Smelting

High temperatures are required to render metals into a viscous state. A constant airflow helps raise the temperature of a fire. As such, smelting facilities were often located in areas exposed to wind. Ancients also employed bellows to manually pump air into a furnace to raise the temperature. Archaeologists have discovered evidence for ancient bellows in Egypt dating as early as 1580 bc (Healy, Mining and Metallurgy, 193). Industrial bellows were constructed out of a pair of pipettes leading to ceramic pots lined with a leather bag. A smelter would lift and press the leather bags in an alternating motion to provide constant airflow to a fire. Archaeologists have shown that by using clay-tipped pipes inserted through the ground at the fire basin, several people could provide consistent airflow to smelt ore for copper sulphides (Levy, 1–110).

The amount of fuel and time required to smelt metal depends on the properties of the metal and the conditions in which it is treated. Soft metals such as copper, gold, and lead have very low melting points. Iron has the highest melting point, and ancient foundries could not reach a high enough temperature to smelt it. Instead, ancients would heat iron to expunge impurities and other minerals, often in a bloomery. This method left behind an amorphous bun that could be worked by hammer and heating. This became the common method during the Roman period (Healy, Mining and Metallurgy, 177–79).

Different types of furnaces could be used to smelt metals. At the small scale, a simple stone vessel called a crucible sufficed. Full-sized ovens were required for smelting larger quantities of metal and producing ingots.

• Crucibles or bowl furnaces were used for small-scale production. They were open-air fire pits that allowed for a vessel to be inserted directly into the flames.

• Round furnaces were composed of clay and commonly used for iron-ore production. Multiple layers of charcoal and iron ore were sealed within the bloomery of these furnaces. As the impurities separated, the iron pooled and coagulated at the bottom of the bloomery. Ancients regularly scraped and removed impurities and slag as the iron ingot formed. The final product was a spongy and porous ingot of iron.

• The shaft furnace functioned on the same concepts of the bloomery but had a chimney shaft structure as opposed to a dome. This allowed for a higher temperature to be reached. Shaft furnaces remain from as early as 500 bc (Healy, Mining and Metallurgy, 184–85).

Archaeologists can identify ancient foundries for smelting based on the remains of crucible fragments, slag, and the presence of minerals, ore, or processed metal in the area (Beit-Arieh, “Southern Sinai,” 305–6). The scale of production in metallurgy is evident in the amount of slag left behind.