Soot may be the result of an outdoor barbecue, perhaps on your pergola or brickwork; it may be from the exhaust fumes of your car on your paint work (particularly if it’s an older type diesel); or it may be on the glass door of your wood burning stove.
Whatever the cause, it is unsightly. Luckily, it’s easy to remove, as despite its unpleasant appearance, it’s not stuck very strongly to anything.
Spray some Oven Cleaner onto it (any one will do), leave it for a minute or so, and then wipe away with a wet rag. Rinse and re-wet the rag and repeat until the surface no longer feels slimy.
If it’s on the inside of your stove, another option is to wet a newspaper (or rag), dip it in the ash in the bottom of your stove (which often contains unburnt minerals that can act as an abrasive cleaner) and use it to wipe the glass.
Alternatively, Ajax Powder Cleanser will also do a good job.