|Imam Mosque Dome - Isfahan|
With the importance that the dome and the vault rapidly gained in Islamic architecture, it soon became necessary to devise a solution to the problem of the transition zone between the square chamber and the dome above it, namely from a square plan to a circular one. The basic solution depended on the squinch, an arch set at an angle across each corner of the building. In the 11th and 12th centuries, the corner squinches were generally variants on the following scheme: centred; set over the corner; a small barrel-vaulted squinch flanked on either side by a quarter dome with the whole composition enclosed within a larger broken arch. Between each of these corner squinches, centred above each side of the square chamber, is a similar lobed arch. In this way, an octagon is created over the base of the chamber. Above these squinches a row of smaller arches, set over the angles of the octagon, thus form a sixteen-sided polygon. This so-called muqarna system appeared in a simple form at the end of the tenth century at Tim, in Central Asia, and then at Yazd, in the Davazde Imâm Mosque, built in 1037. It reappears frequently afterwards, particularly at Ardestân and at Isfahan in the Friday Mosque.
The small triangular squinch was to have multiple applications, even covering, in superimposed rows, the entire interior surface of eivân domes. Each row is shifted sideways in relation to the ones below, and each squinch becomes gradually narrower towards the apex of the arch. Under the Safavids, each row was shifted slightly forwards so that it jutted out over the one below it, forming a network of cells, or hanging stalactites.
Another important building technique was the use of a double-shelled dome to lighten and strengthen the structure. In Iran, the oldest examples of this technique appear in the Kharaqân tombs, in Zanjân Province, built at the beginning of the 11th century. This technique enabled the inside and outside of the dome to be designed in different shapes. Good examples are the spectacular Timurid and Safavid domes, such as those of the Gur Emir Mosque at Samarkand (1434) and of the Sheikh Lotfollâh and Imam mosques in Isfahan (1598 and 1611).