What actually happens to an edge when it is sharpened?

Experiment: vad händer när du slipar en kniv med grova diamanter?

OK, this may not look like much to any knife or sharpening expert, but it’s the harsh reality when using 150 micron (0.15 mm) diamonds in a filing motion. I usually do this to quickly remove material with the aim of getting the sides to meet in a clean, sharp edge. In the image below, they hadn’t quite met yet, and we still had some filing left to do.

This was not the main point of the experiment, however, but rather to learn and get answers to a couple of claims I’ve heard from men (yes, only men) over the years (continued below the image):

You should only pull the knife across the sharpening stone – This made no difference at all compared to moving it back and forth
You should only push the knife across the stone – Again, no difference compared to moving it back and forth
You should use a circular motion on the stone – removes material faster than a windshield-wiper motion
If you start too coarse, you’ve ruined the edge and it can never become truly sharp again – no, you can recover it, but it requires more work with finer grits than I expected (this likely comes from using grinding wheels where heat can ruin the temper)

My conclusion is that you can do almost anything and still achieve a razor-sharp edge as long as you do the following:

Sharpen until the sides meet
Use a light touch to avoid pushing the edge over to the opposite side
Switch sides more frequently for the same reason

The edge below is the same as in the first image, but after going through 100, 300, 600 and 1200 grit, and then polishing with a leather strop to remove the burr. There are still some marks from the earlier coarse diamonds, but the small horizontal lines are 0.015 mm to give a sense of scale.

close-up of sharp knife edge — This is the edge after sharpening with 1200 grit and leather stropping to remove the burr

As a comparison, I took an image of something we all consider sharp—a razor blade. This is an industrial razor blade, but brand new and straight out of the box: