Glazed Seljuk wall tiles, painted with an underglaze. First half of the 13th century.

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.
This tile was produced with the so-called ‘Minai technique’, involving the use of colors that have to be fired at different temperatures. First blue, green and turquoise were applied on an underglaze and fired. Other colors such as yellow, red, white, black and sometimes gilt were then applied on top of this, and re-fired at a lower temperature.

Correspondent: J.M.Criel, Antwerpen.
Sources: Wikipedia