Structural clay tile grew in popularity in the end of the nineteenth-century because it could be constructed faster, was lighter, and required simpler flat falsework than earlier brick vaulting construction. Each unit is generally made of clay or terra-cotta with hollow cavities, or cells, inside it. The colors of terracotta transform from gray (raw, moist clay) to orange, red, yellow, and cream tones. This is due to an effect of the firing process which hardens the clay so it can be used for structural purposes. The material is commonly used in floor arches, fireproofing, partition walls, and furring. It continues to be used in Europe to build fire-resistant walls and partitions. In North America the material has largely been replaced by concrete masonry units.