Christian architecture in Rome not being called upon to attempt any such heavy constructions as were required by the use of vaulting on a large scale, and not needing heavy walls for its wooden-roofed churches, did not patronize concrete construction. Stone was used but seldom, in the regular courses of the opus quadratum, in such works of engineering as the bridges of Gratian and Valentinian and in the restoration of such monuments as the Coliseum and the theatres. But
even this was abandoned after the Gothic wars: its latest use being possibly in the bridge by Narses over the Anio.
In religious architecture brickwork was the rule in the body of the structure, for the walls were not heavy enough to allow of a brick facing and a concrete core. The quality of the brickwork varied at different periods. As long as the government factories continued the manufacture of bricks, up to the time of the Gothic wars, they were of excellent quality, the main change between the brick of the Antonines (second century) and those of the fifth century being a diminution in size, a change which is found early in the fourth century, though there were also variations in color and texture.
Mediaeval brickwork was less perfect during the middle period. Heavy beds of mortar and careless laying, which we find as early as the fifth century, with an interlude of excellent work under Theodoric, became the rule between the seventh and eleventh centuries. But in the course of the twelfth century there was a return to better brick-making, more careful laying and thinner bedding, which helped to give
a similar effect to that of the age of Constantine.
In classic architecture it had not usually been permissible to let the brickwork be seen except in works of pure utility; with Christian architecture the treatment was different. The exteriors were carelessly treated, for they were spiritually of no interest; and their brickwork was covered only sporadically, as by a mosaic on the facade. The trimmings of doorways and porches were also of stonework. It was only in the interiors that the brickwork was as absolutely concealed as in classic buildings either by facings of thin marble veneering slabs or by mosaic work.
Two other methods were occasionally used: the opus mixtum and the opus saracinescum or a tufelli. The former consisted of alternate layers of small stone blocks, usually tufa, and brickwork, there being at times two rows of the bricks to one course of stone. This method became popular in the time of Constantine and during the rest of the period before the Gothic war, and again came into vogue during the tenth century. The opus saracinescum was a "petit appareil" of small tufa blocks which is found as early as the seventh and remained popular until the eleventh century.
It was only outside of the city that local stone was substituted for brick, and here the stone was often used in so plain a fashion as to lose its natural advantages over brick, as in the basilica of S. Eli at Nepi, or the tower of S. Scholastica at Subiaco.