This device appears to be a normal container for holding burning coals unless magic is detected for. 

It enables a magic-user to summon an elemental of 12 hit dice strength from the Elemental Plane of Fire. A fire must be lit in the brazier — usually 1 round is required to do so. If sulphur is added the elemental will be of +1 on each hit die, i.e. 2-9 hit points per hit die.

The fire elemental will appear as soon as the fire is burning and a command word is uttered. (See MONSTER MANUAL for other details.)