The spell created a kind of reverse fire, which had a black flame and radiated not light or heat but shadow and cold. This black flame magically reversed the oxidation process, consuming ashes and char and leaving behind the material in its original form. It behaved in all other respects like an ordinary fire. The black fire appeared at a point of the caster's choosing, and was initially around the size of a human hand, then spread over all burnt materials in reach. It would burn for a full day or until no burned materials remained in reach.
Unburn could be applied to the remains of any fire, but its success depended on its age and completeness. A recent fire was usually easy to reverse, if all materials remained present. An older fire was less certain, as ashes may have been blown away or otherwise removed. The spell could not restore lost materials, but it could bring back small quantities of ash that had risen on the hot air created by the original fire.
Interestingly, the unburn spell also reversed rusting and tarnishing of metals, removing corrosion and stains, because rust and tarnish were the result of oxidation, which burning was simply a rapid form of.
An unburn spell could not stop a fire already in progress or prevent one from starting, nor could it heal or restore to life a creature harmed or slain by fire. However, a burnt corpse could be restored to an unburnt state.
The unburn spell was mostly used to reverse the destruction of a burnt item that the caster wished to recover, such as a book or scroll, a painting, or a house.
In addition to the usual verbal and somatic components, the spell required a small vial filled with water.
The spell was developed by the Halruaan mage Daltim Flamefist sometime in the mid–14th century DR. He included it in his spellbook Daltim's Tome of Fire, which he lost in Tethyr in the 1360s DR. It remained a unique spell at the time.