Fig. 2

Educts/raw materials for making alkali glass. Analyses in general: Ref. Brill (1999); Maus and Jenisch (1999); Wedepohl (2003). a Geogenic educts N = 102. Analyses from Brems et al. (2012), Brill (1999), Burkhardt (2016, unpublished, see acknowledgement), de Quervain and Friedlaender (1942), de Quervain and Jenny (1956), Gerber and Stern (2012), (Niggli et al. (1930) and Silvestri et al. (2006); b Biogenic educts N = 127. Analyses from Barkoudah and Henderson (2006), Cilová and Woitsch (2012), Stern and Gerber (2004), Gerber and Stern (2012), Tite et al. (2006), Verità (1985), Wolf et al. (2005) and Wolff (1871)