black soot on your fireplace glass or logs indicates that you did not serviced your fireplace properly .
natural gas fireplaces as a source of heat needs doing Anual service and cleaning
first reason of having black soot on your fire place after not doing Annual service is doing wrong service by some
technicians. when they do service they suppose to put fireplace logs in the exact place of manufacture setting otherwise it will make black soot
so having an experienced fireplace technician is too important
the other reason for having black soot on your glass and logs is blocked Air shutter ! when the Air shutter gets blocked and dirty the gas not gana receive enough Air so uncomplete combustion will happen and you will see the main burner flame getting red and red and at the end glass and logs getting black soot witch is so dangerous because your fireplace will make CO