shielding gas protects the weld by being a pocket of gas around the weld. the purpose of this gas is to keep normal air out, especially oxygen. it is different from flux because there is no liquid on the weld. there is only a gas around the weld. because there is no liquid, it will not clean up dirt and other things on the metal. this means that the metal has to be clean before it is welded. if it is not, the dirt and other things could cause problems. the gases that are usually used are argon, helium, and a mixture that is 3 parts argon and one part carbon dioxide. other mixtures of gases can have nitrogen, hydrogen, or even a little bit of oxygen in them. one kind of welding that uses shielding gas is gas metal arc welding. it is usually used in factories to make things.