The composition of the soda-rich and mixed alkali plant ashes used in the production of glass

Soda-rich plant ashes have been used in the Near East and Egypt in the production of glass and faience from the 4th millennium BC onwards, and mixed alkali plant ashes have been similarly used in western Europe during the 2nd and first half of the 1st millennia BC. In the production of these ashes, the plants of interest are salt resistant, halophytic plants of the Chenopodiaceae family, growing in coastal, salt marsh and desert regions. A primary criterion in selecting ashes for glass and faience production is that the alkalis are predominantly in the form of carbonates, bicarbonates and hydroxides rather than either chlorides or sulphates. In the current paper, previously published data for such ashes are brought together and re-assessed, and new analytical data are presented for ashes produced from plants collected in Egypt, Greece and the UK. For the ashes produced from Salsola kali plants collected from Greece and the UK, the soda to potash ratios (0.3–1.8) do not show any systematic differences between the regions in which the plant was growing, but instead reflect the fact that this species favours the accumulation of K⁺ over Na⁺ ions. Further, the results suggest that S. kali could have been the source of the mixed alkali ashes used in western Europe, if the ashes had first been treated in some way in order to reduce their lime-plus-magnesia contents.