All weapons can be upgraded by talking to the Blacksmith or the Enchanter. To upgrade a weapon, a Crafting Item is required.

Crafting Item Acquired
Pig Iron After defeating the boss during Lord of the Manor
Sword Polish

After defeating Evanie the Sword Saint during No Party Like a Search Party

Mana Stone Shard After defeating the boss during Research Materials
Vial of Demon Blood After defeating the boss during The Virgin Gynocides
Bone Chunk Defeat Zombie Bard in Night of the Raping Dead
Clay Idol Defeat the boss in All's Were That Ends Were
Fairy Dust Defeat Evo during A Forge Too Far
Mechanism Parts Defeat boss in Check Out My Package
Invisible Crossbow Defeat The Invisible Man(forgot his actual name) during Looking for a Cure
High-Grade Steel Bar After getting Gargan to Neutral RP, go to the Guild Warehouse in the Northmarket Slums.
Slime Egg Defeat the boss in One Is The Loneliest Party
Vial of Virgin's Tears Defeat Therese during Manor Invasion
Vampire Fang Defeat Count Corgan during Looking for a Cure
Glowsticks Defeat the Crowd of Fanboys during Rescue Me
Dingly Bell Defeat the Clown in Stuck In The Middle With You
Heavy Bolt Found during Looking for a Cure
Sliver Churchbell After turning in the Night of the Raping Dead quest, talk to the Lumbermill Priest.
Broken Axe Blade Defeat the Orc Boss in Na Na Why Don't You Get A Job
Werewolf Claw Defeat the Werewolves in Looking for a Cure
Fiend Folio Defeat the business Demon of Sacrifices for Algernon
U-238 Shovel Defeat Fionna in it's pronounced Sabotage
Sphinx Organ Defeat the boss of Get Dumped
Strand of Ultrasilk Defeat the boss in It Came From Below the Equator
Championship Award Complete Hour of the Beasts
Killer's Eye Defeat the boss in Finishing Unfinished Business
Dragon Heart Defeat the boss in Family Matters
Permaice Shard Defeat the boss in Bewitchy Women
Chromium Ingot Defeat Golem during Hall Monitor from Hell
Pheonix Feather Defeat Yeon during Hall Monitor from Hell
Chimera Talon Defeat the boss in My Little Chimera
Royal Seal Defeat the boss in Cannibal Corps.
Golden Bat Skull Defeat the Bling Bat during The Phantom Breaker Battleground
Bandersnatch Horn Defeat the boss in A Manxome Foe
God Feather Deafeat the boss in Beyond good and Devil
Firm Whalebone Deafeat the Master Worm during Can’t Handle it Cove
Titan Shaft Found during Elf Half Empty?
Juggernaut Piston Found during Circle of Strife
Dragon Sinew Defeat the boss in Send Newts
Holy Diver[1] Complete Can’t Handle it Cove
Fiend Essence Defeat the boss in Cannibal Corps (part II)
Focus Assembly Defeat the boss in Golem to be Awesome
Jar of Protoplasm Defeat the boss in Are You Afraid of the Dark?
Roc Feather Defeat the boss in Blow Job
Dwarven Mechanism Inside the Green Tower (second floor in one of the green chest)
Keanu's Guide to Acting Defeat Borgan during Fortress of Solitude
  1. This is also a Weapon for Therese