From Rebirth of the Night Wiki

The Crucible is a very, very important crafting pot, created from smelting an unfired crucible in a Kiln. This pot enables the creation of Soulforged Steel and the smelting of manufactured tools and armor down into their components, enabling recycling from say… Mob traps. Or nearly-broken armor/tools you’ve been using for a while now.

The Crucible is also unique in that the majority of its functionality is locked behind having a steady stoked flame. As you will have made the Crucible with a stoked flame in the Kiln, you will likely know how to maintain a stoked flame from that experience, so this will not be repeated here.

Like it’s iron brother, the Cauldron, the Crucible benefits from having more fire underneath it in a 3x3 area. The details on that are talked about on it’s page, but will be repeated here for clarity and having to save you from clicking more: Needs a fire directly under it. Each additional fire in the 3x3 ring surrounding the fire underneath the Crucible. * If you’re using stoked fire, then make sure the fire sources come from Hibachis, or else the fire will get blown out.

Stoked Crucible uses: Creation of Soulforged Steel, utilizing a Soul Urn, Coal/Charcoal Dust, an Iron Ingot, and Soul Flux. 100% return ratio for recycling Diamond/Soulforged Steel tools, armors, and weapons 66% return ratio for every other metal’s tools, armors, and weapons (This 66% is the default, and may have been changed by the modpack author, if this mod was included in a modpack.) Mass smelting of cobblestone and sand into stone and glass, respectively. This saves you on fuel costs in the furnace MASSIVELY. Recycling tons of other metal stuff, also at a 66% return ratio, but with a mostly lossless conversion rate for some stuff. Compressing nuggets into ingots, mainly for use with the above reycling factors, and also mob farms. Supports most modded ingots if they have a nugget form they break down into. * Other mods, such as Better With Addons, may add additional recipes and uses for the stoked crucible![1]