Iznik pottery was produced in Ottoman Turkey as early as the 15th century AD. It consists of a body, slip, and glaze, where the body and glaze are “quartz-frit.” The “frits” in both cases “are unusual in that they contain lead oxide as well as soda”; the lead oxide would help reduce the thermal expansion coefficient of the ceramic. Microscopic analysis reveals that the material that has been labeled “frit” is “interstitial glass” which serves to connect the quartz particles. Tite argues that this glass was added as frit and that the interstitial glass formed on firing.