 Tin oxide is often employed to render glaze opaque but in İznik glazes it remains in solution and is transparent.Abū'l-Qāsim described the use of earthenware saggars with a fitting lid.Although Miletus ware bowls were stacked in the kiln one on top of the other separated by spurs, the lack of spur marks on Iznik fritware suggests that saggards were used.