Electric phosphate smelting furnace in a TVA chemical plant (1942)

Chemical reduction, or smelting, is a form of extractive metallurgy. The main use of smelting is to produce a metal from its ore. This includes iron extraction (for the production of steel) from iron ore, and copper extraction and other base metals from their ores. It makes use of a chemical reducing agent, commonly a fuel that is a source of carbon such as coke, or in earlier times charcoal, to change the oxidation state of the metal ore. The carbon or carbon monoxide derived from it removes oxygen from the ore to leave the metal. The carbon is oxidised, producing carbon dioxide and carbon monoxide. As most ores are impure, it is often necessary to use flux, such as limestone to remove the accompanying rock gangue as slag (also called scoria or cinder).

The 7 metals that were known in ancient times (mercury, tin, lead, copper, silver, gold, iron) can in principle be smelted through similar chemical reactions from their ores:

Mercury Oxide

Cassiterite

Minium

Silver oxide

Cuprite

Hematite

Different ores require different reactions at different temperatures, but almost always the reducing agent is carbon. The list above is sorted in increasing temperature order, so iron is the most difficult metal to smelt from the ones in the list (that's why historically iron smelting was the last to be discovered).

A common mistake is to think that the metal is obtained from the ore because at high temperature the metal just melts out of the ore. That is incorrect: if a blacksmith just heats up the ore without the proper reducing agent (carbon), he will just obtain molten ore. Also, one can smelt some ores at a temperature lower than the temperature required to melt the metal. Usually, though, these reactions happen at temperatures high enough to melt the resulting metal, so the metal can just be cast directly out of the furnace.

The exception to the previous paragraph is that some metal oxides just decompose at relatively low temperatures, so instead of trying to smelt mercury out of mercury oxide, one can just heat up mercury oxide to about 500C, and the oxide will decompose into mercury and oxygen; as mercury boils at 357C, this will cause the oxide to decompose and boil out, producing the highly toxic gaseous mercury. This is possible only for mercury and a handful of other metal oxides; most metal oxides must be smelt with carbon as the reducing agent.

 Copper smelting: kilns

There were in the past some arguments that copper was first smelt by accident also in campfires, but that seems improbable as campfires are about 200°C short of the temperature needed to smelt copper. A more probable path may have been through pottery kilns, invented in Iran by 6000BC. Pottery kilns produce ceramics that can be glazed with colorful earths (mostly metalic oxides) to produce colorful vases; it happens that malachite (copper oxide) is a colorful green stone, so a potter that encrusts malachite in a ceramic vase in a coal-fired kiln will produce a few droplets of metallic copper (ruining his vase). That may have set the way to smelt copper.

The first known cast copper artifact is a mace head found in Can Hasan from 5000BC.

Copper created some impact on the ancient world, as it produces good blunt weapons and reasonable armor, but it is still too soft to produce useful blade weapons. Therefore, the smelting of copper did not replace the manufacture of stone weapons, which still produced superior blades. Bronze smelting

Bronze is a copper/arsenic or copper/tin alloy. The presence of arsenic and tin dramatically increased the hardness of copper, producing war-winning weapons and armor. A noble wearing bronze armor was basically impervious to the stone tools of the times, and his bronze sword kept its edge and shattered the older stone-based weapons. The knowledge of the smelting of copper allowed kings to overcome their enemies, and caused such a revolution that it marked the end of the Stone Age and the beginning of the Bronze Age. It would be millennia, though, until bronze could be used by common soldiers and townsfolk, and for a long time they were luxury items used by nobility.

The first copper/arsenic bronzes date of 4200BC from Asia Minor, and were used for a long time until replaced by the modern copper/tin bronzes by 1500BC. It is unclear that if at some point in time the smiths that produced copper/arsenic bronze added arsenic oxides on purpose, or if they explored some copper lodes that happened to have arsenic as a lucky contamination.

The first copper/tin bronzes date of 3200BC, again from Asia Minor. Copper/tin bronzes are harder and more durable than copper/arsenic ones, and made these obsolete. The process through which the smiths learned to produce copper/tin bronzes is once again a mystery. The first such bronzes were probably a lucky accident from tin contamination of copper ores, but by 2000BC we know that tin was being mined on purpose for the production of bronze. This is amazing, given that tin is a semi-rare metal, and even a rich cassiterite ore only has 5% tin. Also, cassiterite looks like any common rock, and it takes special skills (or special instruments) to find it and locate the richer lodes. But, whatever steps were taken to learn about tin, these were fully understood by 2000BC.

REF : Wikipedia

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