With the ‘Arab’ Conquest of Sindh, not only a new political class and polity were introduced, but there was also the introduction of new types of building material and concept of architectural planning. Stone was gradually replaced with bricks and brick-tiles and lime mortar and gypsum to bond these bricks, was used for the first time.
In a number of palatial and residential buildings at Mansura in Sindh the excavators found below ground level a basement ‘built of brick and white lime mortar’, as well as gypsum along with lime at the upper levels of construction. Similarly in the mosques of Deybul and Mansura founded after the conquest by Muhammad bin Qasim, square sized burnt bricks, brick-tiles and gypsum and lime-mortar are encountered as the building material.
In the Indo-Gangetic plains where the Turkish Sultanate was established, the use of lime-mortar starts from the 13th Century. The distinguishing element at Lal Kot, near the Qutb complex at Delhi, which marks the archaeological remains of the pre-Sultanate from the Sultanate period, is the use of lime plaster.
The use of lime as mortar though known in the ancient period there are extremely few examples which survive. Use of gypsum as mortar had also been known but confined only to the Indus cities of the third millennium BC.6 Similarly prior to its profuse use in Medieval India, surkhi had also been employed only at Mohanjo Daro, after which it is not encountered.
Irfan Habib rightly points out that the medieval use of lime and gypsum mortars was eminently due to the combination of building methods derived from Byzantium and Sassanid Iran under the Arab caliphates.
The use of these new building materials: the lime-mortar, gypsum, surkhi and the brick which they helped to bind, led to cheaper costs of construction. There was a proliferation in the building constructional activity. With cheaper costs, men of lesser means could also indulge in the luxury of building activity.
Thus if in Ancient India only public structures and places of worship were generally constructed, now a large variety of buildings, both religious (mosques, temples, tombs, khānqāhs) and secular (palaces, sarais, bazars and residential structures, works of hydraulics etc) sponsored both by king and laity resulted.
The need for these new building materials had arisen due to the introduction of the arcuate technique of construction. It was a system in which the enclosed space is roofed and vaulted with the help of an arch. The arch itself is a structure, especially one of masonry, forming the curved, pointed, or flat upper edge of an open space and supporting the weight above it, as in a bridge or doorway. This arch when in its true form (the ‘arcuate’ system) is constructed with the help of wedge-shaped stones known as voussoirs and a key stone.
Two spans are constructed, each springing from the imposts on the wall, pier or pillar. At the point of their contact a triangular key stone is added to hold them together. In this technique the durability of the enclosed space was guaranteed till the ‘keystone’ was in place. Secondly, the voussoirs ensured that the weight of the stones radiated in different directions, leaving the ceiling almost weightless. Thus the structure roofed by such a ceiling could be larger and higher. Thirdly the angle or slant of the voussoir could help in getting the desired breadth of the building. In this system, small medium of construction provided flexibility of attaining the myriad shapes and sizes. Thus brick was more suited which in turn needs a good binding material like lime mortar and gypsum.
We have evidence to suggest that these innovations in the field of architecture after the Turkish conquest were soon imbibed and indigenised by the Indian architects and engineers who appear to have incorporated these techniques and principles in their practice and knowledge systems.
A well-known fourteenth century polymath and Jain scholar, Thakkura Pheru (b. 1270), who served Alauddin Khalji at some high position in his treasury, compiled amongst other works, Gaṇitasārakaumudi, a mathematical text in Apabhramṣa, in which he claims to have borrowed material from past teachers and contemporaries, apart from his own personal experience.
In this text Thakkura Pheru clearly mentions the mathematical measurements of not only wells (kūpa, kuva & vavi) and wells (pāya-seva, sovana) but also domes (goṃmaṭṭa), minarets (munāraya) and arch (tāka). The terms goṃmaṭṭa (gumbad), munāraya (mināra) and tāka (tāq) themselves suggest indigenization of a recent import. These domes, minarets and arches, according to him were constructed of bricks (iṭṭa) whose procedure of construction (citi-vyavahara) he goes on to discuss.
Pheru goes on to elaborate the computation of the bricks and mentions that the length, width and height of a brick are half, one-third and one-eighth (hattha) respectively. Regarding the dome, he mentions:
Half the circumference of a dome at its foot, multiplied by one-quarter of the circumference and increased by one-ninth (of itself), is the piling (cayaṇa) (when the circumference is measured) from the inside of the wall (bhiti-gabhāo). (When the circumference is measured) at the middle of the outside (? bāhira-majjhāu), it is the area (khitta)…..The circumference multiplied by half the diameter and increased by one-ninth (of itself) is (the volume of) the empty space (khalla) in a dome. This has been told according to experience. There is no doubt. It should be known thus.
Similarly Thakkura Pheru, in this work, also discusses the mathematics of an arch:
In case of an arch (tāka) above a door, multiply the length and height and further multiply the thickness of the wall. This diminished by one and a half times its one-seventh, is the volume of the empty space (khalla) along its apex (sihā).
We know that it in 1311, during the reign of Alauddin Khalji, employer of Thakkura Pheru, a true dome was constructed which covers the structure known as Alai Darwaza. The arches, squinches and the dome are all of the true order in this building. It was during the same period that the first arcuate mosque is the Jama’at Khana mosque constructed by Alauddin Khalji around 1312 AD. By the period of Akbar, the four-centred Iranian arch had been popularised and came to be identified as a typical Mughal arch.