Charcoal

Peering in, we could see that the only light in the room came from a dull blue flame which flickered from a small brass tripod in the centre. It threw a livid unnatural circle upon the floor, while in the shadows beyond we saw the vague loom of two figures which crouched against the wall. From the open door there reeked a horrible poisonous exhalation which set us gasping and coughing.