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). Material from
Sardis (in modern
Turkey) is evidence of the earliest use of gold and silver parting in the 6th century. 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. 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. 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. The only large group of medieval parting vessels so far discovered were found at
Coppergate and
Piccadilly sites in York. 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.
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.
Post medieval to modern period '' Comprehensive accounts of the salt cementation processes is given by
Biringuccio in his
The Method of cementing gold and of Bringing it to its Ultimate Fineness.; in the
Probierbuchlein – Little Books on Assaying; by
Georgius Agricola in book 10 of
De Re Metallica; and by Ercker in his
Treatise on ores and assaying. This was a period where new techniques began to be explored. Granulation of the gold instead of gold foil increased surface area and therefore the efficiency of the reaction. 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. There are archaeological finds at sites in London of distillation vessels for making acids in Britain from the 15th century which include fragments of ceramic cucurbits (vessels for heating reacting chemicals) which were used with alembics for distillation. By the 18th century cementation was rarely used and had been replaced by acid treatment. Into modern times the acid parting method continued to be used but other methods were discovered. In 1860s Australia the
Miller process was developed, this removed silver by bubbling
chlorine gas through the molten gold mixture. Soon after, in the 1870s,
electrolytic refining of gold was developed, the
Wohlwill process, to deal with the problem of removing
platinum from the gold. This technique is the most commonly used today for processing alloys start with relatively pure gold.
Historic processes Salt cementation This process was used from
Lydian to post-medieval times. 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. Theophilus mentions the addition of urine to the mix. With heating, the silver reacts with the salt to form silver chloride which is removed leaving a purified gold behind. Conditions needed for this process are below 1000 °C as the gold should not melt. Silver can be recovered by smelting the debris. Heating can take 24 hours. Hoover and Hoover explains the process thus: under heating salt (sodium chloride, NaCl) decomposes in the presence of silica and alumina (from the brick dust or clay) to produce hydrochloric acid and also some chlorine. This reacts with the silver to produce silver chloride (AgCl). The urine is acidic and aids decomposition. Silver chloride is volatile and would be removed from the metal. And the container is sealed to stop the escape of the silver which can be recovered later. Notton in experiments found that with one heating the gold content could be taken from 37.5% to 93%
Sulfur and antimony processes 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. The impurities form metal sulfides and the gold is left unreacted. The gaseous sulfide condenses on the crucible fabric. The antimony process is the same but uses
stibnite (Sb2S3) instead of sulfur because stibnite is stable at a higher temperature than sulfur. This is much quicker than the salt process and gave a purer gold, but it could dissolve some of the gold as well. This process is first described in the
Probierbuchlein.
Acid parting The distillation was used in the 12th century Europe after its introduction from the East and after that period more powerful acids could be created. Nitric acid (
aqua fortis, called by
Agricola aqua valens) could be made by the distillation of saltpetre (KNO3) with water and alum (KAl(SO4)2) or vitriol (FeSO4). ==See also==