| NAGURA STONE AND ITS USE |
| Traditionally the Nagura Stone was used to help create a slurry/paste on hard to very hard tomae strata awase-toishi used for sharpening tools. Without the use of the nagura these really hard stones tend to exhibit a skipping type surface during sharpening because very little of the host stone is being relieved or ground off in the sharpening process that would normally provide the slurry paste. Soft host stones will give up or provide their own slurry more quickly through use and you can see that this rate of wear in the grinding action and the volume of released surface material is an indicator of the hardness factor between hard or softer stones. In theory a slurry or paste is a film comprised of loose stone particles that are suspended in water which in turn act 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. A slurry also helps in exposing ever more fresh and sharp stone surface cutting particles through the abrading action. If the slurry is composed strictly of material drawn from the host stone, then the particles of the slurry should be by default of the same granular hardness as the host stone itself and should act and continue to chip off from the host more and more 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 flakes of the abrasive material suspended in the slurry may themselves become ground into a finer dimension grit particle. Although this slurry will abrade the blade and remove steel to some degree, most of the actual controlled sharpening action continues to occur between the steel and the grit still bound up as the structural stone grit particles that comprise the sharpening stone surface. There is however a problem with using nagura on suita strata stones that you need to be aware of. Suita stones are called suita because they contain a "su", a beehive pattern of microscopic holes in the matrix formation of the natural stone. HERE IS A PHOTO OF THESE HOLES. These su holes can aid in the sharpening process by providing water retention, swarf flushing, and in the retention of embedded and desirable active cutting material. These su holes can hold small loose bits of the host suita stone that were loosened in the normal sharpening process and carried by the slurry. This is useful if the small bits are of the host stone, but not if the bits are of a foreign material like a nagura stone. If they are from the host stone the sharpening process will allow them to be draw up again and injected into the slurry again for a second round of work to be ground into an even finer grit particle dimension and size. But if bits of foreign material become lodged in the "su" holes that are of a larger diameter then the optimum grit of the host stone, and are hard enough, they will scratch the steel of the blade in an adverse way. This is what can happen if you use a nagura stone with a suita stone to create a slurry. The hard nagura particles act as they should to create the slurry but remain as foreign material and continue to scratch the blade over a period of time because they become embedded in the su holes and are not flushed out so easily. If the nagura grit particles are of a larger diameter or larger grit than the host stone the scratches will be considered a detractor and not a desirable factor in the sharpening process. This situation should be avoided at all cost. In short, natural stone nagura is not recommended for suita stones. In contrast the advantages of utilizing a diamond plate to help create a slurry now seem obvious. Not only is the slurry itself composed 100% of the host stone, but no foreign 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. The diamond plate when used as a diamond nagura is equally advantageous with both the suita and tomae strata stones because it is a neutral agent. Also a courser 500 grit diamond nagura can be used to build a slurry quickly or to flatten and true up a stones surface, or a 1000 or finer grit diamond nagura can be utilized to build a slurry while maintaining greater stone dimensions. 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 and as described above. By the necessity of the synthetic binder, the man made stones are forced to perform a task that is a chemist 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 particle. Mr. Lee observed this occurrence in the structure of a natural hard Arkansas stone. He referred to this as an anomaly. 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 in their grit diameter, this can be duplicated by machine, but instead in the bonding material itself. In its imperfection and fragility the bond is therefore perfect, thereby releasing the cutting particles to do their duty when they are called upon by the message imparted in the sharpening process of water and steel to stone. |
