When you toast salt in a carbon steel wok, it starts to acquire a yellowish tinge. Any chemists know what that yellow color comes from? Is it an actual reaction between the salt and the metal, or is it just deposits from smoke from surface oil/polymers being vaporized?

Here’s what happens. The control is raw salt. Middle is toasted 5m in stainless steel, bottom toasted in carbon steel. (I don’t have a cast iron pan handy, but it also happens in cast iron)

(Also: the cast iron toasted salt is unaffected by a very strong magnet.)

Ah ha! When you dissolve the toasted salt it dissolves clear but leaves a film of debris floating on the surface, which seems to confirm suspicion that it’s deposits from vaporized oil/seasoning that’s just in the surface of the salt crystals.

Next test: salt in contact with smoke from the wok but not in direct contact with the metal (trying it with a stainless steel spoon and aluminum foil as vessels.)

Ok, this time I heated salt in a cast iron wok under a lid, but lifted the salt off the surface using an aluminum foil boat, and a stainless steel spoon. Still changes color. I am quite confident in saying that it’s due to deposits from vaporized seasoning. Thanks everyone!

(Moreover, the lid, the spoon, and the foil all got similar yellow deposits.