APPLICATION OF GEOLOGICAL METHODS TO UNDERSTANDING ARCHAEOLOGICAL HIGH ALKALI GLASSES MANUFACTURED DURING THE 9TH C. BC

Analysis of ancient manufactured glass from northwestern Iran, dated to the 9th century BC, is being carried out in order to investigate what happened to glassmaking technology during what has often been called a "dark age". Electron microprobe with WDS is being used to characterize the glasses both chemically and mineralogically. Analyses of unaltered areas of these glasses show that they are soda-lime-silica in composition, containing about 17-21 wt% Na₂O and 2-8 wt% CaO. Most glasses also contain greater than 1-6 wt% MgO and 1-4 wt% K₂O, with a few containing less than 1 wt% each of MgO and K₂O. These high alkali glasses are easily damaged by an electron beam under standard analytical conditions used on geologic materials. This problem has been overcome by using 15 kV voltage, a beam current of 2.0 nA with a stationary beam of 20 um diameter, and calibration and analysis count times of 20 seconds for each element. Natural and synthetic minerals and manufactured high alkali glasses have been used as primary standards. Analytical accuracy and precision are good have been checked by repeat analyses of glass standards that are replicates of ancient manufactured glasses.

The imaging capabilities of the electron microprobe are necessary to identify and avoid the hydrated or otherwise altered areas commonly found in ancient manufactured glasses. A typical change in major element composition is almost total loss of sodium from a glass and an apparent corresponding rise in SiO₂ content. In addition, imaging has been used to identify partially-reacted batch materials, colorants, and phases that have crystallized from the melt. Remnant batch materials include immiscible alkali sulfates, plagioclase, alkali feldspar, chromium spinel, quartz, and possible nepheline. Dark brown or black glasses contain colorants of polymetallic sulfides of different combinations of lead, copper, antimony, and iron; opaque yellow glasses contain the yellow colorant lead antimonate; opaque white glasses contain the colorant calcium antimonate. These features, along with the chemical compositions and 8th century BC cuneiform texts, are used to infer the original raw materials and the manufacturing technology of glass of this "dark age" period.