Creosote, the black or brown residue of combustion that collects to the internal surfaces of the chimney flue liner, is very flammable. If allowed to Make up, it may catch fire, resulting in cracks in "fireproof" brick, stone or clay flue liners and allow warmth to succeed in nearby wood framing https://ricardolqkgc.blog2freedom.com/28093716/wood-fireplace-inserts-for-dummies