Abstract
This work aimed to investigate the origin of different red hues of Roman copper-based red opaque glass sectilia, to shed light on the production technology behind them. This objective was achieved by the depth study of the samples of glass sectilia, which decorated the villa of co-Emperor Lucius Verus (161–169 AD). These were selected for analysis due to their abundance, the certainty of their date and of their different red and orange hues. Using OM (optical microscopy), colourimetry and FORS (fibre optical reflectance spectroscopy) spectroscopy, four red and four orange hues were individuated. A set of representative samples for each hue was analysed by EPMA (electron probe microanalyses) to detect any correlation between colour and chemical composition. Crystalline phases were investigated through high-resolution FEG–SEM (field emission gun scanning electron microscope), μRaman spectroscopy and XRD, for the identification of colouring and opacifying agents and to understand how the different hues are affected by their shape, concentration and dimension. Sub-micrometric particles of metallic copper and cuprite crystals were identified as both the colouring and opacifying agents. These were not present in the same samples and were manufactured by two distinct colouring techniques, corresponding to two different glass chemical compositions. The size and the number of the colouring particles were the main factors that distinguished one hue from another. Although produced through different colouring techniques, some red samples appeared to be very similar to each other. These data enrich a period of the Roman age through some analyses and allow the identification of the type, as well as some of the production conditions of opaque red glass produced during 2nd century AD, which could be considered to be a period of transition from one technology to another.
Highlights
IntroductionBrief Overview of the Theory of Opaque Red Glass
When copper is dissolved in the melt as a cupric ion (Cu2+ ), a blue turquoise glass is obtained, while the glass is colourless as a cuprous ion (Cu+ )
The multi-analytical approach used to investigate the copper red glass sectilia of the Gorga collection allowed the characterisation of several crucial aspects of this glass, providing precious insight, in order to clarify its production technology
Summary
Brief Overview of the Theory of Opaque Red Glass. The use of copper to colour glass dates back to the beginning of glassmaking technology. Based on its oxidation state, copper-containing glass has a wide chromatic scale of colours. When copper is dissolved in the melt as a cupric ion (Cu2+ ), a blue turquoise glass is obtained, while the glass is colourless as a cuprous ion (Cu+ ). When the reducing conditions are achieved, and copper separates from the melt in the form of metal micro particles (Cu◦ ) or as crystals of cuprous oxide (Cu2 O, cuprite), Minerals 2020, 10, 875; doi:10.3390/min10100875 www.mdpi.com/journal/minerals. Minerals 2020, 10, 875 opaque red glass is generated [1,2]. This process occurs throughout the formation of some minerals
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