Clarity and brilliance: antimony in colourless natron glass explored using Roman glass found in Britain

Abstract

This paper discusses the development of Roman antimony decolourised natron glass, its dominance, and subsequent decline, using new trace element data for colourless glass found in Britain. Experimental glasses are used to investigate the influence of different proportions of raw materials (particularly the ratio of natron to calcium carbonate) on the resulting transparency or opacity of glass when antimony is added. Focusing on the 1st to 3rd centuries AD, the study has found that (1) There are chronological differences in antimony colourless glass compositions including (a) some early vessels have abnormally low calcium, aluminium and barium levels; (b) 1st/mid-2nd-century AD vessels in Britain may also contain up to 0.6 wt% lead oxide whereas mid-2nd/3rd-century AD vessels contain less than 300 ppm, and (c) the antimony content tends to decline over time. (2) These developments can be linked to production and recycling practices; but trace elements suggest that all of these antimony colourless glasses share an origin, probably Egypt. (3) Crucially, production of experimental glasses illustrates the inherent suitability of a sodium-rich, calcium-poor base glass composition for making antimony colourless glass, as it readily dissolves added antimony; conversely lower-sodium, higher-calcium glasses start to form opacifying calcium-antimonate crystals with the same quantities of antimony. Thus, the sodium-rich, calcium-poor glass composition from Egypt was ideally suited for decolourising with antimony and formed a water-clear glass. The calcium-rich Syro-Palestine glasses were more easily opacified with antimony to make opaque glass, but were decolourised with manganese, not antimony.

On the making, mixing and trading of glass from the Roman military fort at Oudenburg (Belgium)

“…In this assemblage, there is only one first to second century sample containing higher lead content (222 ppm). It has been suggested that earlier antimony glass tends to contain higher lead contents (Paynter 2006;Paynter and Jackson 2018), which goes well with the argument that different antimony sources would have been exploited through time (Lobo et al 2014). There are also variations in the lead content of the Sb glass from Bocholtz (Huisman et al 2009) suggesting that the Sb glass reaching the northwestern provinces was likely to be a product of the use of different antimony sources.…”

“…Manganese and antimony have been found together in Roman glass (Gliozzo et al 2015;Jackson et al 2015). As their simultaneous presence is not necessary to decolour glass, this has been considered as a proof for mixing and recycling of different glass types (Braun 1983;Freestone 2015;Nenna et al 1997;Paynter and Jackson 2018). The study of glass recycling has gained importance in the last years as it is a crucial element to understand glass production patterns as well as ancient technology (Ceglia et al 2017;Freestone 2015;Jackson et al 2015;Paynter and Jackson 2018).…”

The Color Palette of the Mosaics in the Roman Villa of Noheda (Spain)

“…Glass recycling appears to have been systematic and large-scale, in particular on the western fringes of the Roman Empire. This is reflected in the mixed composition of numerous glass assemblages and archaeological evidence from, for example, Roman Britain [63] and the seaborne trade of glass cullet [14,83]. Recycling and mixing of both end members of Roman glasses (Roman Sb & Mn) are less evident in areas close to the primary production sites in Egypt and the Levant.…”

“…White glasses with variable amounts of lead have been documented since the Hellenistic period, particularly in cameo and mosaic glass [58,61,62]. Even though it has been proposed that the lead may have been accidentally introduced with the antimony [58], recent experimental work has demonstrated that the presence of as little as 2% lead oxide modifies the viscosity and thus the workability of the glass, and facilitates the formation and dispersion of Ca 2 Sb 2 O 7 crystals during the melting process [59,63]. Elevated concentrations of lead were also found in some of the blue, turquoise, and grey tesserae from Noheda (1.00% < PbO < 2.13%).…”

“…Elevated concentrations of lead were also found in some of the blue, turquoise, and grey tesserae from Noheda (1.00% < PbO < 2.13%). This may be due to the use of a lead rich antimony ore [58], a calcination procedure that involved the use of lead to refine the antimony minerals [63], or simply the result of recycling processes such as the addition of lead-white glass to a translucent colored glass (Figure 4a, Table S1). This practice of adding an opacified glass to a strongly colored translucent glass has been documented in seventh-century mosaics from Rome [64].…”