Before the technology existed to produce finely graded sub-micron abrasives with narrow particle size distributions, the ubiquitous barber hone was commonly used to maintain straight razors.
In comparison to modern synthetic hones, these vintage barber hones seem relatively fine. One might wonder if some undocumented process existed to produce ultra-fine grit hones a century ago. A quick analysis with an electron microscope reveals that these hones are not fine by our modern standards. Abrasive particles are tens of microns in size, many in the region of 30 microns, corresponding to 600 grit.
Although the abrasive particles are relatively coarse, a barber hone is typically “conditioned” to smooth or glaze the hone to a point where the grit is nearly ineffective.
The image above shows a grit particle in the “conditioned” state – smooth and level with the surface of the clay binder.