The new improved air mixer is 100% Brass, making it work more efficient than the older air mixers. If you have decided to use the H Fireplace burner or fire pit ring with fire glass and want to use propane instead of natural gas, we recommend using the Propane Air Mixer. This piece is designed to allow air to enter the gas stream in order to reduce the amount of soot created by propane. This air mixer valve will keep your Urban fire glass looking cleaner and free from the soot that is normally created when you use propane versus natural gas.