What is the purpose of coating around heating element in soldering airgun?
Mica sheets are used as electrical insulators for heatsinks , heater guns and  microwave ovens , maybe (salvage) from old toasters



It prevents the body plastic from melting and is rated by thickness.  The heater coil also has high AC voltage on it.

Looks like you cooked it maybe by blocking the intake air or cheap quality inadequate thickness.

Good luck.