What causes black soot buildup on propane gas logs?

The combustion process can leave behind black soot on propane gas logs. This usually adds to the logs’ realistic appearance and does not pose a problem. Liquid propane log sets typically produce more soot than natural gas log sets. This is completely normal. If a vent-free log set is covered in soot, however, this is not a normal condition and a fireplace professional should be consulted. The vent-free log set may not be operating properly if there is a heavy soot layer present. Normal soot can be gently brushed away with a paint brush if desired. Do not remove the log set from the fireplace or use water or abrasive cleaners.