Read the other installments in the “Sharpen This” series via this link.
Let’s pretend I want to break one of your fingers. The job would be easy if you held up your fingers in the air with them spread apart.
It would be more difficult to harm you if you clenched your fingers in a fist. And it would be almost impossible if you stuck your fist inside a shiny and hard vase.
Violent fantasies aside, this exercise demonstrates one benefit of polishing your edges. A steel edge breaks down quickly when the iron atoms aren’t well connected to one another. So when there are deep abrasive scratches in your tool’s edge, those act like the space between your fingers. Without good support among the atoms (or your fingers) it’s easy to break them off – making a dull edge and a howling reader.
Clench your fingers in a fist, and you have created a durable structure that can keep your fingers intact. Sheathe your fist in something, and it’s going to take me a while to punish you for the naughty things you’ve done.
In sharpening, the act of polishing removes the deep scratches that separate the iron atoms, which are in a matrix with carbon. The fewer and shallower the scratches, the more durable the edge. This is, I think, easy to understand.
But what makes some people do silly things is the idea that they can create the ultimate cutting edge by polishing to finer and finer grits using particles that are less than 1 micron across.
In theory, sure. Polishing can refine an edge to an incredible degree. But it’s unlikely in the real world using real-world abrasives.
I’ve used sub-micron sharpening equipment (less than half a micron) that costs a stupid amount of money. I worked with these stones and slurries for months to get comfortable with them and improve both the sharpness of my edges and the finish on the wood. I concluded it was a fool’s errand.
In a real workshop, there are just too many abrasive particles on every surface to make this sort of crazy sharpening a practical thing. And the purity of the sharpening media itself – despite the manufacturer’s claims – plays a big role in the results.
Also, these super-fine abrasives cut so slowly that you might want to have “Heaven’s Gate” on your shop’s TV while you polish that one holy edge.
So What is a Practical Polish?
Believe it or not, the cutting edges of woodworking tools haven’t gotten insanely better in the last 200 years. That’s because we have always had abrasives that get the steel to the same approximate level of sharpness and polish. The pre-Industrial craftsman might have had stones that were inferior to modern stones, but he or she also had a strop, which is the great leveler among the sharpening cultures.
Strops are charged with fine abrasives – a micron or so – that break down to even finer particles with use. And so a fine polish and a wicked edge have been available for many generations. We know this from books, of course, but also from the furniture record. Visit the Winterthur Museum some time and observe the pieces made using ribbon-stripe mahogany that are nicely planed. Those cutting edges were plenty sharp.
In my experience, the sharp edge that can handle anything in woodworking is made with an abrasive that is about 1 or 2 microns – give or take. After that point, the time and care required to take the edge to a noticeably higher level simply isn’t worth it (unless your hobby is sharpening or you participate in Japanese planing contests).
If you want to polish beyond 1 micron, you’re not hurting anyone. So feel free. But for people who want to get back to the work as soon as possible, 1 or 2 microns is the sweet spot for an edge that is sharp and durable.
— Christopher Schwarz