Refining is the separation of a metal from its impurities. As such it is applied to a wide range of different processes for the different metals.
Copper
The copper that was produced by the last stage of the smelting process was, in general, rather impure (black copper with copper content as low as 90%). This had a high iron content, which would have to be reduced by two stage refining process if the copper was to be useable for most purposes. This was done by first an oxidizing process to convert the metallic iron in the metal to iron oxide (usually magnetite), and then a reducing process (poling) to remove the excessive amount of oxygen introduced by the iron removal stage.
Refining black copper
The magnetite would float on top of the metal where it could either be slagged by the addition of silica and tapped off, or the pasty magnetite could be physically scrapped off the top of the liquid metal to form a high magnetite smithing hearth bottom type slag. Unfortunately, the tapped refining slag would be difficult to distinguish from other of copper smelting slags, or even iron smelting slag – however the presence of a significant amount of partially digested crushed quartz fragments is often an indicator of a copper smelting or refining slag.
The high magnetite smithing hearth bottoms, or skulls, have been recorded rarely from archaeological contexts. This is almost certainly because the number of sites on which refining occurs would be limited as –

• Either the slag would be produced on the original smelting site, where the small amount of such slag produced would be swamped by the much large amount of ‘normal’ smelting slag. Thus not easily recovered and recorded.
• Or the refining occurred away from the smelting site, that is, the copper was traded as black copper ingots. In this case, the slag might be mistaken as a slightly unusual iron smithing slag.
Poling
This was the final stage of the refining process designed to reduce the oxygen content of copper to a reasonable level. It was carried out by plunging a pole of wood into a bath of molten copper this produces hydrogen and other reducing gases by the distillation of the wood. These in turn reduce any copper oxide present.
Alloying copper with tin or zinc would also deoxidise the metal but at the expense of loss of expensive alloying element to slag or vapour. It is, therefore, more economical to pole the metal before alloying.
Silver
The main method of separating silver from the majority of its impurities was by cupellation of the impure metal with lead. However, cupellation would not separate gold from silver.
Gold
Cupellation
Cupellation was used to separate gold from those alloys containing copper but as silver is not oxidized during the cupellation process, it would not separate gold from silver. Thus, if high purity gold was required for jewellery, or coinage the metal would have to be further refined by parting.
Mercury amalgam refining
In the classical period mercury amalgamation refining was used to recover and refine gold from worn out gold embroideries and gold leaf gilding. Gold dissolves in the mercury to form a pasty amalgam. The majority of the mercury was remove by squeezing the mixture chamois leather leaving the amalgam behind. The gold was then recovered by heating amalgam so that the remaining mercury was lost by evaporation.
The same technique was later used to recover finely divided gold from crushed ore.
Parting
To get rid of silver from gold it was necessary to use a parting technique. The earliest of the heated the impure gold with salt in combination with other chemicals to convert the silver to silver chloride which diffused out of the metal. This method appears to be first used to produce the gold for the Lydian coinage produced at Sardis in the 6th century BC (Rampage and Craddock 2000).
Later other parting methods were used to treat impure gold alloys; these included acid, and the antimony or sulphide parting.
Related term: Parting
Iron
Bloomery iron and steel did not undergo a specific refining process other than by forging and possibly folding and welding to break up the slag inclusions and homogenize the metal.
However, as the major use of iron was in the malleable form, the majority of cast iron was (re)fined to wrought iron by oxidizing carbon, and when present the silicon, out of the metal. Initially in Britain, this was carried out in the finery forge using the Walloon process where both a finery and chafery hearth were used. After Henry Cort’s use of the reverberatory furnace to convert the iron and grooved rolls to convert the resultant bloom to bar, the puddling process generally replaced the finery forge.