Instructions

1. Use an old floury potato rather than a waxy new one. Floury varieties collapse into fluff when they rehydrate, which is what a gratin wants; a waxy one comes back rubbery.
2. Peel them or scrub them, but be consistent across the batch. Skin dries harder than flesh, and a mixed tray leaves you with two doneness tests to run instead of one.
3. Drop the cut pieces into cold water as you work. Potato flesh goes pink and then grey within minutes of meeting air, and the water keeps the air off until you are ready.
4. Cut slices at 3 mm for scalloped potatoes, or cubes of about 9 mm if they are heading for stews. Do not mix the two shapes in one load, because they will not finish together.
5. Boil the pieces for about five minutes, then drain and cool them. This is the step nobody should skip: it kills the enzymes that would otherwise go on darkening the potato in the jar for months.
6. Pat the pieces dry and spread them without touching, seasoning them now if you want them seasoned. Wet starch glues slices to the mesh.
7. Dry at 54 °C for 9 hours. Slices turn translucent and glassy, cubes go hard and faintly yellow, and both shrink far more than you expect them to.
8. A finished piece is brittle enough to snap and makes a hard click when dropped on the bench. Anything leathery is holding water at its centre, and potato is a starchy low-acid food where that matters more than it does in fruit.
9. Cool them right down and seal them in jars in the dark, where they keep for a year. Cover them with boiling water for twenty minutes to bring them back, and use that water in the dish — it carries most of what leached out.