Years ago one of my jobs was cleaning our old coal burning kitchen range and I had to use some stuff called I think Zeebo used to polish up lovely, don't know if it's still available or where you get but it did the job.
Remember to get your flue cleaned professionally too - technically it should be twice a year if you are using it extensively. A lot of folk don't realise that your insurance policy can be negated if you have a chimney fire and you have no proof or out of date certificate/receipt from your chimney person.