How does a Belgian Blue Whetstone compare to a Coticule? (a story about garnets)

The Belgian hones (Coticule and Belgian Blue Whetstone, commonly abbreviated to BBW) are very closely related. They are mined together. Actually is is impossible to extract any Coticule without also extracting massive amounts of BBW. Traditionally, Coticules were always backed by a piece of BBW. Some bonded together by nature, but most of them glued together by man. The main reason for this is to reinforce the Coticule part with the stronger Blue stone. Nowadays, Coticules are backed with a piece of Portuguese Slate, because it is more cost-effective to use easily available slate tiles for backing a Coticule than to produce labor intensive backing stones out of BBW rocks. The BBW stones are now sold separately, because they posses honing qualities of their own. Both the Begian hones are used with "slurry" This is an abrasive milk raised on top of the hone by moistening the surface with water and rubbing it with a small piece of Coticule/BBW The slurry of the Belgian hones contains spessartine garnets. These garnets are extremely hard mono-crystals with a very typical shape.

Highly magnified representation of a Spessartine Garnet

They are hard enough to scratch steel. A slurry is not  made of garnets only. It also contains water (obviously), and a great deal of phyllosilicates. Phyllosilicates are minerals that are much softer than steel. These minerals bonded the garnets together before they were abraded off the stone by rubbing with the slurry stone.
On a hypothetical note, that explains what we have found in frequent observations, small garnets can easily sink in the steel. Larger garnets can't sink so deep in the steel, because the honing pressure is spread out over a wider surface area. Try scratching a table with a spoon. Now try the same with a fork (Don't send your wife or mother to me when you run into trouble over this important experiment;-)) Back to the garnets: with the size of the segments at the garnets' surface being roughly the same, smaller garnets are less rounded than larger garnets. That makes smaller garnets more "spiky".
Conclusion: large garnets leave slightly wider but also more shallow scratches, while small garnets cut more aggressively and deeper.

Coticules contain a high concentration of small garnets (5 to 15 micron in diameter): up to 40% of the entire volume of the rock.
BBWs contain a lower concentration of wider garnets (10 to 25 micron in diameter): up to 25% of the entire volume of the rock.

That makes BBW's slower than Coticules and if we are aiming for serious steel removal during the first stages of honing, the average Coticule will perform a great deal faster than any BBW. Hence, if the choice to do serious bevel correction is between Coticule and BBW, pick the Coticule.

But there's more to the picture:
The loose garnets in the slurry also abrade the very edge, while it plows through the fluid. The thicker the fluid, the higher the concentration of garnets (less water) and the more the thinnest part of the edge deteriorates from the impact with the garnets. That is a dulling action. At the same time, the garnets remove steel from the bevel sides, which is a sharpening action. As long as the edge is not very sharp earlier during the honing process, the dulling effect is negligible, because the tip is not as fragile. When the edge becomes sharper, the tip becomes more fragile and more prone to "slurry deterioration". At a given point, there's a limit where the edge looses as much keenness as it gains. You could hone the razor into oblivion and not ever get a sharper edge than this limit. One of the advantages is that you'll never "overhone" a razor on a Belgian slurry, allegedly a common danger on synthetic water hones. With normal pressure and regular razor honing methods, we have never been able to see any evidence of "overhoning" on the Belgian hones.

At equal slurry thickness, a BBW with its lower concentration of less aggressive garnets, has a sharper limit than a Coticule.

Of course you could use thinner slurry on a Coticule, but for the inexperienced user it's still easier getting more keenness of a BBW with slurry than hitting the same level on a Coticule with thinner slurry. With experience it can be done, however, and the BBW becomes pretty redundant for razor sharpening at that point.

When no slurry is used, but plain water instead, we get a different story. The garnets remain halfway or more embedded in the surface of the hone. A Coticule with its small garnets in higher concentration offers a very finely textured surface. The BBW, having more sparsely spread bigger garnets offers a less fine surface. Both hones become very slow. The BBW becomes so slow, that it seems to loose most of it's honing qualities. It almost behaves like a piece of marble. It seems to very slowly dull the edge rather than sharpening it any further. That's why it is not recommended to dilute the slurry to plain water on a BBW.
But Coticules just keeps on going, albeit at an extremely slow rate.

No slurry, no slurry deterioration either.

Obviously the edge won't gain sharpness infinitely. As with any hone there still is a limit. That limit is defined by how deep the hone cuts in the steel. A hone that digs 0.5 micron into the steel will never define an edge thinner than those 0.5 micron. That's one good reason to use minimal pressure during the final stages of honing. Coticules on water are slow. They remove almost no steel with each honing stroke. For that they can define very sharp edges. But they are so slow that it takes ages to catch up if you weren't already at a very decent keenness when you decided to start working on water only.
A Coticule with water is a finishing hone. The majority of them can't be used to make up for much neglected keenness earlier on in the honing process.

Last update on 2010-02-14 by Bart Torfs.

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