On the picture: A close-up of some tiles adorning the mihrab (prayer niche) inside the mausoleum of Şehzade Mahmut, a son of Bayezit II. The mihrab and its tiles are original (1506).

Regarding the 15th century Iznik blue-and-white pottery:
İznik work, named after the town in western Anatolia where it was made, is a decorated ceramic that was produced from the last quarter of the 15th century until the end of the 17th century. İznik town was an established centre for the production of simple earthenware tiles and pottery with an underglaze decoration when in the last quarter of the 15th century, craftsmen in the town began to manufacture high quality tiles and pottery with a fritware body (frit being added to clay to reduce its fusion temperature), painted with cobalt blue under a colourless lead glaze. The meticulous designs combined traditional Ottoman arabesque patterns with Chinese elements. The change was a result of the active intervention and patronage by the recently established Ottoman court in Istanbul, who greatly valued Chinese blue-and-white porcelain.

Correspondent: J.M.Criel, Antwerpen.
Sources: ‘Islamic Architecture: Ottoman Turkey’ (Godfrey Goodwin) – London 1977 & Wikipedia
Source: ‘Vakıf Abideler ve eski Eserler III’ - Vakıflar Genel Müdürlüğü, Ankara 1983 .