Iron pans are standard across Indian kitchens, yet certain dishes are always cooked in something else. The rule is not superstition, and it follows from how seasoning on iron is built and destroyed.
What seasoning actually is
Heating a thin film of oil on iron drives a chemical change in which the oil molecules link into a hard, plastic-like layer bonded to the metal surface.
That layer is what makes a seasoned pan release food, and it is also what stands between the iron and anything in the pan. Bare iron would react far more readily.
It builds up over many uses, with each thin application adding to the last, which is why a pan improves for years with regular cooking.
How acid breaks it down
Acidic ingredients slowly attack the bonds in that polymerised layer, thinning it and eventually exposing metal underneath. Long simmering with tomato or tamarind is the usual culprit.
Once metal is exposed, iron dissolves into the food in small amounts, which gives a metallic taste and can turn light-coloured dishes grey or blue-tinged.
The damage is cumulative rather than immediate, so a quick acidic step does little while an hour-long simmer strips a noticeable amount.
Why some traditional dishes ignore this
Certain regional preparations are cooked in iron deliberately, on the understanding that a dark leafy dish or a heavily spiced gravy masks any colour change.
Those dishes also tend to be cooked briskly rather than held over long simmering, which limits the contact time available for the reaction.
The practice is old and specific rather than general, which is why it appears as a named tradition rather than as ordinary everyday cooking.
Where the alternatives sit
Stainless steel is unreactive but conducts poorly and sticks readily, which makes it good for wet acidic cooking and awkward for anything needing browning.
Brass and copper vessels traditionally carried a tin lining for precisely this reason, since the bare metals react strongly with acidic food.
Modern kitchens usually solve it with one steel pot reserved for tomato and tamarind dishes, leaving the iron for dry and short cooking.
How to repair a stripped pan
A pan that has lost its seasoning shows dull grey patches and starts sticking. The layer has to be rebuilt rather than restored, which means fresh oil and heat.
Very thin applications heated past the point where the oil polymerises work better than thick ones, since a heavy coat stays tacky rather than hardening.
Cooking fatty, non-acidic food in it repeatedly does the same job more gradually, which is how most pans in daily use maintain themselves without any deliberate effort.