A petrological approach to the study of ancient glass.

In glass archaeology, "production indicators" are those remains which testify to specific operations carried out during the productive cycle; they allow us to reconstruct the processes and technological expedients used in the past to produce glass. The intermediate products of glass melting reveal textural and chemical inhomogeneities which may be used to infer parts of the production history. Samples of glass masses belonging to an early stage of glass making were unearthed in the archaeological sites of Lomello (province of Pavia, Italy) (I-IV centuries A.D.) and Val Gargassa (Genova) (late XII Century A.D.). In these materials, unmelted mineraI phases, banded textures, phase separation, and crystallisation alI produce different textures which supply information on provenance of raw materials; components added and the efficiency of the melting process; liquid immiscibility and refining processesc; ooling rate of vitreousp roducts. The composition of glass influences the different arrangements of tetrahedral-fold cations (SiO4-, AIO3-) and determines the various properties and features of the glass itself. The low effective diffusivity prevents the composition from becoming homogeneous during glass melting, and consequently evidence of early production steps is retained. Mineral relics present in the micro-textures of materials (i. e., fining slags) have great archaeometric value, because they may indicate the provenance of raw materials. The composition of relic mineral phases also supplies important analytical grounds on which to define the recipes followed during glass making, in terrns of vitrifying, stabilising, flushing and additive components.