PNNL

Abstract

Partially vitrified archaeological samples, produced by firing quarried rocks in charcoal fires, have been obtained from Broborg, a vitrified Swedish hillfort. The melting conditions would have influenced the oxidation state of iron in the glass melt by consuming oxygen for combustion and releasing water vapor into the melting atmosphere. The oxidation state of iron and presence of water vapor have often been linked to the relatively low temperatures that were employed at the Broborg Hillfort, by the Iron Age (Migration Period) inhabitants, to produce these melts. The archeological samples excavated from the site are the result of near-surface alteration for approximately 1500 years. As such, they have been identified as analogues for the investigation of the long-term alteration of glasses used for nuclear waste immobilization. However, the influence of iron oxidation state on the alteration behavior of these Broborg hillfort glasses is poorly understood and is complicated by the heterogenous nature of the archaeological samples. To address this, synthetic analogues have been produced using either fully oxidized or reduced iron precursors to investigate the influence of glass composition and iron oxidation state on the degree of network connectivity. The melting behavior, glass transition temperature, oxidation state, and chemical durability of these high-silica, low-iron, low-alkali glass analogues have also been assessed. We investigate how the degree of network connectivity (interpreted using fraction of non-bridging oxygens) may be correlated with: i) the behavior of the glass-forming melt; and ii) the chemical durability of the final glass. Glasses with a lower degree of melt connectivity have a lower viscosity, resulting in a lower glass transition temperature (Tg) softening temperature (Td) as well as in a lower temperature of foam onset (TFO) and temperature of foam maximum (TFM). This lower degree of network connectivity played a greater role in accelerating the conversion of batch chemicals into glass than the presence of water vapor in the atmosphere. The oxidation state of the low concentration of iron in this set of high-silica glasses did not significantly affect chemical durability