Interactions between silicate and salt melts in LBA glassmaking

Abstract

A series of glass-making experiments have been done to explore the influence, if any, of the presence of alkali chlorides in a typical soda-lime-silica glass batch on the final composition of the glass. The experiments have shown that concentrations of chlorides up to the limit of solubility of chlorine in the melt are actively contributing alkali ions to the glass-forming process, and that at higher chloride concentrations in the batch a separate salt melt forms, known as galle. At equilibrium conditions, the alkali ratio in the galle is different from the initial alkali ratio in the batch and differs from the ratio in the co-existing silicate melt. Significantly, there is full exchange of alkali ions between the two melt systems and the addition of pure potassium chloride to a batch containing pure sodium carbonate (= soda ash) results in the formation of a mixed alkali glass and a mixed alkali galle, with complementary alkali ratios. The alkali earth elements (i.e. calcium and magnesium), in contrast, are not taking part in these ion exchange reactions, and seem not to be affected by chlorides present in the batch. These findings are particularly relevant when comparing analyses of different plant ashes with archaeological glasses; the alkali ion ratios between ash and glass are only likely to be similar when no galle is forming together with the glass melt; in the presence of more than a few weight percent of chlorides in the batch, it is likely that the alkali ratio in the glass will be increasingly shifted away from the total alkali ratio in the batch as the chlorine content increases. While the argument ManuscriptPublished as Tanimoto & Rehren (2008) is developed specifically for Late Bronze Age halophytic plant ash glasses, the results are likely to be valid in principle also for any other glasses based on halogen-rich batches and containing more than one alkali metal.

Glassmaking using natron from el-Barnugi (Egypt); Pliny and the Roman glass industry

“…These experiments support the previous findings on the limited solubility of chloride and sulphate in the melt, corresponding to the results of Turner (1956) and Tanimoto and Rehren (2008); they also show that the presence of one affects the solubility of the other. Up to 2.6 wt% chlorine was detected in the melt when sodium chloride alone was added to the batch, but this was reduced to 1.9 wt% chlorine in the batch that originally contained equal amounts of both sodium sulphate and sodium chloride.…”

“…Tanimoto and Rehren (2008) also propose something similar, that a glass with a eutectic mixture would result (a mixture with a lower melting temperature than any other combination of the same constituents). The eutectic composition would be dependent on the melting temperature and the raw materials used (Rehren 2000); an immiscible, chlorine-rich layer would also form on the surface of the glass, although alkalis could exchange between these two melts.…”

“…Bateson and Turner (1939) determined that in a soda-lime-silica matrix, the maximum concentration of chlorine in the resulting glasses, regardless of the amount of sodium chloride added, was 1.42 wt% (melted at 1400°C); a concentration of up to 1.8 wt% was observed by Tanimoto and Rehren (2008).…”

Glass Making in the Greco-Roman World: Results of the ARCHGLASS project

“…During the glassmaking process, excess sodium chloride and sulphate forms a second melt phase on the surface of the molten glass. This immiscible 'galle' could be skimmed off or discarded after the glass has cooled (Freestone, 2006;Gerth et al, 1998;Shugar and Rehren, 2002;Tanimoto and Rehren, 2008).…”

Potash – a key raw material of glass batch for Bohemian glasses from 14th–17th centuries?

“…Bracken potash contains P 2 O 5 and chlorine, neither of which is present in substantial amounts in wood ash potash. The higher P 2 O 5 or chlorine (in the form of KCl) contents in bracken ash can cause glass opalescence; modern studies (Bateson and Turner, 1939;Gerth et al 1998;Tanimoto and Rehren 2008) provide more information about the behaviour and limited solubility of chlorine in glass. Moreover, bracken is a seasonal plant and changes composition through the season, which is reflected in the plant ash; therefore, the concentrations of alkali, iron and manganese (and possibly Cl) change throughout the season (Smedley and Jackson, 2006;Jackson and Smedley, 2008).…”