Potassium-Calcium Glass: New Data and Experiments

• Published in: Archaeometry Vol. 46; no. 1; pp. 137 - 156
• Main Authors: Stern, W. B., Gerber, Y.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01.02.2004; Wiley-Blackwell
• Subjects: CHEMICAL COMPOSITION; COLOUR; Excavation and methods; GLASS; Laboratory methods; MELTING TEMPERATURE; Methodology and general studies; MINERALOGICAL COMPOSITION; Physical and chemical analysis; POTASH; POTASSIUM-CALCIUM GLASS; Prehistory and protohistory; WOOD ASH; Composition; Temperature; Chemical analysis; Sand; Wood; Mineralogy; Glass; Experimentation; Colouring; Raw materials; Laboratory; Quartz
• ISSN: 0003-813X; 1475-4754
• DOI: 10.1111/j.1475-4754.2004.00149.x

The chemical composition of potassium–calcium ‘wood‐ash’ glass reflects the elemental pattern of the involved non‐volatile base materials in quartz sand, wood ash and possibly potash. The essential elemental ratio K2O/CaO of wood ash varies between 0.2 and 0.8, and depends on the habitat and geological substratum of the wood rather than on the tree species; ratios between 1.0 and 3.0 in wood‐ash glass are only possible when potash is added as a third base material. Melting temperatures of wood‐ash glass sensu stricto, termed K–Ca‐2, produced with the two raw materials quartz sand and wood ash, are between 1250°C and 1400°C, while those of three‐component‐glasses, termed K–Ca‐3, are between 900°C and 1250°C, according to the amount of added potash. Experimentally produced glass displays different hues, from colourless to brown, olive‐green and pink, according to the chemical composition of the wood ash. Elevated MnO concentrations between 0.5 and 3 wt% may originate from wood ash and are hence not necessarily an indicator of colour‐inhibiting additives. Phosphate stemming from wood ash is an essential discriminator between wood‐ash glass and potash–lime glass. Because wood ash contains only minor amounts of sodium, wood‐ash glass with equal concentrations of potassium and sodium is a hybrid glass type, where besides quartz sand, wood ash, possibly potash and also soda‐rich cullet have been applied for glass production.