Bare cast iron vs lined copper

Which one heats faster?

Lined copper, by a wide margin: the copper body of lined copper conducts at about 401 W/m-K against 52 for bare cast iron. Conductivity is how quickly heat moves through the metal, so a high figure means the pan reaches an even temperature sooner and spreads the burner's heat across the base rather than concentrating it.

Conductivity is not the same as heat retention, and confusing the two is the most common misunderstanding about cookware. Cast iron conducts poorly at about 52 W/m-K and retains heat extremely well, because retention comes from mass and specific heat rather than from conductivity. That is precisely why iron sears well: the pan barely drops in temperature when cold food lands on it.

Which one suits which job?

Bare cast iron is best for searing, cornbread, shallow frying, anything that benefits from a pan that does not lose heat when cold food lands in it. Cast iron is not a good conductor. Its advantage is heat capacity: a thick, hot pan holds its temperature when a steak hits it, which is what actually produces a crust.

Lined copper is best for sugar work, delicate sauces and anything where the pan must respond the instant the heat changes. Copper conducts about twenty five times better than stainless and roughly eight times better than cast iron. You are buying control, and paying for it in price, weight and upkeep.

Practical differences that decide it

Induction: Bare cast iron works on induction and lined copper does not work on induction. That alone settles the question for anyone with an induction hob.

Acid: Bare cast iron is reactive, meaning long acidic cooking strips seasoning or discolours the surface, so tomato and wine braises belong in lined copper. Oven ceiling: 700F against 450F, remembering that the handle usually decides the real limit rather than the material.

Weight and upkeep: bare cast iron is very heavy and lined copper is heavy. Bare cast iron needs a maintained seasoning layer, which is oil polymerised onto the metal by heat, built up with use and stripped by detergent or a dishwasher.

The honest conclusion

Most kitchens should own both rather than choosing. They are good at different things, and neither is a substitute for the other. A mixed kitchen outperforms a matched one at every budget, which is the single most useful thing to understand about buying cookware.

If you genuinely have to pick one, pick on the job you do most often. For searing and browning, that is bare cast iron. For sauces and anything that needs quick control, lined copper spreads heat faster.