A fireproof unit for a sliding closure at the spout of a container containing metal melt, including a fireproof adapter block that can be placed on the container spout, a fireproof bottom plate connected thereto, to which bottom plate a slidable fireproof sliding plate can be connected, wherein the sliding plate allows the sliding closure to be opened or closed. In addition to the first bottom plate connected to the adapter block, a second bottom plate adjoins the adapter block. The second bottom plate can be slid in under the adapter block to replace the first bottom plate. The adapter block has a length in the direction of the second bottom plate such that the adapter block is in sliding contact with the second bottom plate at the lower sliding surface of the adapter block.