When firewood is burnt in the furnace it usually leaves incomplete combustion due to which stains appear on the surface. The creosote not only is dangerous but gives a bad smell to the room. The stains are not cleaned easily and it can create a problem for you. In order to clean the surface you need water and add trisodium phosphate to it. Mix it properly and dip a cloth in this liquid and apply on the surface which has stains. Rub hard on the surface so that it gets cleaned up.
Scrub as much as you can because you need to apply a lot of pressure on it. The steps have to be repeated unless it is cleaned and you get back the same surface. Take a neat cloth and wipe all the excess water and rinse it completely. So you can follow the above steps whenever the surface gets dirty with creosote.