#16 Mikawa Shiro Koma Nagura
This misshapen piece of koma would normally have been cut while in a
raw larger form into smaller nagura pieces. As it stands, the thickness was sawn
and the top lapped so it now makes an excellent bench shaped stone. I am
tempted to trim the ends and make smaller pieces but I am going to restrain
myself although there would be plenty of real estate between those cut lines.
It is very pure white and not overly hard like some koma is.
There is a solid inclusion that is not toxic in any way and is bound with the
same mica that you see on the back side of what used to be the kawa skin.
Well below the pay grade because of the odd ends and the low
riding top/left corner. Tons of good grit ready to be used.
This shiro white nagura makes an excellent middle stone, inserted
between a bevel set and the final finishing stones, or it can serve
as a giant nagura rubbing stone. It will remove metal
from the hardest razors leaving a more polished surface.
I found that these base stone nagura are easier to use if you soak
them for 1 to 2 minutes so the hydration becomes consistent. Splashing
clear water as you go along will keep them cutting.
Becoming very scarce in this size, I do not know how long I can continue
to source them from my nagura miner who keeps telling me that these
will be the last he will every see. He is old so I believe him.
These and all of my other bench size Mikawa nagura do not have Asano ink
stamps or any other attributions because I circumvented the stones before
Sakamoto-san the owner of the Asano stamps had a chance to make his a bid.
The stone (toishi) has been lapped perfectly flat and the corners and
edges beveled unless otherwise noted. All of my stones come with a
money back guarantee minus the return shipping.
|All stones are sold with a
money back guarantee
minus the return shipping.
||Size in mm
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
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
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
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
#30 205 75 8.07 2.95
#60 185 70 7.67 2.75
#80 180 63 7.08 2.48
#100 160 58 6.29 2.28
Razor 136 82 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
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
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