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 hard or very hard base, main or host 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 stones moistened surface
as the nagura was rubbed along 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 nagura slurry
medium of girt particles is present.

Soft host stones will give up or provide their own slurry more readily during use, and you can
monitor this rate of wear during the grinding action by the volume of released stone surface
material, 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 characteristics.

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
space between the blade and the hard flat stones surface is narrow, and being filled with only water
a greater suction is created which may overcome and be too strong of a force for some clays.  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 adjusting to alternate blade to stone angle presentation.

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 host 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 continued direct aggressive abrasion. If on the other hand the swarf is not
evacuated and persists near the stones surface 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. 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 steel.    

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 original and drawn
from the host stone, then the particles of the slurry should be by default be of the same granular
hardness, size and character as the host stone itself. Under the best of circumstances these host
stones particles would be physically able to act in a way that continues to abrade off from the host
stone a perpetual well of very fine stone particles as compression takes place during the
sharpening process. Sort of like cutting diamonds with diamonds under pressure.

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. 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 silica form of
chert. Chert is known to contain high levels of
Radiolarite and the chert rich stratas near Kyoto where the best known awase toishi are found are
particularly rich in these forms of ancient radiolarian silica. This microscopic type of chert derived
from Triassic/Jurassic era micro shell sea animals has irregular shapes that are jagged and sharp
and often appear in flattened plate like shapes rather than the round ball shapes of sand. This plate
shape is more prone to cleave or break into smaller forms if stressed under pressure.

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
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
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 dissolve. This is
followed at the mechanical level through the unmistakable signal of friction, steel to stone. These
properties are not unique but are usually expected of natural Japanese awasado. 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 things to these
stones that they seem fit to do, short of dropping them on the hard floor.

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
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 ocurrence might not be all that rare in both man
made and natural stones.

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 in a laboratory, 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.