Micro-textures recording melting-history of a medieval glass cake.

Glass cakes are particular historical glass artefacts, the use of which is not ascertained yet. They are semifinished product ingots traded from suburban glasshouses as raw material. Whether they were used to produce worked-glass, jewellery, or enamelware or they were tools used to sleek skin, paper, and textiles, is still debated. An analytical investigation of a glass cake, combining microtextural studies and modelling using pertinent phase diagrams has demonstrated that microtextures of mineral phases within the glass mass result from a complex melting process implying: peritectic-type reactions giving rise to wollastonite crystallization and subsequent reabsorbing of wollastonite when the peritectic temperature is exceeded. Analogous microtextures are shown by K–Al-phases. The melting behavior of a batch, to produce glass, thus represents a complex process that is not possible to model linearly, as commonly believed. It consists of a reaction-path where partial melting is accompanied by separation of crystal phases that, later on dissolved. The composition of the volumetrically dominant glass infers a melting temperature of about 1150 °C.