Creosote, the black or brown residue of combustion that collects within the interior surfaces of a chimney flue liner, is highly flammable. If allowed to Create up, it could catch fire, resulting in cracks in "fireproof" brick, stone or clay flue liners and allow heat to succeed in nearby wood framing https://chimneycleaningnearme13319.ja-blog.com/28115639/5-simple-statements-about-wood-mantel-explained