Two overriding practical constraints influenced early church architects: the need to use stone for roofs because of the lack of suitable timber, and the need to withstand fairly frequent earthquakes. The necessary strength was provided by the use of an early type of concrete in a method probably copied from Roman architects. The earliest churches were built of massive stone blocks, with mortar separating them, forming the outer and inner surfaces of the walls; between them was a thin layer of concrete. This concrete was compounded of broken tuff and other stones, lime mortar and eggs. During the 5th and 6th centuries the technique evolved, the slabs of the stone shell becoming thinner and the cement core thicker. The method of construction was then to erect finely cut slabs of tuff or other stone a few rows at a time and without mortar to form the surfaces of the outer and inner walls after which concrete was poured into the cavity between them. This concrete adhered to the facing slabs and formed a solid strong core and it is this core, rather than the thin slabs of tuff which forms the building's major support. The slabs were varied in size and height to break up the vertical and horizontal rows and thus provide protection against parts of the concrete core falling out during earthquakes. Great thought was given to enhancing the artistic appearance of churches, and different churches had the tuff slabs erected in different ways. In some churches the slabs were carved and either different colours of slab might be employed to provide a contrast or else a uniform colour might be used, sometimes with mortar applied between the slabs to give a completely uniform appearance. As the technique evolved the largest stone blocks gradually came to be reserved for the lowest courses of stonework as well as for corners and smaller
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