A muffle kiln is a type of furnace used for firing ceramics and other materials. However, if you're looking for alternative terms to describe this specific device, there are a few similar options. One alternative could be a "retort kiln," which also operates within a closed container, maintaining a controlled environment for firing processes. Another synonym is a "sealed kiln," referring to the sealed chamber in which the firing takes place to prevent the escape of heat or gases. Lastly, a "ceramic kiln" can also be used as a synonym for a muffle kiln since both terms describe the same function of firing pottery or ceramics under controlled conditions.