Cupellation

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The Kupellation is a method for the separation of precious metals , for example gold or silver , alloys containing noble metals. The cupellation process is also known as abortion or driving work , historically also as purification . The contaminated metal is alloyed with lead ; the latter absorbs the impurities. The resulting lead oxide, together with the less noble metal oxides , is absorbed by a porous crucible, the cupelle or chapel . It is a crucial process step in silver smelting. However, gold and silver cannot be separated from one another in this way.

Areas of application

  • Tasting beings , for determining the silver content in lead ores
  • Analysis of the precious metal content (fire test) of metal alloys
  • Recycling of precious metals
  • Silver smelting

Procedure

The cupellation uses the fact of the different oxygen affinity of the metals involved. Noble metals like silver and gold are very difficult to oxidize. Base metals, on the other hand, can be oxidized very easily in their liquid state . Precious metals can be cleaned up to 99.99% in this way.

To carry out the cupellation, the metal to be tested is melted with about twice the mass of lead in a cup in a so-called test furnace in an oxidizing atmosphere. The following chemical reaction takes place:

In this way, the concentration of the noble metal increases steadily during the oxidation, until all of the lead is converted into lead (II) oxide (black lead) and the accompanying metals are also oxidized. Due to the lower surface tension of the oxide melt, it is absorbed by the cup, while the precious metal remains as a small pearl, the Güldisch silver . Lead can be produced from black lead by melting in a reducing atmosphere. However, the part of the lead that escapes as lead oxide or lead vapor is lost.

The cup is a crucible-like vessel, but made of a porous material. The oxidation processes are accelerated due to the much larger surface of the melt in contrast to the usual crucible. Cups are made from plant ash , bone ash and magnesia to prevent a chemical reaction with the lead oxide. Georgius Agricola also reports of the need to anneal the cupelle before use to prevent the lead from splashing out when melting. This was known as breather .

Calculation of the fineness

The metal to be tested is weighed before and after cupellation. The difference can be used to calculate its fineness:

Here, m 1 is the original alloy mass and m 2 is the mass of the regulus.

history

Precious metal and lead finds in Anatolia from the third millennium BC, but at least 2500 BC BC, make the use of the cupellation for their extraction appear credible.

In the Bible , refined silver is mentioned in Psalm 12: 7: "Silver, melted in the furnace, separated from slag, refined sevenfold".

Cupels are known to us from Roman times. They are dated to the first century BC. Since the cupels are soaked with lead oxide, they are very well preserved in the soil. Pliny the Elder reports in his Natural History of the use of cupellation for silver smelting.

literature

  • G. Agricola: De re metallica , 1556.
  • PT Craddock: Early Metal Mining and Production , pp. 205ff, 1995.
  • Marcos Martinón-Torres, Thilo Rehren, Sigrid von Osten: A 16th century lab in a 21st century lab: archaeometric study of the laboratory equipment from Oberstockstall (Kirchberg am Wagram, Austria) . In: Antiquity . tape 77 , no. 298 , September 2003 ( online - free full text).
  • E. Pernicka / G. Wagner: Thasos as a raw material source for non-ferrous and precious metals in antiquity , DER ANSCHNITT, Supplement 6, pp. 224-231, Association of Friends of Art and Culture in Mining eV, Bochum 1988, ISBN 3-921533- 40-6
  • Theophilus Presbyter: De diversis artibus . 12th Century
  • Th. Zimmermann: On the earliest lead and precious metal finds from Anatolia - some thoughts on context and technology. Der Anschnitt, 57th year, pp. 5–6. German Mining Museum, Bochum 2005.

Individual evidence

  1. ↑ Th. Zimmermann: On the earliest lead and precious metal finds from Anatolia - some thoughts on context and technology. Der Anschnitt, 57th year, pp. 5–6. German Mining Museum, Bochum 2005.
  2. ↑ Psalm 12: EU

See also