Chemistry and Technology of Prehistoric Glass from Lower Saxony and Hesse

Abstract The investigated collection of glass beads and bracelets represents a cross-section of the most important glass types found for the Bronze Age and Iron Age in Central Germany. Among the artefacts are examples of different chemical compositions, colours and varying degrees of opacity. The artefacts were dated from the 14th to the 1st century bc , directly by their archaeological context, or indirectly through findings of contemporary representatives of similar fragments. The glass fragments were found in burial, settlement and oppida-like contexts. In order to highlight some of the principle characteristics of ancient glass manufacture, raw materials and recipes used, the major, minor and trace elements of the glass were determined by electron microprobe analysis, inductively coupled plasma-atomic emission spectroscopy and ICP-mass spectrometry. According to our investigations, the silica content in ancient glasses ranges from 67 to about 72%. The predominant alkali is Na, with a concentration between 14 and 17 wt.% Na 2 O. CaO ranges from 6 to 8%. The chemical analysis of Bronze Age glasses from Central Germany has indicated a chemical change of this soda-lime composition. Until c . 800  bc plant-ash glass with high Mg soda-lime composition was in use, which was manufactured from plant ash, like Salicornia herbacera and calcereous quartz sand. On account of the plant ash used, relatively high and correlating values of MgO (3·8 wt.%), K 2 O (1·5 wt.%) and P 2 O 5 (0·3 wt.%) were determined in this glass with soda-lime composition. In contrast, the natron glass with low-Mg soda-lime composition of Late Bronze Age and Iron Age from about 800  bc onwards was assumingly manufactured with the mineral raw material natron/trona and quartz sand. Due to this raw material used, relatively low MgO (≤0·5 wt.%), K 2 O (0·9 wt.%) and P 2 O 5 (0·06 wt.%) concentrations were determined. The fact that these Bronze and Iron Age glasses found in Central Germany were manufactured in the circum-Mediterranean area (Near East, Egypt, Syria or Mesopotamia) or even, but less probably, with the use of raw materials which were imported from there, is suggested by their relatively high Cl concentrations, which range from 0·9 to 1·3%. The Cl was probably introduced into the melting process by the Cl-containing natron/trona mixture or the Na-dominated plant ashes. In addition to this circum-Mediterranean plant-ash and natron glass, fragments of Bronze Age glass were found with a totally different chemical composition, an indication for an independent glass manufacture in Europe from the 12th century bc onwards. The artefacts of this glass type contain only traces of Cl. The low P 2 O 5 , MgO and relatively high K and Na contents demonstrate that these glasses were produced using processed plant ashes (like potash) and albite-rich quartz sands.