Sharpening: Belgian whetstones

Coticules (Belgian whetstones) are found in only one place in the world: the Belgian Ardennes. European Blades Mag. explains the different types and what they are used for.
Text and pictures: Bas Martens

You can look at sharpening a knife in two ways. One sees it as a necessary evil, a job that, while it must be done carefully, ultimately serves only to regain a sharp knife. The other sees the sharpening process quite differently: a Zen-like ritual, in which man and knife fuse into one. Well, that might be a bit of an exaggeration, but you get the idea.
The first group does not care much what they sharpen with; as long as it is quick and easy. For the second group, time does not matter. They are looking for beauty and perfection, and then, according to many, there is only one way: sharpening with stones.
For this, you have Japanese waterstones and Belgian waterstones. The latter will be described here. The Belgian stones are among the best in the world in terms of quality. There are two types: the real Coticule (or Coticule Jaune) and the Belgian Blue Whetstone (or Coticule Bleu). To avoid any misunderstandings: the Coticules we are talking about here are not grindstones but whetstones. So they are only used for the final stage of the sharpening process. They are not there to correct a frayed cutting edge, but to turn sharp into super-sharp again. The exception are the Coticules Pyrénées: a combination of the Belgian Blue Whetstone and a coarser 'Pierre des Pyrénées', suitable for pre-grinding.
Three types of Coticules and Belgian Blue Whetstones. On the bottom a BBW (Coticule Bleu), in the centre
a Coticule Pyrénées, and on top a Coticule Jaune with a slate base. All three are 100 x 40 mm.

Coticules and Belgian Blue Whetstones (BBW) are natural whetstones, which have been mined in a slate quarry in the Belgian Ardennes since Roman times. It is sedimentary rock formed over 480 million years from greyish-yellow volcanic ash and clay with a fine grain structure of garnets, which are almost as hard as diamonds. The garnets are 5-15 micrometres in diameter (a micrometre or micron is one thousandth of a millimetre). The Coticule Jaune has a yellow colour and at least 40% garnets. The Belgian Blue Whetstone is blue (because of the iron oxide) and has at least 30% garnets. The Coticule Jaune therefore has higher quality than a whetstone, but is also scarcer. Moreover, not all Coticules are completely pure. Only the high quality ones have a homogeneous stone with equal hardness everywhere.
The Belgian Blue Whetstone and the Coticule are mined from the same locations. The BBW occurs in broad vertical layers with the thin Coticule layers in between. Mining is very labour-intensive and is done virtually without machines. One kilo of Coticules requires moving a ton of rock. Because of the thin layers, Coticules are very fragile. To use them, they are glued to a layer of slate. The BBW is harder and thicker and usually does not need such an additional underlayer.
The Coticule and BBW are suitable for all common steels, up to powder metallurgical steels, and HHS steels up to a Rockwell hardness of 64. To use the stones, they must be wetted. Sprinkling with some water is sufficient. The stone is not porous, so it does not absorb water.
During grinding, the garnets are released and form a grinding paste. This paste does not scratch, but gives the blade a tiny burr. The maximum sharpness is obtained by honing the blade on a leather strop. This also makes the stones very suitable for sharpening razors. After honing, the stone should be rinsed well to remove the rubbed-off metal and stone particles and then dried. With regular use, the stone can be hollowed out. Those hollows can be rubbed away with a sheet of glass paper.
With thanks to Belgischer Brocken for providing the whetstones. The complete range can be found on their website,
Photo captions:

Water, grinding paste and metal particles are clearly visible on the yellow surface. They can be quickly
rinsed off with water.

The Coticules are available in many different sizes. This one, 100 x 40 mm, is perfect for pocket knives.