HAuCl4
Well-known member
Found this in wikipedia and other web search. Maybe just general interest or maybe a trigger for ideas. Enjoy!. 
Theophilus was a 12th century German monk and in his book ‘’De Diversus Artibus’’[12] gives the clearest description of the salt cementation process.
“ break into tiny pieces a tile or piece of burnt and reddened furnace-clay and when it is powdered, divide it into two equal parts by weight and add to it a third part of salt of the same weight. It should then be likely sprinkled with urine and mixed so that it does not stick together but is just moistened. ”
—Theophilus[13]
This mixture is then added to an earthenware pot and layered with thin sheets of gold fold. The pot is then sealed and heated in a furnace.
“ Then put the fire and wood below and see that a plentiful fire is not lacking for the space of a day and a night. In the morning, however, take out the gold and melt it again, hammer it, and put it into the furnace as before. After another day and night take it out again, mix a little red copper with it, melt as before, and put it back into the furnace. And when you have taken it out a third time, wash it and carefully dry it. Weigh it, when dried, and see how much has been lost, then fold it up and keep it. ”
—Theophilus[14
The Method of cementing gold and of Bringing it to its Ultimate Fineness-Vannoccio Biringuccio
http://books.google.it/books?id=ruBbKRKGeOwC&pg=PA202&lpg=PA202&dq=The+Method+of+cementing+gold+and+of+Bringing+it+to+its+Ultimate+Fineness&source=bl&ots=hiijeUwTzS&sig=tp9DXpELlCoqPTUMy955BxI33S0&hl=it&ei=jwxcTMnCJ4P_8AaElfDuAg&sa=X&oi=book_result&ct=result&resnum=1&ved=0CBUQ6AEwAA#v=onepage&q=The%20Method%20of%20cementing%20gold%20and%20of%20Bringing%20it%20to%20its%20Ultimate%20Fineness&f=false
Modern re-creation of salt cementation methods by Notton:
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.[18] Heating can take 24 hours. Hoover and Hoover[19] 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%[20]
Theophilus was a 12th century German monk and in his book ‘’De Diversus Artibus’’[12] gives the clearest description of the salt cementation process.
“ break into tiny pieces a tile or piece of burnt and reddened furnace-clay and when it is powdered, divide it into two equal parts by weight and add to it a third part of salt of the same weight. It should then be likely sprinkled with urine and mixed so that it does not stick together but is just moistened. ”
—Theophilus[13]
This mixture is then added to an earthenware pot and layered with thin sheets of gold fold. The pot is then sealed and heated in a furnace.
“ Then put the fire and wood below and see that a plentiful fire is not lacking for the space of a day and a night. In the morning, however, take out the gold and melt it again, hammer it, and put it into the furnace as before. After another day and night take it out again, mix a little red copper with it, melt as before, and put it back into the furnace. And when you have taken it out a third time, wash it and carefully dry it. Weigh it, when dried, and see how much has been lost, then fold it up and keep it. ”
—Theophilus[14
The Method of cementing gold and of Bringing it to its Ultimate Fineness-Vannoccio Biringuccio
http://books.google.it/books?id=ruBbKRKGeOwC&pg=PA202&lpg=PA202&dq=The+Method+of+cementing+gold+and+of+Bringing+it+to+its+Ultimate+Fineness&source=bl&ots=hiijeUwTzS&sig=tp9DXpELlCoqPTUMy955BxI33S0&hl=it&ei=jwxcTMnCJ4P_8AaElfDuAg&sa=X&oi=book_result&ct=result&resnum=1&ved=0CBUQ6AEwAA#v=onepage&q=The%20Method%20of%20cementing%20gold%20and%20of%20Bringing%20it%20to%20its%20Ultimate%20Fineness&f=false
Modern re-creation of salt cementation methods by Notton:
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.[18] Heating can take 24 hours. Hoover and Hoover[19] 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%[20]
