Every wood fire sends more up the flue than heat: water vapor, tar droplets, and gases that didn’t finish burning. When that mixture hits a flue wall cooler than about 250°F, it condenses and sticks. That residue, brown-to-black, sticky-to-glassy, is creosote, and your chimney manufactures a little more of it with every fire.
Why it’s genuinely dangerous, not just dirty
The danger curve: it gets worse non-linearly
Fresh deposits are soft and sooty, easy to brush out. Left through more fires, they densify into a crunchy tar layer. Left further, they cook down into a shiny glaze that’s essentially solidified fuel bonded to the liner, the stage that causes serious chimney fires and no longer responds to ordinary sweeping. The full breakdown is in our stages guide, but the short version: cleaning early is trivially easy, and cleaning late is a specialist job.
What to do with this information
Nothing dramatic, just measurement on a schedule: an annual inspection reads the deposit, a sweep clears it when it crosses the 1/8-inch line, and deep removal exists for flues that got away. If you can’t remember your last cleaning, that’s your answer, book the look.