Fireclay Basics. As the name implies, fireclay is a clay-based material that is fired in a kiln at very high temperatures. Fireclay is a type of ceramic; however, the firing process fuses the glaze to the clay and vastly improves the strength and integrity of the material.Fireclay sinks are extremely versatile, since the clay can be poured and shaped in any configuration, including the incorporation of artistic embellishments.ireclay is a type of ceramic; however, the firing process fuses the glaze to the clay and vastly improves the strength and integrity of the material. Fireclay sinks are extremely versatile, since the clay can be poured and shaped in any configuration, including the incorporation of artistic embellishments.