How it began
In the first half of the sixth century BCE a refinery worked beside the Pactolus stream at Sardis, the Lydian capital in what is now western Turkey. It stood outside the city walls in a suburban quarter that archaeologists call Pactolus North. A. Ramage of Cornell University supervised its excavation in 1968 and 1969. The furnaces, the fragments of vessels, some still holding drops of gold, and the scraps of gold sheet found there are the earliest known evidence for the cementation process anywhere.
Before it existed
The gold of Lydia came out of the ground as electrum, a natural alloy with silver, and the first coins were struck from it as found. Its gold content varied from one deposit to the next and could be lowered by adding silver, so a piece of electrum of known weight was still of uncertain value. There was no dependable way to issue metal of a consistent purity while gold and silver stayed locked together.
How it works
Larger pieces of electrum were hammered into thin sheets to give the salt a broad surface to attack. The sheets and grains of river gold were packed between layers of salt, perhaps mixed with clay or brick dust, in coarse pots like ordinary Lydian cooking pots. The pots sat in small brick ovens at about 800 degrees Celsius for many hours, perhaps days. Salt vapor turned the silver into silver chloride, which soaked into the pot and the furnace bricks and left the gold nearly pure. The silver was won back by smelting that debris with lead and then heating the alloy to about 1100 degrees in a bowl hearth under a blast of air.
What changed first
Coins issued separately in pure gold and pure silver appeared shortly before the middle of the sixth century BCE, under King Croesus. The excavators tie the refinery directly to that change: coinage accepted as a medium of exchange needed metal of consistent purity. The same installation may also have been used to break down electrum coins and other objects, now obsolete, into their gold and silver.
What lasted
Sardis shows the full chain of an ancient precious metal works: parting by cementation, recovery of silver by smelting and cupellation, and testing of the gold for purity before it left the site. The excavation and the scientific study carried out with the British Museum were published as the book King Croesus' Gold, which sets the Lydian process alongside ancient, medieval and modern methods of refining gold. The site is described there as the only known ancient refinery.
