Seasoning a French Top, and the Mistake That Cracks One

How to clean La Cornue stove top: three rules, and the reasoning behind them

The plaque coup de feu is a slab of cast iron heated by one 7,500 BTU burner under its center. Looked after, it will outlast the kitchen it stands in. Mistreated once, it cracks — and a replacement is a made-to-order part ordered against the range's own size and configuration, with a lead time rather than a shelf.

The three rules are: clean it warm, keep it dry, and never let anything wet touch it hot. Everything below is why.

Cleaning: scrape, do not wash

Warm — not hot, not stone cold. Cook, then let it come down for twenty minutes.

A metal scraper or a bench scraper across the surface takes off what cooking left. A dry cloth takes off the rest. That is the whole routine, and it is how these plates have been cleaned since 1908.

Soap and water strip seasoning, which is the polymerized oil film that keeps bare iron from rusting and makes cooking directly on the plate pleasant. So a wash costs you a re-oil, and a plate washed often enough is a plate you are permanently rebuilding. If you do need to wash it, do it cold, dry it immediately and completely, and season it again.

Seasoning: thin, and repeated

A little oil on a cloth, worked over the whole surface, then wiped back until it looks almost dry. Then let the plate come up gently on a low burner.

Thin is the entire technique. A thick film does not polymerize properly — it goes sticky and stays sticky, and then it carbonises into a rough black layer that is genuinely hard to remove and which insulates the plate slightly. Several thin layers over several cooking sessions beat one enthusiastic one.

A well-seasoned plate is black. Bare iron is gray. Neither is a problem; gray simply means you have some seasoning to build.

The mistake: thermal shock

This is the one that brings us out.

A cast-iron plate at searing temperature has a considerable gradient across it — hottest above the burner, much cooler at the corners. Put anything cold and wet onto the hot part and the metal there contracts against metal that has not, and cast iron is strong in compression and brittle in tension.

The specific versions we see: a damp cloth used to wipe a hot plate. A pan of cold liquid set down on the center. Somebody washing the cooktop down at the end of the evening before it has cooled.

None of those is careless in any ordinary sense. They are all things people do to every other cooktop they have ever owned.

So: nothing wet until it is genuinely cold. Not warm — cold. If you want to clean it at the end of cooking, scrape it warm and dry, and leave water out of it entirely.

The underside, which nobody looks at

Once a year, look at the face that points down.

Rust and scale there insulate the iron from its own burner, and the complaint that produces is a plate that has quietly lost its performance — slower to come up, a smaller hot center, corners that never get usable. Owners describe it as the plate wearing out. It is a layer of oxide.

It is also where corrosion starts in a coastal kitchen, because it is the surface that never gets wiped and never gets hot enough for long enough to stay dry.

Cleaning it back is part of a French top service, and it is one of the more satisfying things we do: a plate that had been declared dead comes back because the iron can feel its burner again.

Crazing is not a crack

Let the plate go completely cold and clean it back dry, then look in daylight.

Crazing is a fine spider web in the black seasoning layer. It does not catch a fingernail. It is the normal appearance of a well-used plate and it means nothing at all.

A crack is a single line, usually running from an edge or from the hot center outwards. It catches a nail, and it often shows a rust trace along it because moisture has got into the casting.

If it is through the iron, stop using the plate — a cracked plate can move, and a moving plate over a lit burner is not worth the risk. If it is a stabilized hairline at the edge away from the hot center, that is something to keep dry, keep seasoned and monitor, and it can go on for years. We will tell you which you have rather than condemning a plate for having a line on it.

And if the heat has gone rather than the plate

Worth saying, because it is the more common call. Most cold-spot complaints start underneath: a weak, rich or partly blocked burner cannot load a plate this heavy, and a blocked drip or air path starves it of combustion air.

So before anyone talks about the iron, light the burner and look at the flame. Blue, even, and the right size. That is where these visits are usually solved.