You’re asking about the Adamantine Forge in Baldur’s Gate 3—specifically what goes in the crucible and how it works.

Direct answer

In BG3’s Adamantine Forge, you put Mithral Ore (often written as Mithril/Mithral) into the crucible at the center of the forge.

Separately, you place a mould (e.g., for armor or a weapon) into the mould chamber before using the forge lever.

Quick Scoop: How the Crucible Works

Think of the Adamantine Forge as a two‑slot crafting machine:

  • Crucible (center of the platform):
    • Put one chunk of Mithral Ore in here.
* The ore is **consumed** during the forging process.
  • Mould Chamber (side slot):
    • Insert one mould for the item you want (e.g., scale mail, shield, etc.).
* This tells the forge _which_ adamantine item to create.

Once both are loaded:

  1. Use the Forge Lever to lower the platform into the lava.
  1. Use the Lava Valve to pour magma, which also starts the Grym boss encounter.
  1. Hit the Forge Lever again so the giant hammer smashes down on the crucible, shaping the item and dealing huge damage to Grym if he’s standing on it.
  1. After the animation, loot your new adamantine item from the mould chamber.

Mini FAQ & Forum-Style Notes

“I put a mould in, but nothing happens. What am I missing?”

Most forum threads point out that both pieces are required: the mould and a chunk of Mithral Ore in the crucible.

“Can I use other ores or items in the crucible?”

No—guides and wikis only list Mithral Ore as valid material for the crucible in the Adamantine Forge.

“Why is this a big deal?”

The Adamantine Forge is a big Act 1 set piece in Grymforge, and properly using the crucible with Mithral Ore not only crafts powerful gear but also lets you cheese the Grym boss with the hammer slam.

SEO-style meta description:
In Baldur’s Gate 3, the answer to “what goes in the crucible BG3” is simple: place Mithral Ore in the central crucible and a mould in the mould chamber to use the Adamantine Forge and craft adamantine gear.

TL;DR:
What goes in the crucible in BG3’s Adamantine Forge? Exactly one chunk of Mithral Ore —nothing else will work.

Information gathered from public forums or data available on the internet and portrayed here.