Known to be in Daltim's Tome of Fire:
The unburn spell calls into existence a reversed form of fire, one with a black flame that radiates cold and shadow rather than heat or light. While a normal fire requires flammable material for fuel, the black fire created by unburn feeds on the ashes and charred remains of normal fire. The most common use for unburn is to reverse the flame effects on something the caster wants such as an important scroll, a painting, or a home.
There are some restrictions to this spell. The results of a recent fire are relatively easy to undo because all the ashes and other materials are still there. Unburn can also be used on older fires, but there is a risk that some of the raw material has been carried off or blown away in the wind. The magic of unburn does not cause most of those materials to return to the scene, though it will call back minor amounts of ash that were wafted away by the hot air of the normal fire. The exact amount of material left unrestored is up to the DM and should be based on the size of the original fire and how long ago it happened.
Unburn is effective only on burned materials. It will not prevent a fire, restore hit points lost to a fire-based attack, or restore to life a creature slain by fire. (It will, however, restore a dead body that was consumed by flames.)
Since rust (and other tarnish) is produced by oxidation, and fire is a form of rapid oxidation, the black fire of unburn will also undo the effects of rust or tarnish on metal, removing corrosion and stains
