Soot

Was at a customer's house to see if they had a gas leak on their gas water heater. Didn't find a leak but noticed soot on top of the water heater. Dirty Soot

The wrong mixture of air and gas is the cause of soot produced by a Gas Water Heater. Build-up or debris can get caught in the burner, disrupting the proper flow of gas. Debris also can block the vent or flue, which doesn't allow carbon monoxide to escape.