..Coticule.. the sequel

by Mathieu

After writing the second post of this blog, where we discussed the Belgian Coticule, I continued my research on the topic and came to the conclusion that there is much more to be told on the subject. In order to give a complete and in depth report on these stones I have a lot of work ahead. I also noted my poor writing style and some grammatical errors. I hope to improve myself but this will take time and effort.

First I should address some errors, some of the statements I previously made are just plain wrong. My apologies for that! I could have chosen to edit the previous post but instead I will leave it as it is and present some more accurate information in this sequel. The original Coticule post will remain a silent witness of my steep learning curve in blogging.

Coticule is a composition of two Latin words cotem or the feminine diminutive form of cos, cotis which means whetstone and ‘novacula’ which means razor. A whetstone for razors so to speak. (Not to be confused with the Novaculite which is a family of American oil stones, the famous Arkansas Stone is one of them but these have very different properties.) It is mainly in this subculture of straight-razor users that this stone is still extremely popular and they are probably the largest community who still uses them today. We will not discuss the Coticule in the light of shaving, only in relation to sharpening tools for woodworking.

The active sharpening particles are properly referred to as ‘spessartite garnet crystals’ or simply garnet. I referred to them as ‘granaat’ which is the Dutch equivalent and therefore it was printed Italic. These particles turn out to be larger then I thought and instead of 2 micron they are actually 10 to 15 micron (other sources state 5 to 20 micron). Because of the shape of the crystals, which is to be compared to a rhombic triacontahedron, it is only the edges and corners of the crystals that scratch the steels surface. It is the depth of the scratches created by the garnet crystals that is about 2 micron deep within good quality stones. This makes sense if compared to a Shapton Glass stone where the abrasive particles are 1.84 micron in their #8000 stone.

On the Mohs scale of mineral hardness, Talc has a hardness of 1 and Diamond has a hardness of 10. Garnet is rated between 6.5-7.5. Other abrasives commonly used in synthetic sharpening stones are aluminum oxide with a hardness 8.0-9.0 and silicon carbide which scores 9.0-9.5 . This might give you an idea of the particles hardness but to me it remains subjective how this translates to the quality of an edge sharpened with a Coticule.

My little research revealed another interesting fact. I have always been told that Coticule was only found in Belgium but apparently this is not the case! In Nova Scotia there are very similar veins of Coticule to be found. The veins are intricately folded in most places and not thicker then 1/3 inch leaving them useless for the purpose of mining sharpening stones. It also remains unclear to me if the content is the same within the stones from other regions. The Belgian Coticule layers are originated around 480 million years ago and I couldn’t find any dating regarding the Nova Scotia rock. Further reading revealed that there are similar rocks to be found in Massachusetts, Wales, Ireland, Norway, New England and Newfoundland. So much for the ‘only in Belgium myth’. Eventually it became clear to me that many rock types which are slightly different in origin and content are referred to as Coticule. According to J. A. Thompson, “Coticule’s are garnet-rich quartzite’s and are chemically distinctive lithologies of controversial origin.” It is a field term to describe fine-grained garnet-rich granulites which leaves room for interpretation. To make a fine sharpening stone the garnet crystals must be sufficiently small and the quartz must be finely grained, this is only the case with Belgian Coticule’s I suppose.

The sole miner of the Belgian Coticule’s only makes a distinction in two qualities which he refers to as “standard” and “selected” (see this page). I believe this is rather inaccurate and contrary to what is stated I think there is a clear distinction in the quality of these stones. I have used many different qualities of Coticule and I could easily place all the stones that I have ever used in order of hardness, edge quality and consistency. I use a handheld x200 microscope to evaluate my edges and this is already sufficient to see a clear distinction. When the edge quality obtained with a certain stones is evaluated it becomes clear that stones form different strata have different properties.

One of the employees of the mine who is a geologists is actually trying to establish a grading system based on the different geological layers which occur in the region. This clearly indicates that my personal conclusion might hold some truth. The owner of the mine also employs a more distinctive grading system which is only used internally in his business. He distinguishes “3th quality” for stones with severe cosmetic flaws, “standard quality” for stones with some cosmetic flaws, “selected quality” for stones with no cosmetic flaws and then a 4th grade called “Kosher” for those rare specimens that are absolutely impeccable. His grading system relates only to the appearance of the stones but also this can be an indication of their performance.

If we would like to come to an indisputable conclusion it would require some serious and complicated scientific research taking into account all possible variables. It would be an interesting study but good old wisdom based on experience is sufficient for me at the moment. I will happily revise my conclusion since I always seek to learn and broaden my views but so far I could not conclude that the amateurish research on the performance of the sharpening stone is convincing in any way. There are to many variables which have been left to personal interpretation. Of course this is also the case with my own conclusions but nonetheless I stick to them for now.

The Belgian Blue Stone, originally known as ‘La Dressante au Blue’ , is worth discussing as well. Let’s look at the etymology first and see what we can conclude from there. ‘Dresser’ could mean as much as ‘set up’, ‘lay’, ‘set right’ etc. . This leaves me to speculate that the Blue Stone originally was used as backing material for the fragile white Coticule layer. This is backed up by the fact that until recently there where companies specialized in cutting the Blue Stones used for backing material. And they where able to use the best quality Blue Stone the could find for it. Those where found adjacent to the Coticule layers because only there the Blue Stone contains sufficient garnet crystals to make for a decent sharpening stone on its own. There are only a few blue layers that are suitable for mining Blue Stones that are to be used for sharpening and  small pink dots in the surface of are an indication for good quality. Today they use a Brazilian slate flooring tile as a backing material because it is more economical and easier to acquire in large quantities. Therefore it remains to be seen if the ‘modern’ backing stones can be used for sharpening because of their different properties.

If you are looking for a fast and homogenous natural sharpening stone which will give you a fine edge on chisels and plane blades not used for finishing, then almost any Coticule will do except the lowest grades. The Belgian Blue Stone has less garnets then a Coticule and is therefore much slower and since they tend to be quite soft and less fine I categorize them in a different league. When you are looking for a really fine stone of exceptional quality, my suggestion is that you can delve into the subject yourself, gain experience with using them and then endeavor on a search which might take some time. Maybe we can look into the different types of Coticule but I will leave that for the next chapter on this marvelous stone…