Creosote only occurs from burning wood, so that's not likely. I have seen some pretty "sparkly" displays though, when someone fired up a water heater with mud dauber nests in the fire tube. As Old Biscuit said, cleaning the tube with a wire and rag is an effective way to clean them out.
Dutch
2001 GBM Landau 34' Class A
F53 chassis, Triton V10, TST TPMS
Bigfoot Automatic Leveling System
2011 Toyota RAV4 4WD/Remco pump
ReadyBrute Elite tow bar/Blue Ox baseplate