The building was restored and repaired numerous times by Ottoman architects. The most famous and extensive work was done by Mimar Sinan, one of the most famous Muslim architects in history (who incidentally was converted to Islam from Christianity when he was young), in the 16th century, which included the addition of structural supports to the exterior of the building, the replacement of the old minarets with the minarets that stand today, and the addition of Islamic pulpits and art.
For almost 500 years the principal mosque of Istanbul, Ayasofya served as model for many of the Ottoman mosques such as the Þehzade Mosque, the Süleymaniye Mosque, and the Rüstem Pasha Mosque.
Mosaics
Following the building's conversion to a mosque in 1453, many of its mosaics were destroyed or covered with plaster, due to Islam's ban on representational imagery. This process was not completed at once, and reports exist from the 17th century in which travellers note that they could still see Christian images in the former church. In 1847-49, the building was restored by two Swiss brothers, Gaspare and Guiseppe Fossati, and Sultan Abdülmecid allowed them to also document any mosaics they might discover during this process. This work did not include repairing the mosaics and after recording the details about an image, the Fossatis painted it over again. This work included covering the previously uncovered faces of two seraphim mosaics located in the centre of the building. The building currently features a total of four of these images and two of them are restorations in paint created by the Fossatis to replace two images of which they could find no surviving remains. In other cases, the Fossatis recreated damaged decorative mosaic patterns in paint, sometimes redesigning them in the process. The Fossati records are the primary sources about a number of mosaic images now believed to have been completely or partially destroyed in an earthquake in 1894. These include a great mosaic of Christ Pantocrator in the dome, a mosaic over a now unidentified Door of the Poor, a large image of a jewel-encrustred cross and a large number of images of angels, saints, patriarchs, and church fathers. Most of the missing images were located in the building's two tympana. The Fossatis also added a pulpit (minbar) and the four large medallions on the walls of the nave bearing the names of Muhammad and Islam's first caliphs.
20th-century restoration
The interior of the dome undergoing restoration.A large number of mosaics were uncovered in the 1930s by a team from the American Byzantine Institute led by Thomas Whittlemore. The team chose to let a number of simple cross images remain covered by plaster, but uncovered all major mosaics found.
Due to its long history as both a church and a mosque, a particular challenge arises in the restoration process. The Christian iconographic mosaics are being gradually uncovered. However, in order to do so, important, historic Islamic art would have to be destroyed. Restorers have attempted to maintain a balance between both Christian and Islamic cultures. In particular, much controversy rests upon whether the Islamic calligraphy on the dome of the cathedral should be removed, in order to permit the underlying Pantocrator mosaic of Christ as Master of the World, to be exhibited (assuming the mosaic still exists).
