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.mHighly 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 burnishingmaction 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 ormmetal 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 Su.. 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 chemistmengineers 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 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 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 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.
(The photo shows "su" holes in the stone.