Are you sure that is the buffing compound and not burnt horn???
Horn is protein based and if you raise the temp too much it will burn. Buffing horn requires a light touch, after all, it is much softer than metal or even paint.
If it is buffing compound on the horn, you should be able to scrape it off with a thumbnail. If it is burnt, you will have to scrape down thru the horn to fresh material underneath. I darn near ruined a beautiful powderhorn I had spent 30 hours carving when I kept pushing for a more lustrous shine!