Gold parting

Silver can be dissolved selectively by boiling the mixture with 30% nitric acid, a process sometimes called inquartation.

Affination is a largely obsolete process of removing silver from gold using concentrated sulfuric acid.

The main ancient process of gold parting was by salt cementation, of which there is archaeological evidence from the 6th century BC in Sardis, Lydia.

[7] Literary sources and the lack of physical evidence suggest that gold-silver parting was not practised before the mid first millennium BC.

The first possible literary reference to the salt cementation parting process is in the Arthashastra, a 4th-century BC treatise from India, that mentions heating of gold with Indus earth.

[8] An experiment recreating the process as described by Diodorus Siculus by heating a mixture of the gold and salt in a sealed pot for 5 days was done by Notton and was found to be successful.

[9] Parting vessels used for refining gold with the cementation process have been found in London, Lincoln, York and Winchester.

[10] Gold parting had been well used throughout the ancient times but only in the Medieval period were clear and detailed descriptions of the processes written.

All the archaeological finds of Roman and early medieval parting point to a solid state process using common salt as the active ingredient.

[11] The only large group of medieval parting vessels so far discovered were found at Coppergate and Piccadilly sites in York.

[12] The pinkish-purple discolouration of the vessels showed them to have been used with the salt cementation process which removes iron from the clay as ferric chloride.

[13] Theophilus was a 12th-century German monk and in his book De Diversus Artibus[14] gives the clearest description of the salt cementation process.

Weigh it, when dried, and see how much has been lost, then fold it up and keep it.It was during the medieval period that distillation was discovered and the first description of nitric acid production was given by Pseudo-Geber in the Summa perfectionis, 1330.

Salt cementation continued to be the main method of parting until the 16th century but in later Middle Ages processes using sulfur, antimony and mineral acids began to be used.

In 1860s Australia the Miller process was developed, this removed silver by bubbling chlorine gas through the molten gold mixture.

It is a solid state process relying on common salt as the active ingredient but it is possible to use a mixture of saltpetre (KNO3) and green vitriol (FeSO4).

The basic process involved the mixing of argentiferous gold foil (in later periods granules were used), common salt and brick dust or burnt clay in a closed and sealed container.

Notton in experiments found that with one heating the gold content could be taken from 37.5% to 93%[22] This is similar to the salt cementation process but creates sulfides instead of chlorides.

Finely divided impure gold and elemental sulfur are reacted together under moderate heat in a sealed crucible.

This process is also well suited to recycling consumers' used or broken jewelry directly back onto the global market 24kt inventory.