Impurities and imprecise firing methods caused voids and lamination in the terra cotta pipe placed in the late 1800s and early 1900s. Photos: National Clay Pipe Institute
Modern manufacturing is precisely controlled to quickly bring the clay from a drying room temperature up to 500º F before slowly raising it to 1,100º F to burn out impurities. Once the pipe body reaches 1,100º F the firing process can proceed quickly to 2,000º F, the point at which the pipe is vitrified — converted to a hard, glass-like state. To maintain dimensional stability, cooling also is controlled in steps.
CLAY VS. THE COMPETITION Specifiers tout vitrified clay as all-natural as well as corrosion-resistant. Other materials are available in longer lengths, such as 13.5 feet for plastics and more than 20 feet for fiberglass compared to 10 feet for clay. But as a blend of clay and shale with water added, only one chemical — hydrofluoric acid — attacks clay pipe compared to the 150 solvent-based chemicals that attack plastics. Source: Colleen Harold