I had an oven that collected a brown coating on the glass window, I used a razor blade type scraper, get the angle right and it comes quite clean.
Another option is to wipe it with caustic soda solution, but do not let it dry on! leaves a residue that is difficult to remove if you are not careful with it, it also tends to destroy your gloves/trousers/fingers too.
Definitely try this first - it’s what we do, costs nothing and comes up a treat,
Scrunch up a sheet of newspaper, dip it in some warm water then dip it in some ash from the fire.
Scrub away and marvel at the results!