Viscosity of soda-lime glass and potash-based glass of the Carolingian period (8th – 9th century CE)

• Temperature-viscosity models of modern glass are of limited use for historic glass due to their different compositions. • Temperature-viscosity data were evaluated on an experimental basis for historical glass compositions. • Early wood-ash glass requires higher melting temperatures than soda-lime glass. • The viscosity behaviour of wood ash glass results in a shorter processing time compared to soda-lime glass. The soda-based composition of the standard bulk glass of the 8th century in Europe was changed to a potash-based composition in the 9th century ( Wedepohl 2010 ). This change in composition leads to a ∼ 200 °C increase in furnace temperature required for melting from the ∼ 1000 °C required to melt the sodium-rich glass. These are the temperatures required to produce a bubble-bearing melt after 30 min at temperature. Increasing these temperatures by 100 °C produces a bubble-free melt after 30 min. Similarly, increasing the amount of time at the lower temperatures would also produce bubble-free melts as the bubbles require more time to rise in more viscous melt. At a temperature of 1250 °C the viscosity of the potassium melt is 1.5 log 10 units higher than that of the sodium-based melt (10 1.5 to 10 0 Pa s). The difference in viscosity of the two melts changes from 2 log 10 units at 1100 °C (10 1 to 10 3 Pa s) to 3 log 10 units at 800 °C (10 4 to 10 7 Pa s). A consequence of the different viscosity/temperature relationship of the two melts is the shortening of the estimated “working time” from 30 s for the sodium melt to 20 s for the potassium melt, taking the working range to be 10 2 -10 6 Pa s. (If a maximum viscosity of 10 7 Pa s is taken for the working range, the working times vary from 40 s for the Na-rich glass to 25 s for the K-rich glass). The sodium melt was synthesized in air at 1100 °C, resulting in a transparent glass deep aubergine in colour. Heating the glass to 1400 °C produced a transparent pale blue-green glass as the Mn 3+ atoms were presumably reduced to Mn 2+ at higher temperature. As medieval furnaces were powered by wood burning and therefore operated at reducing conditions, the pale transparent Mn 2+ -sodium glass was the standard glass of the time. It was known at the time that adding oxygen to the furnace would produce the aubergine coloured glass ( Dodwell, 1961 ; see also citations in Bidegaray et al., 2020 ). The potassium glass which was synthesized in air at 1400 °C is transparent and yellow-olive in colour.