For many years we were reluctant to offer natural Japanese sharpening stones. We had several reasons.
For one: at Fine Tools we offer a very wide variety of man-made waterstones, from a number of different manufacturers that cover any possible functional need many times over, and so we just felt no need for natural stones.
Secondly: with man-made stones we know that they are going to be consistent and will always offer the same sharpening characteristics, from the beginning to the end of their working life when they have become thin. With natural stones, we cannot know this, because with a stone - even though it looks really good - we can only see the surface. As the stone wears in use, sometimes the next layer can have much different sharpening characteristics, or there can be a very hard inclusion or other irregularities that can greatly limit the usefulness of the stone.
And thirdly: 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 since every stone is different and as we said, can change with use. If we buy a box of 20 artificial stones, and we test one and it is of excellent quality, we can normally assume that the other 19 will be the same. With natural stones, this is not so. It would theoretically be possible to test each stone, but this would drive the 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)? With manufactured stones, we can expect very little new in the future. And to just sell our current line, and discover nothing new, is for me, to be honest, just too boring in the long run. As it happened, I found a man who makes 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 judges to be exceptionally good. This sparked my interest, and so now I am 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 largely unsung, but big, role in the technological development of humans. When one thinks about sharpening stones, there is a real sense in which they made it all possible especially in terms of working with metal. No farming without the proper tools, no woodworking without sharp planes, chisels, and saws, mealtimes would be very different without sharp hunting and cooking knives. Sewing requires sharp needles. Cutting tools and their sharpening stones built the basis for the development of many kinds of traditional handwork and specific skills. The need to find a suitable abrasive is very closely intertwined with the development of tools in general and dates back many thousands of years. Even today “high tech” man-made sharpening stones are praised by saying "It grinds like a real stone," that is, the manufacturer found the right materials to allow their stones to mimic the "HighTech" natural stones in use for thousands of years.
Who are these stones for? For beginners, it is best to work at first with relatively inexpensive artificial stones. With these stones most people can achieve a very respectable and useable edge on knives and woodworking tools. But a fair number of people over time become unsatisfied with “useable,” and as their working skills improve, so does their feel for sharpening, and they need better sharpening stones. For these folks, we recommend our high-end artificial stones, and for many these 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 into prehistoric sites have not only revealed an active tradition of blacksmithing dating back to the late Jomon Period (1,000 - 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 normally 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 natural Japanese sharpening stone, which can be considered perfect and has all the necessary attributes, can cost a fortune. It would have a very consistent grain, be uniform in texture and color (preferably yellow), would have no cracks, stains or other blemishes, and is over all a very beautiful stone. Such a stone is known as a "practical Diamond" in Japan, and they turn up maybe once a century. Normally natural stones are nowhere near perfect in appearance. The color can be various shades of yellow, brown or gray, can be different in different areas of the face of the same stone and 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 of the so-called "perfect" stones.
Origin and Cut: Take into account that the quarries for natural stones have often been mostly exhausted, and it is very difficult today to get really fine stones. The names of the stones generally come from the mountain where they were quarried. Here are some examples of names from Kyoto and its surrounding region:
Western mountains: Ohira, Shinden, Mizukihara
Central mountain: Atago
Eastern mountains: Nakayama, Ozuku, Kizuyama, Okudo, Shoubu, Narutaki
Currently nearly all quarries are closed. But there still exists a lot of raw stone material mined in the past waiting for being cut to appropriate sizes.
The stones are cut horizontally, so that the layers of sediment are 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: The ideas on how long before use one should put natural sharpening and honing stones in the water are, even in Japan, very different. 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: put the natural stones in a water bath for at least 30 minutes before use. If once you have used the stone for a while, you have the feeling that it is not enough, then you can try up to 24 hours or even longer if needed. The proper watering time serves to stabilize the structure of the stone to allow consistent results. It is important to remember that these are natural stones, every 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, expand, and this will damage or break the stone. Also, avoid rapid changes in the temperature or drying them out too quickly. Never put one on or close to a strong heat source! Naturally, they can also break if dropped onto a hard surface.
But, recognized and respected experts in Japan also - contrary to what we have written above - say that regularly-used natural stones can be left permanently in the water. They base this advice on the idea that, contrary to the various kinds of artificial stones which can be damaged if left in the water too long, natural stones can withstand this treatment in that they are in principle stones that were formed under water (sedimentary stones). While all this is true in principle, we feel very strongly that dry storage is the best way 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 binding 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 binding material, which is at least in part water soluable, 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 use every day, it doesn’t matter much at all. But for most people today, to respect the 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 theme is too often, and unfortunately, neglected, even though the lack of advice can mean 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. One must also understand the difference between sharpening knives and woodworking tools like plane irons and chisels. 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 iron can be sharpened straight and flat. This allows the maximum precision when working wood with these tools. A hollowed stone, even when you can barely see the groove, will cause problems with the different types of plane irons and chisels.
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 medium-grit (600-1000) sharpening stone: both natural and artificial stones work well, and harder stones are better than softer varieties. The most effective option is however a diamond plate of between 400 and 600 grit. If the stone has a deep hollow, and a lot of material needs to be taken off, 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 always use a coarse stone, of similar or larger grit size.
It is always a good idea to thouroughly rinse the stone after flattening it. If this is not done, often you will 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, which will tend to polish the edge and leave a mirror surface on the steel.
Tip: It is a good idea to get in the habit of flattening your stones after each time you use them. This takes advantage of the fact the stone is already wet and uses the abrasive slurry from the sharpening session. The stone is then rinsed thouroughly and put aside, ready to go when you need it again. How often do you see hollow stones laying around workshops? This often leads to putting off sharpening a tool that needs it, or the sloppy habit of a hurried sharpening on a hollowed stone because it makes a mess and takes time to flatten the stone. Either way, the job cannot be done right and poor work will be the result.