The use of opus caementicium provided the impulse for the monumental development of Roman architecture during the imperial period, but it was fired bricks that served as the primary means for its advance. In reality, bricks were already part of the oldest building tradition in Rome, but generally in the form of mud bricks made with an argillaceous mixture and left to dry in the sun. This opus latericium was still being put to general use in construction in the first century B.C. with excellent results., at least according to Vitruvius , who emphasized the monetary value that Romans attributed to walls in mud bricks, as compared to those in stone.
At the beginning of the imperial period, perhaps in the wake of the standardization of construction materials that began with the tesserae of opus reticulatum, Roman builders paid particular attention to the possible uses terracotta in wall construction. For centuries fired bricks had been used alongside trimmed tiles in the Magna Graecian cities of southern Italy . The Romans immediately understood the advantages offered by bricks, including the low cost of their production, which was connected to the use of widely available raw materials (clay, sand, and straw) the speed of their installation, and their excellent strength.
Whole bricks were rarely used in masonry and most often reduced to smaller triangular or rectangular elements by breaking or sawing the brick along the transversal axis or the diagonal. With these segments masons made facing walls in opus testaceum, while the hollow of the core was filled with a solid opus caementicium
The use of fired bricks began to take hold in Rome early in the first century A. D. Its initial uses in monumental architecture arose during the reign of Tiberius. when they were employed for the construction of his palace on the Palatine Hill as well as that of the barracks for the Praetorian Guards (Castra Praetoria) northeast of the city. From that time forward the use of opus testaceum spread among he city’s work site, and by the time of Nero’s reign was established as the principal building method. The Domus Aurea, Nero´s gigantic residence, was built in fired bricks as was, later the great palace of the Flavian emperors on the Palatine
