We used to boil
coils and then rinse in alcohol (everclear or vodka). I think back then there was a realistic fear the ultrasonic would break the tiny wires. It couldn't hurt to try if the coil is otherwise useless.
I would fill a small jar with alcohol and set that in the ultrasonic. The vibrations will transfer and the lid keeps it from evaporating.
I know from cleaning other
coils it will probably take some scraping if you can't dry burn the crust to ash first.