Unless you are cutting other ceramics or diamonds, these knives should almost never get dull. Use a wood or bamboo cutting board (ceramic, glass or metal is a no-no) and push less. They cut so well because they are so much harder than anything you will encounter in cooking. Bone has a hardness of about 3.5, steel knives about 6.5 and ceramic knives about 9.5. Diamonds are 10.
If you really want to sharpen them you will need a fine or very fine diamond sharpener. The diamond size should be as small as you can get—1000 or more (6 microns or smaller). DMT makes good ones. You can find diamond sharpeners at better hardware, woodworking, and sporting goods stores. They should cost $20 to $60. The trouble is that ceramic is so hard it will take a lot of effort to restore the edge. Use water to lubricate and clean your sharpener. Be prepared to rub (use light pressure) about a dozen times, rinse the diamond and repeat many, many times. Inspect the edge with a magnifying glass. General knife sharpening rules apply.
If you look carefully at the very edge of the blade you will see that it is sharpened at an angle that is different from the rest of the blade, try to match that angle. A sharper angle will result in a sharper knife for a little while until the edge chips off in your food (generally not good). A more blunt angle will last longer, but will not cut as well.
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