Clay flue lining serves two vital functions in a chimney. First, it contains the by-products of combustion and prevents their penetration into surrounding masonry. Second, it provides a uniform draft. The smooth inner surface helps flue gases to exit freely. THERMAL SHOCK Most arguments against using clay flue lining seem to center on the issue of thermal shock. However, selecting a chimney lining should involve more than just determining which flue lining can absorb the greatest temperature change in the least time. To be effective, a chimney flue lining must also be durable. Barring a chimney fire, a properly constructed and maintained clay-flue-lined chimney will last a lifetime. CONSTRUCTION GUIDELINES Proper construction of chimneys and flues is important. Following building codes and manufacturers' recommendations can prevent many problems with clay flue liners.