Belgian stones are mined from deposits in the Ardennes Mountains region. Which, in turn, are the western end of the Rhine Slate Mountains – one of the largest mountain systems in Western Europe, passing through the territory of Germany, Luxembourg, France and Belgium. The length of the mountain range is 400 km, with the highest point – the Groser – Feldberg mountain with a height of 880 meters. These mountains are composed mainly of shales, quartzites, sandstones and limestones. On the territory of Belgium, the rocks are mainly limestone and shale, formed about 480 million years ago, clay and volcanic ash.

As a result of geological processes over millions of years, in the thickness of so-called metamorphic rocks, which include crystalline schists, the formation of a rock-forming mineral, a semi-precious stone – garnet. During long-term weathering garnets as chemically resistant structures are not destroyed for a long time, but pass into placers and in the form of small crystals are preserved inside shales and limestones. Garnet is divided into many species: andradite, grossular, almandine, pyrope, etc. Depending on the variety, garnet has different hardness (from 6.5 to 7.5 on the Mohs scale) and density (for example, pyrope has a density of 3.57 g/cm3 and almandine has a density of 4.3 g/cm3). Garnet crystals with grain size from 5 to 25 microns are randomly scattered in the layers of Belgian shales and are an excellent abrasive, confidently working on steels with hardness up to 62 HRC. It is garnet that gives shale, which is ineffective at sharpening, the qualities of a good abrasive.

Shales themselves are a variety of rocks with parallel layered assemblages of minerals such as chlorite, actinolite, quartz, staurolite, etc. Under the influence of strong dynamic impacts, the rocks are transformed into crystalline shales that can be easily delaminated into plates and tiles. The ability to split into separate plates and makes it possible to mine them by simple percussion of a tool, without the use of special machines or explosion engineering.

According to local legend, deposits of slate in the Ardennes region began to be mined in the times of the Roman Empire. Slate was used mainly for building applications, as a facing material. It has become popular as an abrasive material in modern times. And it was mined in this quarry first of all Yellow Belgian Coticule slate, the so-called “Yellow Belgian stone”. And then, much later, similar abrasive properties were discovered in another stone – Belgian Blue Whetstone, abbreviated (BBW) – “Belgian Blue Stone”. Its blue-violet color was determined by the presence of iron oxide in the precipitate. It has even more abrasive power than Yellowstone, primarily because of its larger garnet crystals. Yellow Belgian Stone and Blue Belgian Stone are actually mined together. They are arranged layer by layer in narrow beds, with the Blue Stone being the predominant rock and making up the majority of the material extracted.

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The way both stones are used for sharpening purposes is extremely simple. They are water stones, which do not require immersion in water, but only soaking the surface of the stone and applying a slurry. The garnet crystals in the stones are released from the surface, together with the particles of the stone itself, and begin to work due to a fairly uniform and constant supply of slurry. The very structure of the shale acts as a kind of “binder” for the garnet grains. However, it should be remembered that there is no real binder in this stone and it will be produced rather unevenly and regularly require leveling. Belgian natural stones are best suited for pre-finishing sharpening and finishing polishing of the cutting edge. These stones work equally well on carbon and stainless, Damascus and high-speed steels. They can be used for finishing not only knives, but also woodworking tools and dangerous razors.

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The two main types of Belgian stones are:

1.Yellow Belgian stone(Yellow Belgian Coticule)

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The stone contains a high concentration of small garnet grains ranging in size from 5 to 15 microns. It represents 30 to 42% of the grains from the volume of the stone. The metal removal rate is similar to synthetic stones. Coticule is often compared to 6000-8000 grit according to the Japanese JIS system, but this is a very rough approximation. A major factor in sharpening with this stone is the density of the slurry. The thickest slurry can give about 1000 to 2000 grit, and a pure stone with water 16000 grit. Polishing with this stone gives a matte underwater finish, similar in appearance to the result of Arkansas Black natural stone.

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2. Belgian Blue Whetstone(Belgian Blue Whetstone) has a geometric shape of garnet crystals, the dodecahedron, identical to the Yellow Belgian Whetstone. But Blue Whetstone contains lower concentrations of grains up to 25% of the stone’s volume. However, the garnet grains themselves are significantly larger, ranging from 10 to 25 microns. Bluestone has a higher hardness than Yellowstone. The manufacturer indicates the grit of the stone to be approximately 4000 according to the Japanese JIS system, but as in the case of the Yellow stone, it should be borne in mind that the scale developed for synthetic stones cannot fully correspond to the natural ones.

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These stones are manufactured in both hand sharpening stone format and as small stones that fit the abrasive holders of the Profile K03, Blitz, Kadet and TSPROF PIoneer abrasive sharpening devices.

Ceramic bond is a special mixture of different types of bulk crushed components, which is supplemented with the main abrasive material and subjected to special heat treatment. The main abrasive fillers for ceramic bonds are silicon carbide and aluminum oxide.

As a result of heat treatment, ceramic bonds form two types: fusible (vitreous) and sintered (porcelain). After cooling down, melting bonds turn into glass-like, sintering ones are only partially melted and become close to porcelain in their composition. As a result of processing, the ceramic bond acquires such properties as water resistance, fire resistance, chemical and mechanical resistance. Different abrasive materials require different heat treatment.

Abrasive tools based on aluminum oxide (electrocorundum) are made on fusible bonds, and those made of silicon carbide are made on sintered bonds. Fusible bonds provide greater abrasive tool strength than sintered bonds. The disadvantages of the sintered bond are its brittleness and reduced bending strength. However, both bonds are considered to be hard. Under the hardness of the abrasive tool is understood, the ability of the bond to resist the tearing of abrasive grains from the working surface under the action of external forces.

Various raw materials are used for the production of ceramic binders: refractory clays, feldspars, wollastonite, boron and borlithium glasses, silica, lithium-containing materials (petalite, lithium manganate, molybdenum, etc.). All materials used in the production of binders are pre-dried, ground to a given coarseness (usually less than 100 microns) and mixed in various proportions. In order to increase plasticity, adhesives such as dextrin, soluble glass, etc. are added to the ceramic mass. Masks for abrasive tools are produced depending on the purpose of their use. Ceramic bond is marked with the letter K and has additional alphabetic and numeric designations. All bond varieties have additional indexing. For example, fusible ceramic binders have Russian marking K1, K5, K8.

Ceramic bond with silicon carbide powder is the most common, and is used to make most of the tools used for industrial grinding applications. The composition of the bond includes refractory clay, feldspar, talc, chalk, quartz and liquid glass. In Russia such clay grades as Latnenskaya, Polozhskaya, Novorayskaya are most commonly used. At the same time, the maximum effect is given by the use of coal clay or a mixture of refractory clay and coal-humus substances, which provide maximum strength. These types of raw materials give the binder additional porosity of the structure due to the burnout of carbon and organic impurities. This reduces the amount of carbon and in the final product, increases its strength. To improve wetting of silicon carbide grains with the binder, the method of coating the grains with fine powders, glasses of different composition is also used, as a result of which films are formed on the surface of silicon carbide grains, which, interacting with the binder, contribute to increasing the strength of the tool. In some cases, various modifiers, in particular so-called boron-containing fluxes, are used to increase the strength of such a bond. Manganese sulfate and manganese carbonate may be added to the bond as “modifiers” in amounts up to 2% of the total mass, which also contributes to increasing the strength and hardness of such bonds.

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Examples of ceramic bonded products used for knife sharpening include “Profile” stones based on silicon carbide. They demonstrate a good hardness of the bond and confidently cope with any steel.Also ceramic bond is used in the American Boride stones series T2, which are made on the basis of aluminum oxide and demonstrate a very high hardness of the bond. They also work on any steel, quickly remove metal, are productive and have a long service life. We will tell you more about these stones in a separate article.