Creosote, the black or brown residue of combustion that collects to the internal surfaces of a chimney flue liner, is extremely flammable. If allowed to Develop up, it could catch fire, leading to cracks in "fireproof" brick, stone or clay flue liners and allow heat to succeed in nearby wood framing https://elliotvlyjw.blogproducer.com/34322052/mantels-fundamentals-explained