Gold parting is the separating of gold from silver (and other metallic impurities). Gold and silver are often extracted from the same ores and are chemically similar. Their separation therefore is the usual focus of parting the alloy of gold and silver, called electrum.
Contemporary technology Four technologies are dominant. The first two start with relatively pure gold, while the third start with alloys even when there is high silver content. The Miller process affords gold up to 99.5% purity. The process involves blowing a stream of chlorine through molten gold. Impurities in the gold form chlorides, which form a slag that floats on the molten gold. Invented by Emil Wohlwill in 1874, the Wohlwill process produces the highest purity gold (99.999%). It is an electrolytic process using pure gold for the cathode (or titanium as a starter cathode) and chloroauric acid (gold chloride-hydrochloric acid) as the electrolyte; this is made by dissolving gold with chlorine gas in the presence of hydrochloric acid. Gold is dissolved at the anode, and pure gold, traveling through the acid by ion transfer, is plated onto the cathode. Silver forms an insoluble chloride slime and copper and platinum form soluble chlorides that are removed. This procedure is used on a very large industrial scale and has a large set up cost due to the amount of gold that needs to be permanently dissolved in the electrolyte.
The Aqua Regia process has been used since ancient times. It works by dissolving the sample in Aqua Regia. Silver precipitates as chloride while other impurities stay in solution. After filtration a reducing agent is added which selectively precipitates gold as metal, with 99.99% purity achievable. It is fast, cost-effective and preferable for small operations.
The Acidless Separation (ALS), developed in 2014, has been designed to pre-refine doré and jewelry alloys even when there is high silver content, which is normally a problem for existing chemical pre-refinement processes.Boscato, M. (2016). "Presentazione di un nuovo processo di raffinazione privo di acidi per oro e argento". Jewel Technology Forum . The metals are separated by vacuum distillation.Gaskell, D. R."Introduction to the Thermodynamics of Materials"; 5ª edizione, Taylor & Francis, 2008.Ohring, M. "Materials Science of Thin Films" 2ª edizione, Academic Press, 2001.Kubaschewski, O., Alcock, C. B. Metallurgical Thermochemistry. Pergamon Press, 1979.Pfann, W. G. (1958). Zone Melting. Wiley It is currently adopted by the most important international refineries.Srivastava, Ankur, Head-Refinery & Ecology, MMTC PAMP Pvt Ltd. September 11, 2025. Challenges in gold and silver refining. India Gold Conference (IGC) 2025. https://www.goldconference.in/programme.html
Other Alternative methods exist for parting gold. Silver can be dissolved selectively by boiling the mixture with 30% nitric acid. Affination is a largely obsolete process of removing silver from gold using concentrated sulfuric acid.
Cupellation removes gold and silver from mixtures containing lead and other metals, but silver cannot be separated from gold by this process alone.
History Gold parting as a process was invented specifically to remove silver. The advent of coinage required methods to remove impurities from the gold. Over the centuries special means of separation have been invented.
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. In the post-medieval period parting using antimony, sulfates and mineral acids was also used.
Early history The very earliest attempts at refining gold can be shown by the surface enhancement of gold rings. Gold quality was increased at the surface by 80–95% gold compared to 64–75% gold at the interior found in Nahal Qanah Cave dated to the 4th millennium BC. Further evidence is from three gold chisels from the 3rd Millennium BC royal cemetery at Ur that had a surface of high gold (83%), low silver (9%) and copper (8%) compared with an interior of 45% gold, 10% silver and 45% copper. The surface was compacted and heavily burnished and indicates early use of depletion gilding.
Ancient and medieval world
Separation of gold from silver was not practised in antiquity prior to the Lydian Period (12th century BC to 546 BC).Craddock, P. T. 2000a Historical Survey of Gold Refining: 1 Surface Treatments and Refining Worldwide, and in Europe Prior to AD 1500. In A. Ramage and P. T Craddock (eds) King Croesus' Gold; Excavations at Sardis and the History of Gold Refining. London: British Museum Press pp27 Material from Sardis (in modern Turkey) is evidence of the earliest use of gold and silver parting in the 6th century.Rehren, T. 2003. Crucibles as Reaction Vessels in Ancient Metallurgy. In Craddock, P. T and Lang, J. (eds.) Mining and Metal Production through the Ages. London: British Museum Press pp207 Literary sources and the lack of physical evidence suggest that gold-silver parting was not practised before the mid first millennium BC. Gold parting came with the invention of coinage and there is no evidence for the use of a true refining processes before the introduction of coinage. As refining gold (as opposed to surface enhancement) results in a noticeable loss in material, there would have been little reason to do this before the advent of coinage and the need to have a standard grade of material.
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. Indus earth is taken to mean soils high in salt, nitre and ammonium salts and therefore ideal for the cementation parting process. A better known and more detailed early description is given by Diodorus Siculus in the 1st century BC quoting an earlier lost book, On the Erythraean Sea from the 2nd century BC by Agatharchides of Cnidus. 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.
Pliny in his Naturalis Historia mentions the purification of gold a number of times and makes reference to the salt cementation process of gold parting. He says that the gold is "roasted with a double weight of salt and three times the weight of misy (ferric sulfates) and again with two portions of salt and one of the stone which is called schiston." Here he is describing the heating of gold with salt and iron sulfates which act to dissolve the copper and silver in the gold.Craddock, P. T. 2000a Historical Survey of Gold Refining: 1 Surface Treatments and Refining Worldwide, and in Europe Prior to AD 1500. In A. Ramage and P. T Craddock (eds) King Croesus' Gold; Excavations at Sardis and the History of Gold Refining. London: British Museum Press. Pp38 Parting vessels used for refining gold with the cementation process have been found in London, Lincoln, York and Winchester. The London vessels, dating from the Flavian period (c. 70–85 AD), were sealed using luting clay; XRF analysis detected gold and silver, with highest concentration around the sealed region showing possible escape of silver as the volatile silver chloride.Bayley, J. 1990. Archaeological Evidence for Parting. In E. Pernicka and G. A. Wagner (eds) Archaeometry '90. Basel; Boston : Birkhäuser Verlag
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.Bayley, J. 2008. Medieval precious metal refining: archaeology and contemporary texts compared. In Martinon-Torres, M. and Rehren, T. (eds) Archaeology, history and science: integrating approaches to ancient materials. Walnut Creek: Left Coast Press. Pp142-3 The only large group of medieval parting vessels so far discovered were found at Coppergate and Piccadilly sites in York.Bayley, J. 1992. Non-Ferrous Metalworking at 16–22 Coppergate. The Archaeology of York 17/7. London:CBA 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. Other vessel fragments are known from Carlisle and Winchester.Bayley, J. 2008. Medieval precious metal refining: archaeology and contemporary texts compared. In Martinon-Torres, M. and Rehren, T. (eds) Archaeology, history and science: integrating approaches to ancient materials. Walnut Creek: Left Coast Press pp143 Theophilus was a 12th-century German monk and in his book De Diversus Artibus gives the clearest description of the salt cementation process.
This mixture is then added to an earthenware pot and layered with thin sheets of gold foil. The pot is then sealed and heated in a furnace.
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. Nitric acid is able to dissolve silver. The addition of sal ammoniac to nitric acid creates aqua regia and this acid is able to dissolve gold. Both acids are used in the acid method of parting but the acids were expensive so were not used until the post medieval period.Taylor, F. S. 1956. Pre-scientific Industrial Chemistry. In C. Singer, E.J. Holmyard, A.R. Hall and T. I. Williams (eds) A history of technology: Vol.2, The Mediterranean civilizations and the Middle Ages; c700 B.C. to c, A.D. 1500. Oxford : Clarendon Press pp356-7
Post medieval to modern period