Our modern appreciation of glass as a utilitarian material is focused very much on transparency as the most obvious basic property of silica-based vitreous consumables such as windowpanes or bottles. However, early glass was only very rarely the transparent and colourless material we know today, but rather a dense and shiny compound boosting with vivid and brilliant colours.
Some cultures started out early to develop and use this fascinating material with its singular potential of thermo-mechanical manipulation in the area of what is now known as modern Anatolia, Iraq and neighbouring regions around 3000-2500 BC. These non-transparent vitreous materials were meant as very much treasured imitations of opaque gemstones such as turquoise or lapis lazuli. The opacification by simple addition of (synthesized!) pigments or the well-mastered striking process bear witness to unprecedented skills and knowledge regarding the control of internal redox-systems and annealing procedures. [1]
Opaque, deeply coloured glass was gradually replaced by its transparent counterpart during the 2nd half of the 1st millennium BC, except for beads or some special glass ware. In particular opaque glass beads and enamelled metal-based jewellery exemplify this technological continuity of opaque materials, and specialists of ancient glass technologies are able to reconstruct the underlying expertise of past craftsmen by means of scientific studies of ancient artefacts.
Still, analysing ancient materials bears special challenges compared to modern industrial-standard glasses. Inhomogeneities are expected at different scales as is recycling and the use of impure raw materials. Long-term burial in soil and concomitant corrosion lead to severe alteration. Invasive analytical techniques that need sample preparation are often ruled out due to the cultural value of the items studied. Ancient opaque glass is a multi-phase composite material connecting (different) crystalline opacifiers and pigments included in a base matrix glass. In general, the system is even more complex involving diverse types of base glass and crystalline compounds in one and the same item. [2]
The current paper will give an overview of the state of the art of opacified glass of the 1st millennium AD, and explore instrumental challenges and limits.
Literature:
[1] I. Angelini, EMU Notes in Min., 2019, 20, 87. [2] J. Peake, in: Keller D. et al. (eds.) Neighbours and Successors of Rome, Oxford 2014, 15
