Slow burn causes the fuel source for any non-magical fire to burn at a much slower and much more efficient rate. The heat and light provided by mundane fires subsequently last longer than they otherwise would. Torches, candles, lantern oil, camp fires and other mundane burning sources of light and heat reliant on a fuel source last twice as long under the influence of this spell and the effect lasts until double the burn time has expired for these items. A torch under the effects of slow burn lasts 2 hours rather than 1 and a pint of oil poured on the ground and lit lasts for 4 rounds rather than 2 and so on. The only requirement is that slow burn be cast when these items are first lit. The spell has no effect on a torch that's almost spent for example.

You can also cause the secondary splash damage from fire-based splash weapons such as alchemist's fire to do additional damage as well. While the initial damage caused in the first round is unaffected the next round's direct and splash damage are both extended for an additional round. The spell need only be cast on the splash weapon after it has been lit and readied for throwing or on an already burning victim. In such cases, items and creatures burn for a third round unless extinguished as usual. For example, alchemist fire would cause 1d6 damage in the first round plus 1 hit point of splash damage to targets within 5 feet, then 1d6 in the second round plus an additional 1 hit point of splash damage (which normally doesn't burn a second round), then 1d6 in the third and final round (the splash damage isn't extended into the third round).

Finally, this spell can also be used to add next round damage to such spells as burning hands or fireball. If slow burn is cast prior to casting a fire-based spell, that fire spell causes an additional dice of burn damage the round after it's cast. Slow burn is simply cast on your hands before you use another fire-based spell. So burning hands would cause an addition 1d4 points of damage the next round, even beyond its 5d4 maximum as long as slow burn is cast beforehand. Fireball would cause an additional 1d6 points of damage the next round and so on.

The spell has no effect whatsoever on monsters of the fire subtype and cannot be used to augment inherent fire-based spell-like abilities or burn damage from their strikes. Slow burn also doesn't work on magic items such as the elixir of fire breath or flaming weapons.