Keen Edge

Level: Mage 3; Components: V, S; Casting Time: Attack action; Range: Close (25 ft. + 5 ft./2 levels); Targets: One weapon or 50 projectiles, all of which must be in contact with each other at the time of casting; Duration: 10 minutes/level; Saving Throw: Will negates (harmless, object); Spell Resistance: Yes (harmless, object)
This transmutation doubles the threat range of the weapon. A threat range of 20 becomes 19-20. A threat range of 19-20 becomes 17-20. A threat range of 18-20 becomes 15-20. The spell can be cast only on piercing or slashing weapons (and it does not stack with itself). If cast on arrows or crossbow bolts, the keen edge on a particular projectile ends after one use, whether or not the missile strikes its intended target.
This spell cannot be cast on a natural weapon, such as an unarmed strike.