Cobalt glass

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Cobalt glass
Cobalt glass vials

Cobalt glass , also written as cobalt glass , is a silicate glass that has been colored blue by cobalt (II) oxide . An intense dark blue color is often used. Depending on the cobalt content, the blue is more or less strong and the glass is more or less transparent. Powdered cobalt glass smalt called, it served mainly from the 16th to the 18th centuries as a pigment in painting. Co 2+ ions, which are surrounded by four tetrahedral oxygen ions, are responsible for the coloration .

history

The first known cobalt glasses can be found under some glasses that are among the first known glassware. They were made during the reign of Pharaoh Thutmose III. (around 1486 to 1425 BC). In the early 14th century BC In BC , cobalt glasses made in Malquata (Malkata) were widespread. Later, among the glasses made in Amarna , blue was the most common color, mostly as cobalt glass. Alums containing cobalt (in the form of magnesium aluminum sulfates) from the western Egyptian oases of Dachla (ad-Dakhla) and al-Charga (El-Kharga) were used as raw material for the cobalt coloring . In contrast to other cobalt ores, e.g. B. from Persia, these alums contain no arsenic. Cobalt glass can be obtained from cobalt ores containing arsenic or sulfur by first roasting the ores. When the roasted product, the Zaffer , is melted into a glass with potassium carbonate (potash) and quartz sand , it has a cobalt blue color. Cobalt was stained on ceramics in Syria and Iran in the 12th century, and more than a hundred years later on Chinese porcelain: in China it was used in ceramic glazes during the Tang Dynasty (618–906 AD) used. Zaffer was used to color glass in the Mediterranean region . In the Middle Ages it was used for Venetian glass , and in the 16th to 18th centuries blue paint works in the western Ore Mountains were the European center for the smelting of cobalt ores and the production of cobalt blue, followed by blue paint works in the West Harz, in Thuringia and some other Central European mining areas. Since 1722 the Norwegian blue paint factory Modum reached the second position in European production. Cobalt blue was used to color Bohemian crystal, to paint Meissen porcelain and increasingly to color textiles.

Another blue color of cobalt was produced in 1775 or 1795, the opaque and crystalline Thénards blue . It was first synthetically produced in the Vienna porcelain factory by the chemist and porcelain painter Josef Leithner by annealing aluminum and cobalt salts. The industrial production of the deep blue pigment was initiated by the Paris university professor Louis Jacques Thénard . Because of this and the availability of synthetic ultramarine , the extraction of smalt cobalt blue was pushed back more and more in the 19th century.

The very impure cobalt oxide mixture Zaffer, which contains arsenic and whose cobalt content can fluctuate, is no longer used in the production of cobalt glass. Purified cobalt (II) oxide is used instead. This ensures consistent, arsenic-free quality.

Use of cobalt glass

Sometimes the coloring is purely for decorative purposes. In addition, cobalt glass - similar to amber glass - was also used to package light-sensitive substances, such as B. Medicines, such as the medicine bottle shown in the picture.

Cobalt glass filters out yellow light when seen through. This is why it is used in chemical analysis to optically separate the pale violet flame color of the potassium salts from the strong yellow color of the sodium compounds in the flame color.

Cobalt glass is also used as a filter in photometry .

Toxicity

Cobalt oxides themselves are classified as harmful ( H phrases H302 harmful if swallowed and H410 very toxic to aquatic organisms with long-term effects ). In the glass, however, the cobalt is built into the glass and, due to its stable structure, cannot simply be removed. This could also be confirmed in Danish studies on porcelain painters. Here, however, toxicologically relevant exposures were observed after exposure to slightly soluble cobalt-zinc-silicate paints.

Individual evidence

  1. ↑ Laurianne Robinet, Marika Spring, Sandrine Pagès-Camagna, Delphine Vantelon, Nicolas Trcera: Investigation of the Discoloration of Smalt Pigment in Historic Paintings by Micro-X-ray Absorption Spectroscopy at the Co K-Edge . In: American Chemical Society ACS (Ed.): Analytical Chemistry . tape 83 , no. 13 , July 1, 2011, ISSN  0003-2700 , p. 5145-5152 , doi : 10.1021 / ac200184f .
  2. ↑ a b c AJ Shortland, MS Tite, I. Ewart: Ancient Exploitation and Use of Cobalt Alums from the Western Oases of Egypt * . In: Archaeometry . tape 48 , no. 1 , February 1, 2006, ISSN  1475-4754 , p. 153-168 , doi : 10.1111 / j.1475-4754.2006.00248.x .
  3. ^ Andrew J. Shortland: Lapis lazuli from the kiln . glass and glassmaking in the late Bronze Age (=  Studies in Archaeological Sciences ). Leuven University Press, Leuven 2012, ISBN 978-90-5867-691-7 , pp. 109 ( limited preview in Google Book search).
  4. ^ Robert B. Heimann, Marino Maggetti, Gabriele Heimann, Jasmin Maggetti: Ancient and Historical Ceramics: Materials, Technology, Art and Culinary Traditions . Schweizerbart'sche Verlagsbuchhandlung Nägele and Obermiller, Stuttgart 2014, ISBN 978-3-510-65290-7 .
  5. ↑ Hans-Heinz Emons, Maria Emons: "Cobalt blue colors". In: Meeting reports of the Leibniz Society of Sciences in Berlin .
  6. ↑ H. Behnken, E. Brodhun, Th. Dreisch, J. Eggert, R. Frerichs, J. Hopmann, Chr. Jensen, H. Konen, G. Laski, E. Lax, H. Ley, F. Löwe, M . Pirani, P. Pringsheim, W. Rahts: Production and measurement of light (= H. Geiger, Karl Scheel [Hrsg.]: Handbuch der Physik . No. 19 ). Julius Springer, Berlin 1928, Methods of Investigation, Photometry of Different Colored Lights, p. 532 ( limited preview in Google Book search).
  7. ↑ E. Prescott, B. Netterstrøm, J. Faber, L. Hegedus, P. Suadicani, JM Christensen: Effect of occupational exposure to cobalt blue dyes on the thyroid volume and function of female plate painters. In: Scand J Work Environ Health. 18 (2), Apr 1992, pp. 101-104. PMID 1604269