Cooking science

Thermal mass

Thermal mass is how much heat something can store. A heavy pan stores a lot and therefore changes temperature slowly; a light one stores little and changes fast. Neither is better in the abstract, and each is much better for particular jobs.

Storage, not conductivity

Two properties get confused. Conductivity is how fast heat moves through a material — copper and aluminium are excellent, cast iron and steel are mediocre. Thermal mass is how much heat a material stores for a given temperature rise, which depends on its specific heat and, crucially, on how much of it there is.

A thick cast-iron pan is a poor conductor with enormous thermal mass. It heats unevenly and slowly, and once hot it stays hot when cold food lands on it.

A thin aluminium pan is an excellent conductor with very little thermal mass. It heats evenly and almost instantly, and it drops the moment anything cold goes in.

Where each wins

High mass — searing

A steak needs the pan to stay hot. Cast iron or thick steel.

High mass — bread and pizza

A stone or a steel gives the base a burst of stored heat, which is what makes the crust. A thin tray cannot.

High mass — braising

A heavy casserole holds a steady low temperature and evens out the hob's fluctuations.

Low mass — delicate sauces

A thin pan responds immediately when you turn the heat down, which is exactly what you want when something is about to split.

Low mass — anything that needs control

Caramel, custard, chocolate. You want the pan to obey the dial.

Low mass — boiling water fast

Less metal to heat first.

Practical consequences

• Preheat heavy things for longer. A cast-iron pan needs five minutes or more; a stone needs three quarters of an hour.
• Turning the heat down under a heavy pan does very little quickly. Take it off the heat instead.
• A heavy pan browns better because it recovers when cold food goes in.
• A heavy pan burns things more readily once it is too hot, for exactly the same reason.
• Ovens have thermal mass too. A full oven holds temperature better than an empty one, and a heavy oven recovers faster after the door opens.
• Preheating the roasting tin with the fat in it is a thermal-mass trick — it means the potatoes meet stored heat rather than cold metal.
• Warm plates are thermal mass working for you at the very end.

Matching the pan to the job

A short version of a decision that comes up constantly.

• Steak, chops, anything seared — cast iron or heavy steel. It has to stay hot.
• A stew or a braise — a heavy casserole. It smooths out the hob and holds a steady low temperature.
• Bread and pizza — a stone or a steel, preheated for at least half an hour.
• Custard, caramel, chocolate, a delicate sauce — a thinner pan you can control from the dial.
• Boiling water or blanching — thin and light, since there is less metal to heat first.
• Roast potatoes — preheat the tin with the fat in it, so the potatoes meet stored heat.
• Anything you keep burning — take it off the heat rather than turning the dial down, particularly with a heavy pan.

Common questions