpening. The smaller Apex is rugged and uses
relatively large 1 x 6 inch aluminum oxide waterstones. The angle guide is
continuously adjustable for any angle from 10 degrees to 35 degrees, with
marks at 10, 15, 18, 21 and 25 degrees. My measurements
confirmed that the marks were accurate. The larger Professional model uses
the same stones and angles, but is more stable and also has a scissor
sharpening attachment. Both units come with good instruction
(1) The Razor Edge Book of Sharpening by John Juranitch
(2) The Complete Guide to Sharpening by Leonard Lee