Smut is composed of aluminum and magnesium oxide.  Smut itself does not reduce the mechanical properties of the weld but it does indicate high porosity levels which can reduce the welds strength.   

• All welds, whether produced with GTAW or GMAW, should be bright and shiny, with no gray or black coating on the weld itself.
• You also should be able to see a shiny cleaning stripe, about 1/16 to 1/8 inch wide, on either side of the weld. These cleaning stripes indicate the area where the reverse-polarity GMAW arc-or the reverse-polarity portion of an AC GTAW arc-has removed the oxide coating cathodically from the aluminum during welding.
• If smut is just aluminum and magnesium oxide, why don't we see it on every weld? If we have good shielding gas coverage, the evaporated aluminum and magnesium are contained in the gas shield and transferred across the arc as weld metal.
• It's only when the gas shielding becomes marginal that sufficient quantities of aluminum or magnesium are oxidized outside of the gas shield to become weld smut.
• So the key to minimizing weld smut is to maintain a good gas shield during welding.
• So what is smut, and what causes it?
• It's often black, so many welders assume that smut is carbon or some other sooty contaminant. This is incorrect. Many people over the years have analyzed weld smut, usually by X-ray techniques.