Coticule, a Belgian sharpening stone sold worldwide

On the roads on the plateau, you can easily find a dull white stone among the blue-purple slate. This is the famous 'coticule'.

Coticule comes from the Latin coticula; from cos, cotis, “razor stone”)

This is a stone with a light yellow paste that contains numerous microscopic garnets (spessartine). These garnets give it an abrasive quality that undoubtedly surpasses that of all artificial materials. Coticule is a sedimentary rock that underwent metamorphosis when the slate layers were folded. It is found in the form of fairly thin veins (about 15 centimeters) in purple-hued schists. Occasionally, much larger pieces are found, but this is rare. Density: 3.2. Mohs hardness: 7 on a scale of 10. Therefore, for a collector, it is not a “pretty piece.” On the other hand, it is a mineral that is unique in the world. To date, there is only one company that extracts coticules in Lierneux, specifically in Sart, at a place called 'Thier del Preu.' It is Ardennes Coticule SPRL, led by Maurice Celis.

Coticule is sold in Belgium and abroad, primarily for sharpening precision components, among others in the medical sector. However, to ensure that the quarry remains profitable, everything that is extracted is brought to market.

History

According to Michel Caubergs, the first coticule operations date back to 1625 or slightly earlier. He explains that in 1686, the trade in coticules began to flourish and attracted foreign merchants. Old Rock, the mine that is particularly important to us, opened in 1800. Around 1860, about 50 workers were employed full-time in the vicinity of Vielsalm in the extraction of the ore. The decline of the coticule industry began in 1945 and the last mine (Old Rock) closed in 1982.

The current coticule extraction is not done underground. Maurice Celis operates in open quarries. He must process a ton of waste rock to obtain one kilogram of usable sharpening stone.

Shaping the stone
Coticule is extracted with a 'slate base.' In other words, about half of the extracted volume is kept in the slate. If the slate is ever damaged or if the surrounding ground does not allow for processing, a slate base is re-glued with resin. The slate layer might be too thick, for example. In that case, it is cut into different pieces. The well-known slate layer is then re-glued on top.

The blocks of coticule are cut into standardized sizes that correspond to the needs of the industry. Generally, these are cubes of 10 to 20 centimeters.

Diamond saws cut out more or less regular blocks. The irregularities are then removed using a turntable covered with abrasive cloth. This rubbing process produces perfectly flat shapes. Finally, the stone is sweetened and glazed. The stone is made as soft as possible by using a much finer abrasive cloth. All scratch marks must be gone. Glazing is the final polishing operation.

Jan Rymenams