How to Fire an Unfired Clay Bucket into a Clay Bucket
Smelting Setup
Place a furnace and add fuel (coal, charcoal, planks, or any burnable item). The Unfired Clay Bucket is processed through the smelting recipe type, so it goes in the top slot of the furnace, not the fuel slot.
The Smelting Step
Put your Unfired Clay Bucket in the input slot. After the smelting completes, you receive a Clay Bucket. This is the finished, usable container.
The Clay Bucket is the base item for all filled variants. It can be filled with water to become a Water Clay Bucket, or with lava to become a Lava Clay Bucket. The smelting step is the only way to convert the unfired form into a working bucket — there is no crafting recipe that produces a Clay Bucket directly.
If you have multiple Unfired Clay Buckets, smelt them one at a time or in batches depending on your fuel supply. Each one consumes fuel and furnace time. Once you have a Clay Bucket, you can move on to filling it with water or lava.