Glazed Seljuk wall tile from the sultan’s summer residence at Kubadabat, near Beyşehir. Since the residence was built by sultan Alaaddin Keykubad I (1220–1236), this tile - painted with an underglaze - must be dated from that period.
Underglaze is a method of decorating ceramic articles; the decoration is applied to the surface before it is glazed. As the glaze will subsequently cover it such decoration is completely durable, but because the subsequent glost firing is at a higher temperature than used in on-glaze decoration the range of available colours is more limited. It uses pigments derived from oxides which fuse with the glaze when the piece is fired in a kiln.
Correspondent: J.M.Criel, Antwerpen.
Sources: Wikipedia & Personal visits (several times between 1978 and 2001).