The Magic Art of Metallic Air combines two Elements: Air and Earth. There is only one text that mentions this Art in the Library of All Worlds. The Book of the Diggers lists this form of magic as one of the dangers that miners face when delving in the Dragon- and Ifrit-infested Hot and Cold Realms.
Its powers manifest in various ways, including gaseous-metallic clouds (this is one way to create mirrors or coat other objects in a metallic sheen), serpent coil air (a striking weapon), and deadly coagulated metallic air - a favorite of the magical assassins from the realm of The Rust Gate.
The Magic Art of Metallic Air has ties to the Dragon Trade: The Book of the Diggers states that metallic coins from Dragon hoards are required to cast and control this form of magic. Bronze coins work as well as gold coins for this purpose. Practitioners of this Magic Art wear strings of Dragon-backed coins when they cast their magic. For exceptionally powerful spells, a mage may wear entire shirts and skirts made of coin-mail.
One way that realms may protect themselves from this form of magic is by promulgating and enforcing sumptuary laws. Unfortunately the realm of The Rust Gate has long since lost the political will to regulate its citizenry in this way.