Diachronic change in imperial-era waterproof lining technology: Evidence from Pompeii, the Bay of Naples and Rome

The monumental water features of ancient Rome and the Bay of Naples represent the largest Imperial-era investment in public hydraulic infrastructure known from the ancient Mediterranean. Underlying and enabling all these works were highly specialised waterproof linings, the composition of which reflect the know-how and intentions of ancient builders. To investigate if and how waterproof lining technology changed through time, and what broader factors motivated their development, 14 mortar specimens dating between the 1st century BCE – 4th century CE were collected from water features found across Pompeii, the Bay of Naples and Rome. Each specimen was analysed using established archaeological and instrumental techniques, including ultra-high resolution mosaic scans of thin sections and polarised light microscopy (PLM). This was paired with a novel use of Micro-X-Ray Fluorescence Microscopy (μXFM), which allowed the elemental composition of whole thin sections to be rapidly and accurately visualised and compared. These analyses produced several data sets that were combined to identify clear change in the composition of Imperial-era waterproof linings. These findings were then contextualised to explore how contemporary socio-economic and political factors may have directed technological change in Imperial-era waterproofing technology.