In central IL they started heating flint regularly in the middle archaic. Extremely rare to see any paleo or early archaic flint that was heated. Won’t say they never did it, but we never see it. With a higher population using flint sources that had been picked over for 5000 yrs, the middle archaic was forced to use more common but lesser quality flint. Heating improved it considerably with easier chipping, increased gloss and color. As a flintknapper, I’ve cooked a ton of flint. At first I buried it in sand and started a wood fire on it. Later I bought a kiln for better control. 550F is the magic number for heatable midwestern cherts. Western material like agate takes less. There’s an art to it, about half guesswork, and a lot of material gets accidentally destroyed trying to heat it.