|A Few Notes About Sharpening Stones
In traditional Japanese craft use, the Nagura Stone is a secondary stone used to help create a slurry/paste medium on the working surface of a
hard or very hard base stones as they were being used for sharpening edges including but no limited to those of razors and all manner of
woodworking and metalworking tools. The slurry deposited on the base stone's moistened surface as the nagura was rubbed on the surface
provides an alternative grit particle paste that begins the sharpening process, helps to energize and encourage the release of the base stones own
slurry compounds while at the same time acting to lubricate the base stone surface to relieve suction that developed between the tool and the stone.
Without the use of the nagura these really hard base stones tend to exhibit tendencies that are difficult for even the more advanced craft persons to
overcome. One such example is the skipping type surface action transmitted through the blade during a sharpening session. This skipping action,
an uncontrolled non-productive negative sharpening motion is disconcerting and can result in deformed bevel profiles as the blade tends to jerk
forward, often in a nose dive direction resulting in convex shaped bevels or micro chipped edges.
This skipping action is the result of the intermittent relief of the partial vacuum created by the blade-fluid-stone marriage as the resistance created
through suction inhibiting the blades forward sharpening stroke suddenly overcomes the vacuums suction. As the suction is overcome by the force
of the forward motion of the tool, the sudden release of the blade freely projects the blade forward in an uncontrolled jerky movement. This
phenomenon is nearly eliminated if a sufficient nagura slurry medium of girt particles is present.
Soft base stones will give up or provide their own slurry more readily during use, and you can visually monitor this rate of release during the grinding
action, and the amount of slurry self generated through friction is an indicator of the hardness factor of any given stone. The physical grip that the
binder-clay material has on the silica cutting particles is the definition of a hard or a soft stone. Highly compacted stones with low ratios of binder
clays exhibit hard stone characteristics, soft stones have higher binder ratios and their ready release of free floating grit particles exhibit softer stone
Besides the suction skipping action felt when matching very hard stones with very flat bevel blades, another more problematic circumstance can
occur, that being the sudden appearance of loose large rogue grit particles that can scratch the blade leaving chips on the cutting edge. The
presence of a single or small group of abnormally large grit particles that are bound together as one larger unit when honing or sharpening with just
clear water can occur when the force of the vacuum is strong enough to overcome the organic binding power of a clay binders. Under the best of
sharpening conditions the slurry will, as a homogeneous pool, abrade the steel and the stone's bound up surface grit at the same time, encouraging
the steel to reduce to be sharper and the stone to reduce in order to shed old and expose newer fresh grit particles. With just clear water the void
between the blade and a hard and very flat stones surface is narrow, and being filled with only water a greater suction is created which may
overcome the clay binders hold on the silica grit. The blade user will find some relief from this occurrence if he/she takes on a slower relaxed
sharpening stroke and or a lighter hand pressure, or by making an adjustment to the angle of blade presentation in relation to the sharpening stroke
and the stones surface. A side sharpening stroke, where the blades edge is pushed forward while presenting a parallel attitude to the stones side
edge would be the extreme example, a 45 degree attitude would be the median, while with the blades edge presented perpendicular to the stones
side edge/forward motion would be the neutral attitude.
In practice a slurry or paste is a film comprised of loose stone particles, both grit and clays, suspended in water which in turn acts as a buffer or
medium between the base stone and the steel. Through this slurry the swarf, or ground steel waste may be carried off and away from the cutting
surface which allows for a continuous direct aggressive abrasion action. If on the other hand the swarf is not evacuated and persists near the stones
surface the relationship between steel and stone is minimized and over time the metal filings can be forced on to the stones surface and embedded
or almost welded into the pores of the stone thereby impeding the cutting action of the grit matrix altogether. If left to continue, this attached layer of
swarf will create a film of metal over the stones surface and from then on at best, only a burnishing action will occur between the stone and the
A slurry also helps to generate and to expose ever more fresh and sharp stone surface cutting particles through the abrading action. If the slurry is
composed strictly of material originally drawn from the base stone, then the particles of the slurry should be, by default, of the same granular
hardness, size and character as the base stone itself. Under the best of circumstances these base stones particles would be physically able to act
in a way that continues to abrade from the base stone a perpetual well of very fine stone particles as compression takes place during the sharpening
process. This grit cutting grit process keeps the base stone fresh and the cutting aggressive.
Through compression and abrasion, the original cutting particles of the abrasive material suspended in the slurry may now have an opportunity to
themselves become ground into finer dimensional grit particles if those particles are of a friable nature. Not all grit particles are friable or easily
cleaved or broken down under normal tool sharpening actions. As an example Japanese awasedo stone particles do cleave and crush, but Belgium
Coticule grit particles tend not to reduce in dimension. A slurry solution can be a powerful cutting media but it can also be, with friable grit particles, a
media of grit particles that can break down in size to become smaller and finer. The most common form of silica we see around us is sand, we know
that sand can easily etch glass or metal when used as an abrasive when wind blown in nature. Common silica sand is seen as being a generally
rounded particle, not easily cleaved or crushed. The silica particles found in Japanese awase toishi are a amorphous silica form of called
The sedimentary rock stratas near Kyoto where the best known natural sharpening stones are found are particularly rich in these forms of ancient
radiolarian silica, while also exhibiting a relatively well balanced percentage of clay filler/binder material. This microscopic type of silica derived from
late Triassic/early Jurassic era consists mainly of radiolarian shells called tests, the ancient 225million year old skeletal material consists of
amorphous silica, fragments of shell sea animals having irregular shapes that are jagged and sharp and often appear in flattened plate like shapes.
Very dissimilar to the round ball shapes of sand silica. This plate shape of radiolarian is more prone to cleave or break into smaller forms if stressed
One problem that may be found when using an alternate nagura stone in conjunction with suita strata stones that you need to be aware of is that
suita stones usually have beehive pattern of microscopic holes, the remaining voids of ancient carbolic gases. This void rich matrix formation of the
natural suita stones can trap loose grit particles. These voids in Japanese are referred to as Su.. HERE IS A PHOTO OF THESE SU HOLES. These
su holes or voids can aid in the sharpening process by providing water retention, swarf flushing, and in the retention of embedded and desirable
active cutting base/host stone material which can be useful if the small bits originate from the host stone and not an alternate stone. If of the base
stone, then during the sharpening process these particles can be draw back up into the slurry for a second round of work where they can be ground
into an even finer grit particle dimension and size. Alternately, if the bits are of a foreign or secondary stone, and become lodged in the "su" holes,
these particles can and will scratch the steel of the blade. So be aware if you use a Aichi nagura or a coarse tomo nagura stone to create a slurry on
a suita stone.
Using a slurry generating stone or device like a diamond plate with suita stones should always be done with care. For all of the tomae strata stones
and most suita stones a diamond plate called a Diamond Nagura (DN) can be a very useful tool to say the least. The DN creates a compatible slurry
composed of 100% base/host stone, no foreign grit material is interjected to contaminate the stone. In this way all of the cutting material in the slurry
is compatible with the host stone because it is made up of the host stone.
A note about Synthetic Stones in this context as opposed to Natural Japanese Stones might spark some thought here.
In naturally occurring and mined Japanese water stones the physical release and separation of the grit from the binder during sharpening as
explained above is a necessity in the performance of the stone in order for it to act as a sharpening tool. The progressive process of the grit particles
breaking down into the optimum grit particle size is activated first at the chemical level at the instant a drop of water is applied to the natural water
stone, the binding materials begin to soften. This is followed at the mechanical level through friction, steel to stone. These properties are not unique
but are usually expected of natural Japanese awasado. Some of the binder material in these stones is water soluble and quite often resembles clay.
Synthetic man made sharpening stones will often have a binder material that was formed during a heat treatment process or by using a resin based
moulded type of plastic binder. These processes lock the grit particles in suspension until they are exposed during the active process of sharpening
and wearing down of the stone to expose new and sharper grit, just like with the natural stones. The secret or proprietary manner and material in
these stones is such that often their release is not so automatic or easily triggered as it is with the water soluble binder found in natural stones.
Synthetic stones are often engineered to act or mimic this release mechanism that we associate the natural Japanese stones to possess. By
necessity the synthetic binder of the man made stones are forced to perform a simple task that might be a chemist engineers greatest challenge;
how to synthesize a binder that will dissolve in water when called up to do so, but not decompose in water too readily, while at the same time
allowing the general public to do any and all manner of mean and nasty things to these stones as they seem fit to do, short of dropping them on the
Releasing grit particles as they are being utilized in the sharpening process at a microscopic level apparently is not such a simple task, especially in
the 20,000 to 50,000 grit realm. I have not observed it myself but have seen photos and descriptions in Leonard Lee's excellent book The Complete
Guide To Sharpening of grit particles bonding and acting as one larger abrasive particle. Mr. Lee observed this occurrence in the structure of a
natural hard Arkansas stone. He referred to this as an anomaly. I suggest that this occurrence might not be all that rare in both man made and
It should be noted that in the case where two or more grit particles remain bonded or locked together during an extended duration in the sharpening
process, these grit particles that remain bonded could and should therefore be construed to be acting as a larger grit particle diameter engaged in
the cutting process.
The beauty of natural stones is not necessarily only of their diminutive grit diameter, this can be duplicated by machine, but instead to my minds eye
the intrigue is in the bonding material itself. In its imperfection and fragility the bond is therefore perfect, releasing the cutting particles to do their duty
only and when they are called by the message imparted in the sharpening process of water and steel to stone directed by the human hand. Like a
wish come true for anyone who soothes the heart with such trivia.