The composition of Late Bronze Age Egyptian faience and glass is thought to reflect the composition of the raw material used in their production, particularly plant ash. This paper argues that there are strong and systematic shifts between the plant ash composition and the resulting glass, as a function of inherent technical processes during both the wet preparation and subsequent drying of faience glazes, and the smelting of glass from its raw materials. These factors lead on the one hand to significant differences in glaze composition from identical raw materials due to different glazing techniques being used, and on the other hand to a homogenisation of glass compositions which may obscure more subtle differences in initial raw material composition. The paper aims to explore the various factors at work, briefly summarizing recent publications and current thought on the subject, in the hope to raise awareness of the issues involved, and to stimulate further research.