While analysis of slag material is essential for reconstructing ancient smelting technologies, the interpretation of the analytical data might be challenging. In this paper we address a particular question, namely the interpretation of calcium (Ca) content in copper smelting slag, based on ICP-OES/MS analysis of multiple slag samples from the copper ore district of Timna Valley (southern Israel). The new data indicate a decrease in the average calcium content in slag between the Late Bronze Age (~12th century BCE, 2.38±1.65 wt.% [n=27]) and the Iron Age (~10th century BCE, 1.17±0.91 wt.% [n=15]). Based on several lines of argument we suggest that this difference represents primarily an improvement in fuel efficiency (i.e., less fuel per smelting cycle while similar quantity of copper was produced), rather than a change in the smelting charge (flux, ore and charcoal) or furnace construction materials. The new data demonstrate that a deliberate search for increasing fuel efficiency was a main trigger behind technological progress in the hyper arid zone of the southern Levantine deserts.
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