Glass is a material that most of us encounter everyday of our lives, however, most people have a very slim knowledge of what glass is, or how many different kinds of glass exist in the world. Beads of opacified glass have for example often been met with doubt to their material, called “glass paste” or in Swedish “glas-fluss”. This is because in modern times, translucency is by many believed to be a main feature of glass. (Brugmann 2004:1) Ancient glass however is not naturally transparent, but need to be decolored by adding substances to the recipe that neutralize the effects of impurities. (Janssens 2013:50) Chemical analysis of opaque glass beads leaves little doubt about their material. Brugmann 2004:1)
What is it made of?
Glass has no standard recipe, the content, even in modern times varies depending of the intended use. To present it simple, glass is made from three main components, Vitrifies, Fluxes and Stabilizers. The biggest part of the glass is some kind of vitrifier, normally silica from sand, however crushed quarts has also been used as a vitrifier in prehistory. Fluxes is a collective term for different kinds of components that lowers the melting temperature of silica and makes the glass mixture easyer to fuse. Examples fluxes are for example Soda and Natron; they were historically derived from Egyptian natron, sodium plant ash and potash plant ash. The flux lowers the melting temperature; however, it makes the glass more susceptible to deterioration from humidity and carbon dioxide. The last group, the stabilizers, makes glass more water resistant. Examples of this are alumina and alkaline-earth oxides, mostly introduced into the glass from impurities in the sand, such as feldspar and clay, or by non-purified ashes. (Janssens 2013:28ff) The last few decades, the advancements of scientific methods for the analysis of glass have moved forward tremendously.
Glass has been made from different recipes in different times and places. The recipe needs to change depending on the material available and the different kind of impurities those materials carry. This makes the chemical composition into a fingerprint of the glass (Janssens 2013:312), however, in the period i work with, the reuse and recycling of glass is not only commonplace, it is norm. Glass is imported as material and used in secondary glass working. (Callmer & Henderson 1991:144, Lundström 1976) This makes provenance by chemical composition delicate and limited. (Janssens 2013:312) and should not be considered the only possible future for the provenance of beads. To produce a strongly colored glass of a specific color, glass-makers had to add a colorant to the batch, For instance copper (turquoise, green), cobalt (blue) or manganese (purple and violet when associated with Cobalt) oxides as natural minerals or synthetic products.