The San Saturnino Basilica (Cagliari, Italy): An Up-Close Investigation about the Archaeological Stratigraphy of Mortars from the Roman to the Middle Ages

Fabio Sitzia

doi:10.3390/heritage4030103

Abstract

The manufacturing technology of historical mortars from the Roman to Medieval period apparently has not undergone evolutions. As reported in the literature, a quality decrease in the raw material occurred after the fall of the Roman Empire. During the Roman Age, the mortars presented the requirements of long durability due to hydraulic characteristics, and in later times, the production has only partially maintained the ancient requirements. To focus on the different production technologies between Roman and Medieval mortar, this research presents the case study of San Saturnino Basilica (Italy), where an archaeological mortar stratigraphy from Roman to Middle Ages is well preserved. An archaeometric characterization was performed to compare the mortars of the Roman period with the mortars of the Medieval period collected from the case-study monument. This comparison was carried out by measuring some physical-mechanical, mineralogical, petrographic and thermal features that give more information about the durability and resistance to mechanical solicitations and weathering. After the characterizations, contrary to what is reported in the bibliography, a better quality of Medieval materials than Roman ones is pointed out. This has been highlighted by higher hydraulicity, mechanical performance, and a more appropriated particle-size distribution of aggregates.

Highlights

In antiquity, the ancient workers had the necessity to produce durable mortars, socalled “hydraulic mortars”, capable of remaining hard in high-humidity environments and underwater, too
Because of the absence of specialized technologies, they tested a lot of raw materials, discovering the ones that best fit their needs as fragments of volcanic rocks and volcanic ash, defining them as “pozzolanic mortar”
This technology consisted of the use of pure lime mortars with the absence of impurities, whose hydraulicity was subsequently provided by the use of reactive aggregates [1]

Summary

Introduction

The ancient workers had the necessity to produce durable mortars, socalled “hydraulic mortars”, capable of remaining hard in high-humidity environments and underwater, too. In the absence of natural raw materials, they experimented a mixture with the addition of artificial aggregates (e.g., ceramics rubble, brick rubble, and powder tiles) The hydraulic mortars such as “pozzolanic” or “brick rubble” typology were already known at the time of the Phoenicians and were perfected by Romans. This technology consisted of the use of pure lime mortars with the absence of impurities, whose hydraulicity was subsequently provided by the use of reactive aggregates (brick rubble, pozzolan, obsidian, rhyolite, and coal) [1]. The correct manufacturing and use of mortars started approximately after the publishing of the De Architectura book series, written by Vitruvio Pollione in 15 BC [2], until the western Roman empire fell (476 AD) [3]

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