Metallum is a type of Essentia added by Thaumcraft 3. Associated with metals, it can be created from items like ores or ingots, and is used for crafting Thaumic items with metallic components or aspects.
To create metallum essentia, drop an item containing the aspect into a crucible. Certain items contain more metallum than other. For example, Iron Ingots contain 8 metallum each, but Silver Ore contains 5. Some finished items and mod items will have metallum aspects as well, for example Redstone Energy Cells from Thermal Expansion contain 14 metallum.