Characteristics of stones

Japanese Natural Stones sharpening characteristics.

H or Hardness Scale Explanation.
H Level 6-8 is medium hard and these stones will self slurry under a blade with pressure.
A drop of water placed with a finger tip will sit proud for one minute and then begin to flatten and seep into the stones surface, within five minutes the drop will be gone. When using a diamond plate (DP) to lap or make a slurry the action is easy and fast.

H Level 9 is hard and requires concerted pressure and effort upon the blade to self slurry. A drop of water will sit proud for one half hour and then begin to soak into the surface. With a DP the stone feels hard and the slurry is thin even with 10 strokes.

H Level 10 stones are the hardest and will not self slurry even under extreme hand pressure.
A drop of water will sit proud and round on the surface for one hour or more. A DP glides over the stone, abrasion happens but very slowly and only with much pressure.

PS or Particle Size Scale Explanation.
PS Level 1-3 (500 to 2,000 grit) for bevel creation are best found in synthetic stones or with greater difficulty using Arato coarse stones from Japan from areas outside Kyoto or from individual or successive nagura types stones with appropriate grit levels. At this grit level steels will brighten but with dull finishes. With laminated blades some contrasting finishes can be created especially if slurries are utilized.

PS Level 4-6 (2,000 to 4,000) leave medium deep scratches that are easily removed.

PS Level 7-8 (4,000 to 8,000) leave finer scratches that are easy to remove and difficult to see with the naked eye but can be seen with a quality 15x loupe. These stones can a high carbon steel blade looking like polished aluminum to dull chrome.

PS Level 9-10 (8,000 to 25,000) leave the finest scratches at the lowest levels ie. 8k to 12k of particle size are difficult to see with 15x, under most optical microscopes at 100x are easier to see. The scratches at 12k to 25k or natural stones judged to be on par with these grit levels are difficult to see beyond the 15k level with most optical glass. The carbon steel or stainless will be bright and polished like a mirror.

S or Cutting Speed Scale Explanation. S Level 1-5 at the lowest level have no abrasion power but instead act as burnishing stones and the upper 5 Level are very slow to abrade tough steel.

S Level 5-7 will cut steel and remove previous scratches easily.

S Level 7-9 will cut steel and remove previous scratches easily and quickly.

S Level 9-10 will excel in the above and at the 10 level will amaze. Cutting speed can be quantified for personal reference by stroke count.

Japanese Stone Sizes Explanation Stone sizes in Japan are in millimeters and referred to as "grades" and the sizes stated are
the minimum dimensions which takes into account variables in Length and Width only. This grade system has its roots at the mine entrance, and is based on a working adults ability to carry a certain number of grouped and graded by size stones down an average
mountain trail on his/her back to the valley floor. Working adult refers to a man of average
strength. Women and children also labored but some adjustments of course made.


Grade Length Width Equivalent in inches.
#24 pieces 210mm 78mm 8.26 3.07

#30 205mm 75mm 8.07 2.95

#60 185mm 70mm 7.67 2.75

#80 180mm 63mm 7.08 2.48

#100 160mm 58mm 6.29 2.28

Razor 136mm 82mm 5.35 3.22

Koppa are by definition bits & pieces and are usually in small sizes


Sharpening Characteristics Explanations.

If all things are equal: a grit rich or silica rich stone will remove a measured amount of steel
faster than a grit poor stone will.
A soft grit rich stone will remove a measured amount of steel faster than a grit poor hard stone.
Like wise if all things are equal: a coarse stone will remove a measured amount of steel faster than a finer stone will.
In the same vain a slurried stone will remove steel faster than a non slurried stone will.
And again a stone used under running water (that rinses away any loose grit/slurry particles) will act finer than a stone with standing water, with or without a slurry.

Japanese natural sharpening stones are unique in the world of sharpening for handfuls of
reasons and one of those is their ability to take on different characteristics while in a state of being used dry, wet, with a slurry or under running water like at your kitchen sink.

One example of this would be that if any one particular high Hard Level Jnat(Japanese Natural)is used dry and without water their sharpening qualities are diminished and they will act more like a burnishing stone, but if that same high hardness level stone is used wet it will act as an abrasive stone. The same goes for coarse stones. Japanese awase-do (aka awase-to, tennen-toishi) really only cut with advanced actions when wet.

And about to slurry or not to slurry. A slurry comprised of loosely bound grit particles suspended in water, often acting as bundles of bound grit will act coarser and cut faster strictly depending upon the mechanical action that created the slurry. A #400 diamond plate will make a coarser acting slurry than a #1200 diamond plate. A nagura or slurry stone cut from a similar piece of tennen-toishi can if harder than the base stones encourage the base stone to contribute slurry more freely, or if the nagura is softer it will itself provide the majority of slurry particles. This is the principle for which the Nagura Progression System is based and this system will only work marginally well with other base synthetic or natural stones like Coticules, Arkansas and the other slate stones. The Japanese dedicated natural nagura tend to favor being paired with the natural awasedo found near Kyoto because both the base and nagura components will encourage the crush of the clays that release the silica grit of each other.

In all the world the Japanese tennen-toishi are unique because of the slurry component and the way it can be exploited. For centuries the clay binders that comprise the glue that holds the stones grit in place has been manipulated by sword polishers, carpenters and barbers to suit their needs and requirements. No other stone will react with such characteristic and predictable ways as the natural stones that the mountains in and around Kyoto offer.
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(The photo shows the size of the stones.)