The Tainted Ore is a quest cut from the original Baldur's Gate, but restored in the Enhanced Edition.
After clearing the Nashkel mine, speak to Berrun Ghastkill and tell him about the poisoning of the ore. He will then ask that a sample of the Contaminated Iron be brought to Taerom Fuiruim, and the "mineral poison" that was found to Thalantyr the Conjurer so that they might figure out a way to counteract its effects.
In the Enhanced Edition, speaking to Berrun Ghastkill after clearing the Nashkel mine while in possession of a Vial of Mysterious Liquid will give a journal entry to bring it to Taerom Fuiruim. Two days after delivering it to him, his stock will expand to include the "Quality" weapons (normal weapons that don't break). The contaminated iron is not needed for this quest.
Note: If Taerom is in the process of creating a set of armor for the party, the option to discuss the tainted ore and vials of liquid will not appear. Wait until the armor is received, and then the option will appear. Taerom will take all the vials from the inventory when the issue is discussed with him, but leaves the ore in the inventory.
Notes[]
Baldur's Gate (1998)
This icon indicates content from the original Baldur's Gate campaign. The quest is also restored by the mod Unfinished Business for BG1.