Cupellation

The cupellation (also: the Kupellationsverfahren ). Historically, the process as refining is already mentioned in the Bible (Psalm 12, 7 " ... silver tried in a furnace, divorced from slags, refined ...").

The cupellation is a process for the separation of precious metals, for example gold or silver, alloys of noble metals. The process is also known as drifting or blowing work. The contaminated metal is alloyed with lead and this takes the impurities in it. The resulting lead oxide together with the noble metal oxides of a porous Tiegelchen, the cupel or chapel absorbed. It is a crucial step in the Silberverhüttung. Gold and silver, however, can not be divorced from each other in this way.

Areas of application

• Try being, for the determination of the silver content in lead ores
• Analysis of the precious metal content ( trial by fire ) of metal alloys
• Recycling of noble metals
• Silberverhüttung

Method

The Kupellation utilizes the fact of the different oxygen affinity of the metals involved. Noble metals such as silver and gold can oxidize is very difficult. Base metals, however, can be very easily oxidized in the liquid state. Precious metals can be so clean to 99.99 %.

To carry out the Kupellation the tested metal is melted at about twice the mass of lead in a cupel in the so-called Trial furnace in an oxidizing atmosphere. The following chemical reaction takes place here:

In this way the concentration of the noble metal during the oxidation is continually increasing, until all the lead in the lead ( II) oxide ( litharge ) and converted to the accompanying oxidized metals. Due to the reduced surface tension of the oxide melt it is absorbed by the cupel, whereas the noble metals as small bead that Güldischsilber, remains. From litharge lead can be represented by melting in a reducing atmosphere again. The part of the lead, which escapes as gaseous lead oxide or lead vapor is lost, however.

The cupel is a pan -like vessel, but porous material. By in contrast to ordinary pot much larger surface of the melt, the oxidation processes are accelerated. Are produced from plants Kupelle ash, bone ash and magnesia to prevent a chemical reaction with the lead oxide. Georgius Agricola reported by the need to anneal the cupel before use in order to prevent splashing out of lead upon melting. This was designated as Abätmen.

Calculation of fineness

The metal to be tested is weighed before and after the cupellation. On the basis of the difference can be calculated fineness:

Here m1 is the original alloy mass and m2 is the mass of Regulus.

History

Precious metals and lead discoveries in Anatolia of the third millennium BC, but at least 2500 BC, to leave the use of cupellation for obtaining them appear credible. From Roman times Kupelle are known to us. They are dated to the first century BC. Since the Kupelle are saturated with lead oxide, they get in the floor very well. Pliny the Elder in his Natural History reports from the application of Kupellation to Silberverhüttung.