For many years we were reluctant to offer natural Japanese sharpening stones. We had several reasons.
First: at Fine Tools we offer a very wide variety of man-made water stones, from a number of different manufacturers, that cover all possible functional requirements many times over. We felt there was no need for natural stones.
Second: with man-made stones we know that they are going to be consistent and will always offer the same sharpening characteristics – from the beginning of their working life to the very end when they have become too thin to use. With natural stones we can’t be sure even if the stone looks really good, because we cannot see beyond its surface. As the stone wears in use, the next layer may develop different sharpening characteristics, or a very hard inclusion or other irregularities may appear that can greatly limit the usefulness of the stone.
Third: we don’t know enough about the individual stones to be able to assess them reliably, to know what we would be selling and whether they are really worth your money! Especially as every stone is different and can change with use. If we buy a box of 20 synthetic stones, and we test one and it is of excellent quality, we can normally assume that the other 19 will be the same. Not so with natural stones. It would theoretically be possible to test each stone, but this would drive prices still higher. It would also erase the quarry stamp on the surface of the stone, and with it the proof of the stone’s origin.
So why are we offering them now (1)? The reason is that very little change can be expected from manufactured stones in the future. And just to continue with our current lines, and discover nothing new, to be honest, is just too boring for me in the long run. As it happened, I found a man who does the rounds of the various quarries, and is very good at judging the quality of the stones by sight and feel. He mostly buys individual stones that he considers to be exceptionally good. This sparked my interest, and so I am now offering a few of these stones for sale in my store.
So why now (2)? A while back, I began to think of natural sharpening stones as an enormously important cultural asset. They have played a big but largely unsung role in the technological development of humans. In a very real sense, sharpening stones are what made it all possible, especially where working with metal is concerned. No farming without the proper tools, no woodworking without sharp planes, chisels and saws. And meal times would be very different without sharp hunting and cooking knives. Sewing requires sharp needles. Cutting tools and their sharpening stones were the basis for the development of many kinds of traditional handcraft and specific skills. The need to find a suitable abrasive is very closely linked to the development of tools in general and dates back many thousands of years. Even today, “high tech” man-made sharpening stones are praised by comparisons such as "it grinds like a real stone" – the manufacturer obviously though that he had found the right materials to enable his stones to mimic the "high tech" natural stones that have been in use for thousands of years.
Who are these stones for? For beginners, it is best to work at first with relatively inexpensive synthetic stones. Most people can expect to achieve a very respectable and useable edge on knives and woodworking tools with these stones. But over time a fair number of people become dissatisfied with “useable,” and as their working skills improve, so does their feel for sharpening, and their need for better sharpening stones. For these customers, we recommend our high-end synthetic stones, and for many they are a revelation. We often get correspondence from customers saying that our high-quality stones had allowed them to achieve a precision and sharpness with their cutting tools that they had not imagined possible. If you get to that level, and you are still curious, and you want to explore further, while knowing that natural stones can be unpredictable, then experimenting with these beautiful natural stones might be a very interesting next step.
The three steps in sharpening: archaeological digs on prehistoric sites have not only revealed an active tradition of blacksmithing dating back to the late Jomon period (1000 to 300 BC), but research has also found evidence that even then the sharpening process was divided into the three steps – roughing out, grinding, and honing – that are still used today. One can assume that the sharpness that could be achieved with a fairly coarse or medium grinding stone would have been considered adequate at first, and then as the culture developed, higher standards of sharpness would have been needed. The abrasive material in natural sharpening stones consists of very fine round grains of silicate with a size range from 0.2 to 0.3 microns (according to Japanese sources) and these are combined with a small amount of iron oxide or other natural materials that improve the grinding effect.
Appearance and quality: a Japanese natural sharpening stone, which can be considered perfect and has all the necessary attributes, can cost a fortune. It would have a highly consistent grain, be uniform in texture and colour (preferably yellow), would have no cracks, stains or other blemishes, and is overall a very beautiful stone. Such a stone is known as a "practical diamond" in Japan, and one turns up maybe once in a century. Normally natural stones are nowhere near perfect in appearance. The colour can range through various shades of yellow, brown or grey, it can differ across the face of one stone and it can change with use. The edges and corners are often rough and uneven or chipped. But surprisingly, the quality of edge that can be obtained with these more common stones is often very close to that achievable with a so-called "perfect" stone.
Origin and cut: quarries as a source of natural stones have mostly been exhausted and it is very difficult today to get hold of really fine stones. Stones generally take their name from the mountain where they were quarried. Here are some examples of names from Kyoto and the surrounding region:
- Western mountains: Ohira, Shinden, Mizukihara
- Central mountain: Atago
- Eastern mountains: Nakayama, Ozuku, Kizuyama, Okudo, Shoubu, Narutaki
Nearly all the quarries are currently closed. But there still exists a large amount of raw stone material mined in the past that is waiting to be cut to appropriate sizes.
The stones are cut horizontally so that the layers of sediment end up parallel to the stone’s face in order to present a consistent grit size and quality. This can be compared to flitch-sawn wood.
Wetting the stone: theories on how long before use one should soak natural sharpening and honing stones vary greatly, even in Japan. There are professionals who speak of 2 weeks! Others advise at least 30 minutes, and yet others who say that simply wetting the surface of the stones works just fine. Our advice: soak a natural stone in a water bath for at least 30 minutes before use. If, once you have become familiar with the stone, you feel this is not enough, try soaking it for up to 24 hours or even longer. The right soaking time serves to stabilize the structure of the stone for consistent results. It is important to remember that these are natural stones, each one is different, and so each has a different need for water. Harder stones, for instance, need more time in the water than softer ones.
Storing the stones – wet or dry: in Japan natural sharpening and honing stones are normally stored dry and out of the sun after they have been flattened and the grit paste has been removed. They should never ever be exposed to freezing temperatures when wet. The water in the stone will freeze and expand, damaging or cracking the stone. Avoid rapid changes in temperature or drying the stone too quickly. Never put a stone on or close to a strong heat source! Stones can naturally also break if dropped onto a hard surface.
Contrary to what we have written above, there are recognized and respected experts in Japan who say that natural stones in regular use can be left permanently in water. They base this advice on the idea that, unlike synthetic stones which can be damaged if left too long in water, natural stones can withstand this treatment because they are basically stones that were formed under water (sedimentary stones). While all this is true in principle, we feel very strongly that storage in a dry place is best for the following reasons.
Natural sharpening stones are normally composed of about 2/3 SiO2 (Silicon-dioxide), which provides the sharpening grit, and about 1/3 KAl2AlSi3O10(OH)2 (Sericite, finely rippled Muskovite) which serves as a bonding material. The stones were formed about 70 million years ago. As they were formed from sediment laid under water, one would think they would be stable when wet. But after the stones have been quarried and cut to size this can change, as the bonding material, which is at least in part water soluble, can dissolve over time and the abrasive particles can flake off unevenly. This decomposition process normally takes place over many years, and so if the stones are in daily use, it doesn’t matter much at all. But for most people today, who value their investment in the stones and their quality and rarity, it is best to store them dry to avoid any possible damage.
Dressing and flattening: this topic is unfortunately often neglected, even though the lack of advice can lead to bad or inconsistent results when you use the stones to sharpen tools. By dressing and flattening we mean the long-term care of the stones, and not just tuning them up for their first use. When the stones are used for sharpening, a hollow develops in the surface of the stone, and the hollow causes problems when sharpening tools and knives. There is a difference between sharpening knives and sharpening woodworking tools like plane and chisel blades. Knife blades mostly have a slightly convex shape and so an absolutely flat surface on the stone is not so important. With woodworking tools though, it is important that the stone’s surface is flat so that the bevel and back of the blade can be sharpened straight and flat. This allows the maximum precision when working on wood with these tools. A hollowed stone, even when you can barely see the dip, will cause problems with the different types of plane and chisel blades.
There are different methods to flatten or dress the stones. For us, there are two practical and effective methods: other sharpening stones and diamond plates.
Fine natural honing stones can be flattened with a sharpening stone of medium grit grade (600 to 1000): both natural and synthetic stones work well, and harder stones are better than softer varieties. The most effective option is, however, a diamond plate with a grit grade between 400 and 600. If the stone has a deep hollow, and you need to remove a lot of material, then a coarser dressing stone should be used. Grinding stones of 120 grit or a diamond plate of about 140 grit are good for this kind of job. When flattening coarse natural grinding stones, like Igarashi, Natsuya or Amakusa, you should always use a coarse stone, of similar or larger grit grade.
It is always a good idea to thoroughly rinse the stone after flattening it. If this is not done, you will often find that some of the coarser grit stays on the surface of the finer stone and leaves ugly scratches on your tools or knives. Since the surface or your grinding or sharpening stone was roughened up by flattening with a coarser grit, you can smooth the surface using a Nagura stone, gently, without pressure. The result will be a fine abrasive slurry that is perfect for sharpening. Depending on the type of stone and the steel to be sharpened, sometimes you need to add a bit more water after dressing with the Nagura stone. Toward the end of the sharpening process, it is best to reduce the pressure on the tool; this will tend to polish the edge and leave a mirror surface on the steel.