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Sword Sharpening

Be very careful when sharpening katana.  Please don't think about sharpening a sword that could be a valuable heirloom. You could be destroying an irreplaceable piece of history and costing yourself hundreds of thousands. A Practical Katana or rusty gunto is a good place to start. Practicing with 2000 grit or finer stones makes it harder to destroy the geometry of the blade, but you should expect to destroy the first blades you try to sharpen.

Nihonzashi LLC, its employees, and associated companies are not responsible for any injury, damage or loss incurred by following any advice given on this site.  Katana are dangerous objects and the utmost care should be given when working with them.

Grit

The first thing everyone asks is what grit stones are used to polish or sharpen a sword. What could be easier than selecting grit? Is that US, European, English, or Japanese grit? Grit specified the mesh used to separate the abrasive particles, but everyone uses a different standard and they don't compare well. Things really fall apart at the finer grits since the US standard switches from grit to particle size in microns. The following table will give you some clues on how to relate the different standards. From this point I will use the Japanese grit since I use Japanese water stones graded by this standard.

Equivalent Grit Table

Particle
Size
(Microns)
US
Grit
Japanese
Grit
Examples
0.5 12000 25000 Chromium Oxide Polishing Compound, Moor White Ceramic
1 5000 10000 Honyama Awasi (Brown Stone), Linde C Compound (Aluminum Oxide Powder)
2 3000 6000 Karasu (Blue Stone), Awasi Toshi
3 2000 4000 Uchigumori, Extra Fine White Ceramic, Green Chrome Rouge, Spyderco Extra Fine Ceramic
6 1200 2500 Ao-To (Blue Stone), Spyderco Fine Ceramic
10 1000 2000 Hard Black Arkansas, Extra-Fine Diamond Hone, Lansky Ultra-Fine Hone
15 800 1500 Koma-Nagura, Hard White Arkansas, Extra Fine Diamond, Medium Ceramic, Moor Black Ceramic
20 600 1000 Soft Arkansas, Lansky Fine Hone, Ultra Fine Scotch-Brite Pad, Spyderco Medium Ceramic
25 480 800 Chu-Nagura,  Washita Stone, Fine Diamond
35 320 500 Kaisei (Natural Sandstone), Fine India, Medium Diamond, Super/Extra Fine Scotch-Brite Belt/Pad
45 280 400 Medium India, Fine Crystolon  (Silicon Carbide), Coarse Diamond, Lansky Medium Hone
60 220 300 Binsui Coarse Stone, Extra Coarse Diamond Hone, Very Fine Scotch-Brite Belt/Pad
80 180 260 Fine Scotch-Brite Belt/Pad
90 150 220 Medium Crystolon (Silicon Carbide), Coarse India, Medium Scotch-Brite Belt/Pad
110 120 180 Arato (Natural Sandstone Or Carborundum), Lansky Course Hone
150 100 150 Coarse Crystolon (Silicon Carbide)
180 80 90 Lansky Extra Coarse Hone, Coarse Scotch-Brite Belt/Pad

A series of stones ranging from about 800 to 12000 grit are used to sharpen blades in good shape. If a blade is badly shaped or has chips to remove, stones down to 80 grit can be used. Don't jump too far between grits or you will be unable to remove surface scratches.

Japanese Water Stones

Japanese water stones are either natural or artificial. Natural stones can be quite expensive, but artificial stones can be used for sharpening polish. About half the stone used in a full cosmetic polish can also be artificial, but some steps require specific natural stones. Artificial stones use a graded abrasive suspended in either a clay or ceramic media. The stones use water as a lubricant. Even the inexpensive stones can run $50 each.

Soaking the Stones

Japanese water stones need to be soaked in water to work properly. Stones take from 5 to 20 minutes to become saturated depending upon the stone. While some stones can be stored in water others must be stored dry. I store all my stones dry and soak them for 20 minutes to simplify things. I put about a quarter cup of sodium bicarbonate in the water. Baking soda will change the pH of the water and keep your sword from rusting while you are sharpening it. Some stones like those from Debado should not be soaked but simply sprinkled with water before using.  These stones will deteriorate if soaked.

Shaping the Stones

Japanese water stones constantly wear down during use. This is normal and helps maintain an aggressive cutting surface. Some stones wear down quite quickly while other stones are quite stable. Stones used to sharpen a sword should be slightly convex and have rounded corners. The stones will become concave (hollowed out) during use so they should be reshaped before or after each use. Special stones are available for keeping your stones true, but I just use each stone to flatten the next finer grit stone when I switch grits. Keeping the edges rounded or beveled will keep the stone from fracturing and help keep you from grinding a grove in your blade.

Sharpening Base

Being a westerner with a bad back, I sharpen swords standing. I use a custom wood platform on my workbench to secure the stones to. Some stones are already attached to bases, and I use one of those standard rubber bases for unmounted stones. You need a sturdy platform that stands up to the water. I used a platform that fit over the kitchen sink when I was single, but a couple of buckets for water keep me out of trouble now. You might prefer clamping down your stones and base, but I find it is unnecessary.

Straightening the Blade

You might have to straighten the blade before you sharpen it. This is your last chance. Once you start sharpening, the geometry will be lost. Straightening a sword is an art in itself. The simplest method but hardest to master is simply bending it over your knee after sighting down it's length. There are slotted wood sword straightening tools available that allow you to isolate the bend easier. Whatever method you choose, just remember to take your time.

Getting a Grip

The sword should be disassembled and the bare blade sharpened. Even the habaki should be removed. I use a piece of an old towel about 1x1 foot wrapped around the blade to provide a good grip. It has to be tight so it does not slide (and slice). When I'm working the monouchi I use my right hand to grip the sword and my left hand to counter the weight of the sword and steady the kissaki. Be careful of slipping your left hand off the blade or you could loose a few fingers. Make sure to wipe off any oil. I use two pieces of towel to grip the sword with both hands when working on other parts of the blade. Remember that it is easy to cut yourself badly when sharpening a sword.

Sharpening

The blade should pass over the stone using a uniform even stroke. This is polishing and not grinding! I use both the forward and backward movement to do the work. Go slow and inspect the blade often. Don't worry about how the edge feels. Just make sure you have worked the stone over the entire surface of the blade while maintaining the surface geometry. A good light source is essential to seeing where you have missed. Remember that you are not sharpening the edge. You are removing metal until the edge is exposed.

 

Scratch Pattern

The first stone should be used until the scratch pattern just reaches the edge. The only exceptions are when removing chips or when the edge has been flattened. The geometry should be established with the coarsest stone and subsequent stones should just refine the surface. The angle of the scratch pattern of each stone can be varied to make sure all the scratches from each stone are removed by the next. Holding the blade up to the light will reveal scratches from previous stones cutting across the pattern. Cosmetic polishes alternate the pattern by 90 degrees to make sure every scratch is removed.

Scratch Pattern Scratch Pattern
Scratch Pattern Scratch Pattern
Scratch Pattern Scratch Pattern

Leave the Edge

The most common mistake is paying too much attention to the edge. All your attention should be on the surface of the blade. The surfaces must be removed to reveal the edge. The first stone is key. The surfaces should be worked until the scratch pattern reaches the edge while maintaining the desired surface geometry. You must keep working with the first stone until the scratch pattern reaches the edge. There may be only a very thin polished surface that reflects the light, so look very carefully. If the edge has been flattened or chipped, you need to continue past this point.

100 Strokes

Once I have established the geometry with the coarsest stone, I use 100 strokes of each stone on both sides of the blade to refine the surface. I use long strokes that cover about 15 inches of blade. I find the geometry comes out more uniform this way. That also means that the entire monouchi can be covered in a single stroke. The blade is rotated very slightly to cover the entire surface from the shinogi to ha.

Rounding the Shinogi

It is key to not round over the shinogi. This is the line that runs down the length of the blade delineating the cutting surface. The blade will need to be worked right up to the shinogi line, but it is very easy to turn the blade too much and destroy the geometry of the blade. The blade has a tendency to roll over and round the shinogi due to its curvature. You can hear when the stone is working the shinogi or ha. The tone of the scraping changes slightly as you reach the edge. Be careful!

Lubrication

Slurry is formed on the top of the stone as it breaks down. This paste acts as a lubricant and as an abrasive. The stone needs to be periodically re-wetted. Use water with dissolved baking soda to keep the stone wet. Don't wash the paste off the stone. Use your hand to dip water onto the stone and keep the blade clean. A small rag can be used, but be careful not to leave thread or other pieces of debris on the stone or blade. You will need to switch water when you switch stones or the coarse grit in the water will leave scratches in the blade. The finer stones need to have a paste formed with a nagura stone to work properly. The nagura stone is soaked with the water stones and rubbed on the top of the finish stones to form a paste.

Power Tools

The worst thing you can possible do is using a belt sander, orbital sander, or grinder on your sword. That is the fastest possible way to turn that sword into junk. Power tools can quickly remove the temper or destroy the geometry of the blade. Orbital sanders destroy the geometry by rounding over the ha and shinogi. I use Delta and Makita wet grinders to reshape badly chipped blades, but they need allot of practice to use. The grinding wheels move slow and are water cooled so the blade does not heat up.

Abrasive Paper

Some people use progressively finer grits of silicon carbide sandpaper to sharpen swords. I have no problem with that, but I personally don't use that method. I'll keep my water stones and let others do their own thing. You can get silicon carbide and diamond lapping film that covers the same abrasive grit range as Japanese water stones.

Jigs

You can get a wide variety of sharpening systems designed for knifes. These jigs have one big problem. They are all intended to create an edge with a single angle. A katana should have a slightly curved continuous surface from the ha to shinogi. It is not a big knife and can not be sharpened like one. You can get more data about the geometry of a katana by checking out the edge geometry