How do I flatten a sharpening stone, and how often?

Stones wear where they are used, which is the middle. Once a stone is hollow, a blade laid on it rocks, the bevel becomes convex, and the flat back of a chisel is ground into a curve. Everything downstream of the stone inherits the stone's shape, which is why this small maintenance job has more effect on sharpening results than almost anything else.

What you'll need

• Coarse diamond plate, flattening stone, or wet-and-dry on float glass
• Pencil
• Water and a bucket or tray
• Straightedge (optional)

Step by step

1. Check whether it is out of flat.Lay a straightedge across the stone in several directions and look for light underneath. Or simply pencil a grid on it — after a few strokes on a flat surface, the areas where the pencil survives are the low spots.
2. Draw a pencil grid across the whole face.A criss-cross of lines with a soft pencil. This is the progress indicator: the job is finished when every line has been abraded away, and not before.
3. Choose a flattening surface.A coarse diamond plate is the fastest and most convenient, and it also does duty as a coarse sharpening stone. A dedicated flattening stone is cheaper. Silicon carbide wet-and-dry paper — around 120 to 220 grit — stuck to float glass or a granite tile works well and costs almost nothing.
4. Flatten under running water or in a tray.Water stones produce a slurry that has to be flushed away or it just cushions the cut. Work under a slow tap or in a tray of water, and rinse frequently.
5. Work the whole face evenly.Move the stone in circles or figures of eight over the flattening surface, and rotate the stone regularly so you do not favour one end. Keep the pressure even and moderate.
6. Stop when every pencil line has gone.That is the definition of flat for this purpose. Rinse, dry the edges, and check with a straightedge if you want confirmation.
7. Chamfer the edges.Take the sharp arrises off the stone's edges with a few strokes of the flattening plate. Sharp edges on a stone chip easily and can nick a blade being drawn off the end.
8. Set the frequency by stone type.Soft water stones need flattening almost every time they are used — they cut fast because they wear fast, and that is the trade. Harder ceramic and combination water stones need it every few sessions. Oil stones wear slowly and need it occasionally. Diamond plates and glass-backed stones never need it, which is a real part of their appeal.
9. Flatten before a job, not after.A stone put away flat will still be flat, but flattening at the start of a session guarantees you are working on a known surface. It takes a minute.

Tips

• Never store a water stone permanently soaked unless the manufacturer says it is a soaking stone — and never leave any stone where it can freeze, which cracks it apart.
• A stone that has been dished for a long time may need a lot of material removed to come flat. It is often quicker to start with coarse wet-and-dry on glass than to grind it away on a diamond plate.
• The same principle applies to the flattening plate. Coarse diamond plates do wear, and a worn one will not produce a flat stone.
• If your blades keep coming out with a rounded bevel despite using a guide, suspect the stone before you suspect your technique.

Keeping on top of it

• Flatten soft water stones (Every session, or every few blades)
• Flatten harder combination stones (Every few sessions)
• Flatten oil stones (Once or twice a year in normal use)
• Check the flattening plate itself for wear (Annually)
• Dry stones fully before storing (Every time)

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