In a world where wizards create "Floating Disk" spells so they can haul more treasure out of dungeons ( because killing things and taking their treasure is a valid career path ) there's no way you can make me believe there would NOT be spells that can temporarily alter the material of a weapon.
Well, as it so happens, this is from Arcane Strife, which was written shortly before 3.5 came out; I developed this spell when I heard about the new DR system.
Alter Weapon Material
Transmutation
Level: Sor/Wiz 1
Components: V, S
Casting Time: 1 action
Range: Touch
Area: One weapon
Duration: One minute/level
Saving Throw: Will negates (harmless)
Spell Resistance: Yes (harmless)
The lack of magical weapons during the First Lich King War badly hurt the freedom fighters and rebels who were attempting to throw off the yoke of slavery that the Eternal Kings had laid upon the entire world. Many creatures could only be hurt by a specific type of material or a magical weapon, and many of those materials were rare at best. This spell was devised to ensure that those who faced the minions of the Undying Lords had the ability to stand against them and hopefully defeat them. Since the end of the war, this spellbook has remained in the hands of many mages and is often one of the spells that mage colleges grant young mages before they go out into the world.
This spell alters one weapon's material type, so that it is considered another type of metal for purposes of damage reduction. For instance, a steel sword could be changed to silver, cold iron, or adamantine. The spell does not in any way actually change the material of the weapon; a steel sword altered to silver can still be affected by a rust monster's attack. Note that weapons cannot be made blessed or holy with this spell – it only alters the type of material.
Weapons under the influence of this spell glitter slightly as if the blade were dusted with the substance the spell emulates.
I thought (briefly) about making this a weapon enchantment, but ditched the idea - for one thing, there was no real way to govern how it changed, and for another, it was just too powerful.
And hey, here's another one as a bonus.
Silversteel
Transmutation
Level: Sor/Wiz 5
Components: V, S, M
Casting Time: 1 round
Range: Touch
Target: One object, up to 5 lbs/level
Duration: Instantaneous
Saving Throw: Fort negates (harmless)
Spell Resistance: Yes (harmless)
During the early days of the Human/Vampire War, when it looked bad for those who dared stand up against the undying tyranny of the Vampire Lords, this spell is one of the magics that turned the tide. Silver weapons were soft and melted in the face of arcane fire, while the steel blades of the vampires who chose to wield weapons still sliced souls free of flesh. This spell changed all of that and gave the humans their first edge against their undying opponents.
Silversteel turns normal, nonmagical silver into a substance that has the durability, tensile strength, and high melting point of steel, yet carries silver's corrosion-resistant properties, high sheen, and the ability to strike some undead, lycanthropes, and devils. Only a relatively small amount of silver can be affected (a maximum of five pounds per level), and it must form one whole object. Silversteel has a hardness of 13 and 30 hp per inch.
Material Component: A chunk of silver/steel alloy (at least 5 gp worth of each).