The roots of glazing techniques : copper metallurgy ?

Abstract

This paper reports on finds of greenish, partially bluish and red glazed furnace fragments from 3rd millennium ВС copper smelting sites at Feinan and at Timna, Wadi Arabah which, due to their geographic location, could have been of significance for the production of glazes in Egypt (" Egyptian faience" '). The weathered glazes consist of Ca-Al- siliceous glass originally high in alkalis, the latter extracted by leaching over the millennia. The presently observed green color is due to minor concentrations of divalent copper; the color is considered to have been blue originally, with changes to green due to weathering processes. Glaze formation is the result of furnace-constructions using locally available calcareous sandstone, which during smelting reacted with potassium and additional calcium from charcoal ash and with copper from the ore. Comparable glazes have not yet been observed elsewhere at prehistoric smelting sites, probably due to inadequate composed raw materials. Experimental work was carried out to replicate the formation of glazes during copper smelting in order to prove technological relationships to glaze-manufacturing processes. The experimental results confirmed the archaeological evidence. Decisive points in common between glaze formation in copper smelting furnaces and " Egyptian faience" are a quartz-rich matrix and its chemical reaction with copper and alkali-salts at high temperatures. Major differences are high potassium concentrations in our samples while sodium was found to be a major constituent in "Egyptian faience ". Chronological problems are not yet sufficiently solved, since the earliest glazed beads predate the archaeometallurgical evidence discussed in this paper.