PALEORIENT Vol. 3 1975-1976-1977
SOME EXPERIMENTS
IN PREHISTORIC COPPER SMELTING
W. ROSTOKER
The large scale use of copper has been regarded as a landmark in the development of human societies in different places and at different times. There is general agreement (1-7) that "native" copper was probably the first resource. Metallic (native) copper occurs in many areas of the world as surface or shallow deposits of small pieces dispersed in weathered rocks. With simple abrasion or polishing the characteristic color is visible. Native copper possesses the deformability (malleability) and resistance to fracture characteristic of metals. However the pieces are usually small, allowing ornaments to be fabricated but not tools, weapons or utensils. For these it was necessary to aggregate the pieces in some way.
The most direct approach to aggregation is melting. Small particles which fuse to droplets will run together to form a pool. The pool may be allowed to cool and solidify to a large lump or if properly contained, may be poured into a mold which permits replication to the shape of a preformed cavity in the mold, i.e. a casting.
Melting and casting requires the development of more intense heat and containment than is provided by an open or pit fire. However, the forced draft pottery kiln and fired clay crucibles have histories at least as long as copper metallurgy so it is reasonable to view them as parallel developments.
The discovery of processes for smelting minerals to metallic copper may or may not have preceded the development of melting and casting technology. There is no clear evidence either way. However, the effective recovery of metallic copper from mineraldoes require the achievement of temperatures in a confined zone in excess of the melting point of copper (1083°C).
We may assume that the simplest technology of smelting was the first to be discovered. This would be the charcoal reduction of oxidic minerals of copper. Three minerals, malachite, azurite and chrysocolla,
are reducible by carbon (charcoal). These minerals are scattered throughout the world in significantly large deposits at and near the surface. They have very brilliant and characteristic colors so that hand sorting pieces from roughly mined ores would be relatively simple.
However, in most parts of the world as the demand for copper and bronze grew the deposits of oxidic ores would have become exhausted. The bulk of the world's copper exists in the mineral form called chalcopyrite. This is a combined copper and iron sulfide. Producing metallic copper from chalcopyrite involves surmounting two technical problems. Sulfide compounds are not directly reducible by charcoal so that attempts to utilize prior charcoal smelting technology would have proven fruitless. The product, if fused, would appear stone-like and certainly not metallic. If the sulfide mineral were converted to a charcoal-reducible product, the reduction of copper and iron would occur simultaneously yielding a product which was not fusible nor deformable, being hopelessly intermingled with fused and unfused glassy material.
To solve the two problems of reducibility and the separation of the iron requires a complicated process even by today's standards. There is no real evidence how this was accomplished even as recently as 1,000 years ago. The first technically meaningful descriptions of copper smelting of sulfide ores occur in the writings of the 15th and 16th centuries A.D. (8-10). What is described are somewhat modified versions of what is called "matte smelting" today.
Matte smelting is a multi-stage pyrometallurgical process from which metal does not emerge until the last step. The first stage is called "roasting" in modern parlance. The mineral separated as much is possible by crushing the ore and hand picking (flotation separation today) would have been heated in air. The sulfide mineral at quite modest temperatures (as low as 400° C)
311
RetroSearch is an open source project built by @garambo | Open a GitHub Issue
Search and Browse the WWW like it's 1997 | Search results from DuckDuckGo
HTML:
3.2
| Encoding:
UTF-8
| Version:
0.7.4