Waterstones consist of abrasive particles that have a particular grit size. According to Spyderco, a prominent maker of ceramic sharpening stones, it is not possible to give such a grit rating to the Spyderco ceramic stones. That has caused many questions as to how these ceramic stones work.
These questions were also asked when Wicked Edge introduced their micro fine ceramic stones. These stones are made by the same manufacturer as the Spyderco ceramic stones. Wicked Edge stated that, although the micro fine stones are rated according to a particular grit size, they probably weren’t of that size.
This article aims to provide some insight into the way ceramic stones work and how that is different from waterstones
What is the mystery?
A synthetic waterstone consists of abrasive particles embedded in a binding substrate. A waterstone abrades because these particles remove material from the blade being sharpened.
How a waterstone abrades depends on many factors. However, the size of the abrasive particles is the most important one. This determines the size of the scratches made by the stone when abrading. The grit size of a waterstone is usually defined as the average size of its abrasive particles.
There are many systems for rating grit sizes. Perhaps the most universal rating method is based on the particle size in microns. Another common method is to count the number of holes per inch in a mesh to sift the particles. Here standards from various continents have slightly different numbering systems. A grit comparison chart can be used to compare them.
Enter the Spyderco ceramic stones. When explaining how the Spyderco stones should be rated, Sal Glasser, head of Spyderco, stated:
We’ve spent a great deal of time trying to determine grits for our stones. The manufacturer has also worked with us, to no avail. A guess seems to be best.
Most abrasives are measured by the grit size used in the matrix. Our ceramic doesn’t work that way. Grit size is constant.
We’ve tried to compare scratch patterns […] and this is probably the closest, but nothing that we can say “This is blah blah”.
Uh? Don’t the Spyderco ceramic stones consist of abrasive particles? Or are these particles sizeless?
These questions have come up over and over again on various websites, on various forums and in various chats I have had. How do the Spyderco ceramic stones work?
The issue got into focus again when Wicked Edge introduced their micro fine ceramic stones. These stones are made by the same manufacturer as the Spyderco ceramic stones, Coorstek. When explaining the grit rating of the micro fine ceramics, Clay Allison of Wicked Edge stated:
How the micro-fine ceramics fit into the mix is creating a lot of questions, with me as well. The stone manufacturer uses a different grit rating than what we’re used to (the many we’re used to) call RA, or roughness average. They sent me the specs for the grit early on as 1.4um and .6, but I have doubts that we can really rate it that fine.
Clay also suggested lapping the ceramic stones with a diamond plate to enhance their performance, which is something that struck many people as odd, since this shouldn’t change their grit size.
This picture shows that the abrasive particles of the ceramic appear to have fused together to form larger particles that are flat across the surface. It is likely that the sintering process used to create the ceramic stone causes this.
So it seems we can view this ceramic stone as a rather dense stone in which the abrasive particles have fused together to some extent. (This probably also explains at least in part why these ceramic stones are much harder than waterstones.) Now a clean ceramic stone of this type would not cause many scratches to the edge.
However, by texturing it with a diamond plate, we’d effectively create abrasive “high spots” on the top of the stone. In this way the texturing influences the effect a ceramic stone has on the edge.
At least, that is the hypothesis.
An experiment would have to corroborate this. Would a ceramic stone lapped with a coarse diamond plate leave a rougher finish on the edge of a blade than a ceramic stone lapped with a fine diamond plate?
Clay performed the experiment. He lapped a ceramic stone using a DMT 50 diamond plate and then sharpened a blade edge with it. He then lapped this stone using a progression of DMT diamond plates, finishing at 3μ. Using a microscope Clay photographed the edge at every stage at a magnification of 2000x. The results are shown below.
The first picture shows the edge of the blade before sharpening. It is very smooth.
The next picture shows the edge after sharpening it with a ceramic stone lapped with a progression of DMT diamond plates, finishing at 3μ.
The third picture shows the edge after sharpening it with a ceramic stone lapped with a 50 grit DMT diamond plate (about 330μ).
It is clear that the coarsely lapped ceramic stone results in a much coarser scratch pattern on the blade edge than the finely lapped ceramic stone.
So texturing a ceramic stone definitely impacts the edge finish a lot. And in a way that is consistent with our hypothesis.
The Spyderco and the Wicked Edge ceramic stones (as well as other ceramic stones?) consist of abrasive particles embedded in a binding substrate. However, unlike with waterstones, the sintering process causes the particles of these ceramic stones to fuse together, so that they effectively form larger particles. These larger particles may, like in the picture Clay posted, even be flat, so that they create one flat surface that does not abrade much.
It seems that by texturing such a ceramic stone with a diamond plate one breaks up the surface or the large fused particles, creating abrasive “high spots”. The smaller these high spots are, the more finely the stone will abrade.
This explains why ceramic stones cannot be rated effectively using grit sizes. And why texturing a ceramic stone does matter.
One question remains: won’t ceramic stones eventually become flat again and loose their abrasive power? Or, as Ken Schwartz stated it:
does it imply that as the surface wears the grit changes?
The answer to both questions should be: yes. However, ceramic stones are very hard and it may be a long time before one notices this. I have used my Spyderco ceramic stones for over four years now and my Wicked ceramic stones for over a year. In this time these stones haven’t changed much.
As Sal Glasser put it:
This post is based on a discussion at the Wicked Edge forum.
50,000 years from now when the Pyramids have crumbled and humans are but a memory, Spyderco ceramic stones will be seen sticking out of the earth, ready to work.
Many thanks to Clay, Phil, Tom and Ken, and all other posters.