Architectural terra cotta was an inexpensive, lightweight substitute for stone that was popular in the United States from the 1870s through about the 1940s. It is comprised of a fired clay body or ‘bisque’, and a glazed outer surface. Terra cotta was glazed using either a matte slip or a glossy glaze, both of which bonded to the clay body upon firing in a kiln. Glazes could replicate the rainbow of colors, but most terra-cotta units have stone-colored glazes.
Terra cotta units were made in a variety of lengths, heights and depths, but they all have an open cell structure supported by relatively thin walls. Although some terra cotta was installed as load-bearing masonry, most terra cotta was hung from a building’s steel structure using iron bars, hooks and angles.
Hollow-clay tiles are a type of terra cotta that can be found behind the plaster in many late-nineteenth and early-twentieth century buildings. They are modular units with hollow cell structures, and have ribbed or textured unglazed surfaces that allow mortar to bond to each side. They were a precursor to concrete masonry units (cinder blocks).