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The mysterious "slag"

Researchers from the Dhamurian Society have been delving into the origins of the mysterious "smelting slag"  found at the Gympie Pyramid site. They have concluded that the material was created by an ancient furnacing method called a "bloomery".

A bloomery consists of a pit or chimney with heat-resistant walls made of earth, clay, or stone. Near the bottom, one or more clay pipes enter through the side walls. These pipes, called tuyères, allow air to enter the furnace,
either by natural draft, or forced with a bellows.
In operation, the bloomery is preheated by burning charcoal, and once hot, iron ore and additional charcoal are introduced through the top, in a roughly one to one ratio. Inside the furnace, carbon monoxide from the
incomplete combustion of the charcoal reduces the iron oxides in the ore to metallic iron, without melting the ore; this allows the bloomery to operate at lower temperatures than the melting temperature of the ore.

The small particles of iron produced in this way fall to the bottom of the furnace and become welded together to form a spongy mass of the bloom. The bottom of the furnace also fills with molten slag, often consisting of fayalite, a compound of silicon, oxygen and iron mixed with other impurities from the ore. Because the bloom is highly porous, mixed with bits of slag, partially reduced ore, unburnt fuel and parts of furnace clay, the bloom must later be reheated and beaten with a hammer to drive the molten slag out of it. This results in "wrought" iron.


moundbloomery.jpg (41217 bytes)


Bloomery smelting was a labor-intensive, small-scale process that required a great deal of knowledge and skill.
Since the beginning of the Iron Age, until the advent of the blast furnace in the middle ages, groups such as the Renaissance Europeans, Africans and ancient Romans have used this method to produce iron weapons, tools and
armor.

The pieces from the Gympie Pyramid site match the description of a "bloom" perfectly:  spongy mass of iron, glassy slag, unburnt charcoal and sandy debris.


charcoalslag.jpg (60228 bytes)

 

Limestone was also used as a "flux" in a bloomery to aid in the removal of impurities. Coincidentally, limestone is readily available nearby at Tamaree.

Bloomeries were used up until medieval times when they were replaced by the blast furnace, able to process larger amounts of material, and pour the iron into casts, resulting in "cast iron".

So we have evidence of ancient iron smelting on the Gympie Pyramid site, but the questions remain: Who, When and Why?

© Copyright 2008 The Dhamurian Society