Rationales in Old World Base Glass Compositions

It is long known that most Egyptian and Roman base glass compositions show a remarkably small scatter in their chemical composition. By plotting appropriately reduced base glass compositions in ternary phase diagrams it is demonstrated that the compositional fields defined by the compositional scatter are closely related to eutectic regions within the relevant phase diagrams. This is interpreted as to be due to an eutectic melting regime, i.e. partial melting in the presence of a crystalline buffer or residuum, and not primarily a result of strict recipe and raw material control. Furthermore, it is demonstrated that possibly two independent melting temperature indicators are correlated, suggesting a factual relationship between melting temperature and melt composition. This evidence is taken to develop a “partial batch melting model” for these early glasses, as opposed to the “total batch melting model” of Mediaeval and early modern glasses. Some archaeological implications of this model are briefly discussed.