If welding is carried out in the air, i.e. gas and arc welding, oxides are created which make a weak weld. Oxides create undesirable compounds and gas bubbles in the metal, creating porosity and weakness. Oxides occur naturally on the surface of the metals and should be cleaned off before welding. However, heat from the welding process creates new oxides and these and other impurities can get brought into the molten metals.