The smoke chamber is the funnel-shaped section of masonry above the firebox and below the flue that compresses smoke from the wide firebox opening down into the narrower flue passage.
Inside the smoke chamber, the walls slope inward as they rise, usually built up with corbeled brick that steps in course by course until it meets the flue's dimensions. This section sits directly above the firebox opening and below the flue, so its shape has to transition smoothly from a wide rectangular opening to whatever shape the flue is. A smoke chamber that's too rough, too wide, or built with an uneven taper can hurt draft and leave more creosote clinging to the walls, since smoke needs a reasonably smooth, gradually narrowing path to move efficiently.
A lot of the smoke chamber's interior gets parged smooth with a refractory mortar to close gaps between the corbeled brick and cut down on creosote buildup. Because it sits hidden above the firebox, damage here often goes unnoticed until a chimney sweep or camera inspection flags it, rather than showing any visible sign from the room below. On older chimneys we're rebuilding or relining, we check the smoke chamber's shape and condition along with the firebox and flue, since a problem in one usually affects how the others perform.
More masonry vocabulary from the glossary.
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