One of the essences in Roper Klacks' laboratory that, when mixed, create his potions. Each one can be smelt, handled or listened to, in order to match it to the pictures in Klacks' spell book.
This is an essence of hot coals - therefore, it presents something of a safety hazard. It smells sulphurous, burns to the touch, and 'sounds' like crackling flames.
Obtained on an extremely high shelf in the laboratory's rafters.