A fireplace smoke chamber is the hottest area of the entire system. This area sits immediately above the firebox. If and when a chimney fire occurs, this is the most likely source for ignition. A parged (smooth surface) from the firebox leading to the flue acts as an upside down funnel and can significantly decrease the chance of creosote buildup allowing for proper draw; While also maintaining proper clearance to combustibles (ie. wood), keeping your home safe in the event of a heat transfer issue or chimney fire.