A chimney sweep is a worker who clears ash and soot from chimneys. The chimney uses the pressure difference caused by a hot column of gas to create a draught and draw air over the hot coals or wood enabling continued combustion. Chimneys may be straight or contain many changes of direction. During normal operation, a layer of creosote builds up on the inside of the chimney restricting the flow. The creosote can also catch fire, setting the chimney and the building alight. The chimney must be swept to remove the soot. This was done by the master sweep.