Iznik pottery was produced in Ottoman Turkey beginning in the last quarter of 15th century AD.[1] It consists of a body, slip, and glaze, where the body and glaze are 'quartz-frit'.[3] 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.[3] Microscopic analysis reveals that the material that has been labeled 'frit' is 'interstitial glass' which serves to connect the quartz particles.[3] The glass was added as frit and the interstitial glass formed on firing.