A ceramic sharpening stone is a hard, durable stone that will provide thousands of sharpenings before needing maintenance. Over time, the force exerted on the stone by sharpening blades will cause the surface of the stone to become dished in the middle where most of the sharpening occurs.
In order to continue using the ceramic stone successfully, you will have to flatten it. Due to the hardness of the stone, this is easiest to accomplish by using a diamond or ceramic lapping plate, which is a large square or rectangular plate specially designed to flatten, or lap, sharpening stones.
Place the lapping plate on a stable surface such as a counter top, work bench, or sturdy table.
Grasp the ceramic stone firmly with both hands with the dished side facing down.
Pass the stone over the lapping plate in a figure eight motion while applying even downward pressure to the stone.
Check the stone every so often to see if it is flat. Place a metal ruler or other straight edge across the face of the stone in several directions. Hold the stone and ruler up to a light. If you can see light between the bottom of the ruler and the stone, continue lapping it.
Repeat the process until the face of the ceramic sharpening stone is flat.