..Coticule.. made in Belgium

by Mathieu

Next week I will be giving a workshop for some woodworkers about freehand sharpening and I have been spending quite some time preparing it. True out the last couple of months I researched some related topics and wrote a small essay to illustrate my personal points of view on the subject.

Sharpening is quite  an interesting and controversial subject. Propably every woodworker has his own system or opinion regarding it. Well I plead guilty, I am one of them. I am not going to share my thoughts about sharpening today, instead we will look a bit closer at one of my favorite natural stones The Belgian Coticule.

I happened to have some promising raw pieces of rock laying around that I wanted to prepare for the upcoming workshop. I found them in the area where they used to be mined in the southern part of Belgium called Wallonia. Between Luxembourg and Luik there is a small town called Vielsalm where they have been mined since at least the 16th century, probably earlier then that. It is a sedimentary stone which originates out of volcanic ashes and turned into stone by heat and pressure to create swirling veins held in between layers of slate and blue stone. It is a unique geological creation that is only found in a very small region in Belgium and nowhere else on the planet. The picture shows a Coticule vein.

For people who like sharp edges the interesting part is the content of this stone. It consist of paragonite, quartz, white mica (hence the color) and ‘granaat’. It is this ‘granaat’ which is the active sharpening component. The quality of the stone greatly depends on the percentage of these components. The best quality Coticules will have up to 40% ‘granaat’ particles in them. If this percentage is higher the particles tend to bind together decreasing the effectiveness of the stone. The best stones have ‘granaat’ particles that are 2 micron or smaller. If there is too much quartz the stone will be too hard and barely usable or just too slow. Sometimes you find small black spots in them and I suspect that is charcoal. I was wondering what these where for a while and no one I talked to and was knowledgable about these stones could tell me anything more until last year when I was in Japan and visited a renown toishi dealer in Kyoto. He was showing us his finest stones and together we examined the surface of them with a microscope. He then told us that the black spots you could see where charcoal. I suspect that it is the same case with the Coticule’s, it is still a guess but it would make sense.

Finding stones of really high quality is very difficult and you need considerable experience in sharpening with natural stones in order to determine whether the stone is really fine and suits your needs or not. Last year I visited the only workshop where they still prepare the Belgian stones for retail. I was lucky enough to acquire two very fine Coticule’s of considerable size which left me about 500€ poorer. I never regretted it and gave the best one away to my teacher Kaneko-san.

The problem with finding a good Coticule is that you have to rely on salesmen most of the time and with natural sharpening stones it is important that you are able to test and evaluate them before you buy. Unfortunately this is not always an option. Most of the stones you can find are laminated, which means a layer of the white Coticule is glued with epoxy to a layer of blue-purple slate. Just like you would find them in nature, except a little different. Here you can see them sorted and ready to be epoxied to the blue stone.

The blue stone can also be used as a sharpening stone but personally I find them less suitable. They are never as fine as the Coticule’s, are much slower and too soft for my own taste.

The stones I bought when visiting the stone workshop where  naturally laminated, which are rare. It is advisable to keep a layer of the blue stone attached to them for strength since the white part of the stone can be fragile. At least that is what they say, personally I think it is mainly because of economic reasons that the stones are cut like that and it is not mandatory to glue any backing to them, unless you have a stone with a fault in it.

Today it took me a couple of hours of flattening using a grinder, carborundum stone and diamond plate too flatten some stones to a point where they are usable. I worked on several stones but only one came out really nicely, luckily it was one of the biggest one I have. It is the right one in the picture.

On the left you can see the other large one I worked on. It turns out the be super hard, even the diamond cutting disc had a hard time on it. After an hour I gave up on that one although I might get back to it later but I’m not sure if it will be useable, we’ll see. In the middle you can see a small stone which is a cut-off of the nice stone. It is somewhat harder then large stone itself and will make a perfect nagura. 

Nagura is the Japanese term for a small stone used to create slurry on the surface which can facilitate the sharpening process. These days diamond plates are often used for this purpose. With the hardest stones this is mandatory and might prove the solution for the other stone.

I haven’t used the stone but I sure will tomorrow and I am pretty convinced it will be a winner. I have a lot of experience with these stones and know how to get the most out of them. In general you could categorize Coticule in the #6000 to #8000 range depending on quality which, like I said before, can only be determined by using them and evaluating your edge and bevel. The reason why I like them so much is their speed. They are some of the fastest stones out there and therefore I prefer them as a ‘site stone’. When working onsite I often don’t have much time too spend on sharpening and then they are just unequaled. For sharpening plane blades which I use for finishing wood I prefer a natural Japanese stone like a Honyama but for anything else Coticule’s are sufficient.

It is not so clear in the picture but the good stone on the right is purely Coticule with no blue stone attached to it. It is solid enough to be used as it is and there is no reason to back it up by glueing anything to it.

There is also a museum dedicated to the history of this sharpening stone. I visited it some years ago and for a blinkered sharpening geek like myself it was a very interesting place. It is a small museum but nonetheless I spend a couple of hours wandering around and imagining the days of yore when mining and cutting was done by hand. It is worth a visit if you are ever around there. Besides geological and local history they display many of the old tools that where used for mining and preparing the stones like this grinding wheel used to flatten the surface. I wish I had one like that today…