Japanese Natural (JNat) whetstones are believed, by their devotees, to provide a unique honing experience. It is observed that the slurry changes character during use, supposedly “breaking down” into finer abrasive, and providing a transition from the initial grit to a finer grit.
To test this theory, slurry was produced on a Nakayama Asagi with a piece of the same stone (tomo). The slurry was captured and rinsed with DI water.
A second slurry was produced in an identical fashion, this one was “worked” by honing a carbon steel razor for approximately ten minutes. The slurry turned black with swarf during this time. The slurry was also captured and rinsed in DI water.
Each slurry sample was deposited onto a piece of polished silicon (featureless background for imaging) and images were taken of each sample.
To interpret the images above, we need to identify the different types of slurry particles. This analysis is accomplished within the SEM using EDX (Energy Dispersive X-ray Spectroscopy).
X-ray maps show the composition of each particle.
The data show 3 predominant types of particles; Silicon Oxide (silica or quartz) and two types of silicates; Potassium Aluminum Silicate, Magnesium Aluminum Silicate. A small number of Iron-containing silicates and iron oxide are also visible. A part of the above image is annotated, below, showing various particle identities.
The image and corresponding data confirm that the abrasive component is silica, and that there are several morphologies (shapes) of silica particles. High resolution imaging of the various silicate particles reveals that they are Phyllosilicates (from Greek φύλλον phyllon, leaf, or sheet silicates) e.g. Mica and clay.
From The Wikipedia page on Silicates:
Silicates comprise the majority of Earth’s crust.
In geology and astronomy, the term silicate is used to denote types of rock that consist predominantly of silicate minerals. On Earth, a wide variety of silicate minerals occur in an even wider range of combinations as a result of the processes that form and re-work the crust. These processes include partial melting, crystallization, fractionation, metamorphism, weathering and diagenesis. Living things also contribute to the silicate cycle near the Earth’s surface. A type of plankton known as diatoms construct their exoskeletons, known as tests, from silica. The tests of dead diatoms are a major constituent of deep ocean sediment.
Silica, or silicon dioxide, SiO2, is sometimes considered a silicate, although it is the special case with no negative charge and no need for counter-ions. Silica is found in nature as the mineral quartz, and its polymorphs.
With this information, we can interpret the “raw” and “worked” slurry images above. The silica (abrasive) particles show no evidence of breaking down into smaller particles. The Phyllosilicate particles do break down into individual sheets or flakes; as expected these clay materials are relatively soft.
In conclusion, there is no evidence that the silica (abrasive) particles “break down” or become finer with use. The soft clay binding material of the stone, composed of phyllosilicate material, does break down into individual flakes.