| Al 3+ Co 2+ O 2−|
|Ratio formula||CoAl 2 O 4|
|External identifiers / databases|
|Molar mass||176.894 g mol −1|
4.5 g cm −3
1955 ° C
|As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .|
The pigment Thénards Blau ( Dumonts Blau , Leithners Blau ) is a cobalt aluminate named after the French chemist Louis Jacques Thénard with the formula CoAl 2 O 4 . It is therefore an oxide pigment . The classic variant is listed in the Color Index under CI Pigment Blue 28 , a greenish variant with the formula Co (Al, Cr) 2 O 4 under CI Pigment Blue 36.
Further names and differentiation from other colors
Thénards blue is also called cobalt blue or cobalt blue . Even if one disregards the fact that "cobalt blue" can also be a pure color name independent of the cobalt content, these names are not clear, as the color of the cobalt glass and the pigment smalt cobalt blue are also mentioned. In all three substances mentioned, the blue color is caused by Co 2+ ions, which are tetrahedrally surrounded by four oxygen atoms. However, cobalt glass and the cobalt glass pigment smalt are amorphous glasses that are at least partially transparent and that usually contain very little aluminum. Thénards Blue, on the other hand, is non-transparent and has a defined crystalline structure - it is a cobalt spinel - and has a very high Al 2 O 3 content.
Further names for Thénards blue are celestine blue or cereal blue (from sky blue , from Latin coelestis "heavenly"); however, these names are not unique either, since z. B. Celestine blue B (C 17 H 18 ClN 3 O 4 ) is an oxazine dye .
Trade names for Thénards Blau are z. B. 22-5091, Sicopal blue 6310, Heucodur 552.
Cobalt blue has been discovered several times throughout history: in ancient Egypt , in China and by Louis Jacques Thénard. The first cobalt blue dye, smalt , was used to decorate pottery in Persia as early as ancient times. Cobalt aluminate was used after analyzes of ancient shards in ancient Egypt during the 18th to 20th dynasties (from 1550 to 1070 BC), after which it was forgotten again. For the production of the cobalt aluminate, cobalt-containing alums (in the form of magnesium aluminum sulfates) from the western Egyptian oases of Dachla (ad-Dakhla) and al-Charga (El-Kharga) were probably used as raw material. These were presumably dissolved in water and precipitated as hydroxides with sodium bicarbonate and then calcined above 900 ° C. In China, cobalt pigments were used in ceramic glazes from the time of the Tang Dynasty (618–906 AD). 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 the blue color pigment smalt, which is a silicon glass and thus an amorphous solid that contains practically no aluminum oxide.
The crystalline cobalt-aluminum mixed oxide Thénards Blau was first obtained as a pigment in modern times in the Vienna porcelain factory from the chemist and porcelain painter Josef Leithner , which is why it is also called Leithner's blue. There are different details about the time of Leithner's discovery: especially the years 1775 and 1795 are mentioned. Since the porcelain manufactory kept the recipe for its colors a secret, it is plausible that the Parisian university professor Louis Jacques Thénard rediscovered the pigment in June 1802, at least partially independently of Leithner - however, on behalf of the interior minister Jean-Antoine Chaptal , he searched the porcelain manufactory Sèvres specifically looking for a replacement for the expensive ultramarine . He had worked with the chemist and painter Léonor Mérimée; he published his discovery in 1803. He was able to produce his blue cobalt spinel synthetically by annealing aluminum sulfate and cobalt (II) nitrate (Co (NO 3 ) 2 ) and also initiated industrial production of the deep blue pigment - production in France began in 1807. The Chemist Gabriel Dumont improved the manufacturing process; his nephew, a manufacturer, brought the paint onto the market, with cobalt spinel being sold under the name Dumonts Blau ( Bleu Dumont ). The marketing of Thénards Blau contributed to the fact that the older smelting works of Smalte were pushed back in the 19th century.
The cobalt aluminum oxide CoAl 2 O 4 can be obtained from cobalt oxides and aluminum oxide . At sufficient temperatures it is not decisive whether one starts with cobalt (II) oxide CoO, cobalt (III) oxide Co 2 O 3 or the mixed oxide Co 3 O 4 , since the Co (III) -containing oxides are above Release 900 ° C of oxygen and form Co 2+ . Potassium chloride can be used as a flux . At lower temperatures of 400 ° C or more, cobalt-aluminum oxides with a spinel structure can be obtained from a hydroxide mixture, but these are dark green and also contain Co 3+ . The temperatures of 1000 ° C or more necessary for a beautiful blue are difficult to achieve with a simple Bunsen burner , so a strong Teclub burner is preferred ; ideal is heating to 1000 ° C in an annealing furnace for four hours. If one does not start from oxides, but from aluminum alcoholates (e.g. from 2-butanol) and cobalt nitrate, the blue spinel can be obtained with a sol-gel process at 700 ° C.
Chemistry and properties
Cobalt aluminate is a cobalt aluminum spinel with the following stoichiometric composition:
- CoO · Al 2 O 3 (cobalt (II) oxide and aluminum oxide).
CoAl 2 O 4 is a normal spinel: the Co 2+ cobalt ions occupy one eighth of the tetrahedral gaps between the cubically densely packed O 2− oxygen ions. The Al 3+ aluminum ions occupy half of the octahedral gaps. The formation of CoAl 2 O 4 serves as a detection reaction on aluminum in a melt sample in inorganic analytical chemistry .
At high temperatures (900 to 1100 ° C) CoAl 2 O 4 has an electrical conductivity that is produced by defect electrons at high oxygen partial pressures and by ionic conduction at low pressures.
The special feature of this pigment is an extreme depth of color . The chrominance of about 85% is outside the color space , which in the technical photographic color reproduction or printing process is achievable. Special colors are therefore necessary in printing , as the brilliance lies outside the CMYK color space of classic four -color printing . In the RGB color space of screens , the color is out of gamut and cannot be reproduced. As a powder, the pigment shows a significantly lighter shade than, for example, mixtures in oil .
A characteristic example of the use of this pigment is the painting "La Yole" by the French impressionist Pierre-Auguste Renoir.
In the paint sector, cobalt blue is used where the level of fastness of copper phthalocyanine blue is insufficient. This applies, for example, to the weathering stability of facade paints . The cheaper and at the same time stronger phthalocyanine blue is used for other areas .
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 pigment, however, the cobalt is built into the spinel mixed phase and cannot be expanded due to the stable crystal structure. 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.
- Kurt Wehlte: Materials and techniques of painting. Otto Maier, Ravensburg 1967, ISBN 3-473-48359-1 .
- Cobalt and its blue colors. In: Neue Zürcher Zeitung. 214, September 15, 1993, p. 67.
- A. Roy: Cobalt blue. In: BH Berrie (Ed.): Artists' Pigments. A Handbook of Their History and Characteristics. Volume 4, National Gallery of Art, Washington 2007, ISBN 978-1-904982-23-4 .
References and comments
- BASF Product Safety Data Sheet for SICOPAL® Blue K 6310 , accessed on December 27, 2019.
- Veronica D'Ippolito, Giovanni B. Andreozzi, Ferdinando Bosi, Ulf Halenius: Blue spinel crystals in the MgAl 2 O 4 -CoAl 2 O 4 series: Part I. Flux growth and chemical characterization . In: Mineralogical Society of America (Ed.): American Mineralogist . tape 97 , no. 11-12 , November 2012, pp. 1828–1833 , doi : 10.2138 / am.2012.4138 ( rruff.info [PDF; accessed on February 3, 2018]).
- Gunter Buxbaum, Gerhard Pfaff: Industrial Inorganic Pigments. Wiley-VCH.
- 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 .
- Walter Noll: Chemistry before our time: Ancient pigments . In: Chemistry in Our Time . tape 14 , no. 2 . Verlag Chemie, John Wiley & Sons, Weinheim April 1980, p. 37-43 , doi : 10.1002 / ciuz.19800140202 .
- 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, p. 153-168 , doi : 10.1111 / j.1475-4754.2006.00248.x .
- MS Tite, AJ Shortland: Production Technology for Copper- and Cobalt-Blue Vitreous Materials from the New Kingdom Site of Amarna — A Reappraisal * . In: Archaeometry . tape 45 , no. 2 , May 1, 2003, p. 285-312 , doi : 10.1111 / 1475-4754.00109 .
- William Jervis Jones: Historical Lexicon of German Color Designations. tape 3 : Early New High German – New High German; F-K . Akad.-Verlag, Berlin 2013, ISBN 978-3-05-005953-2 .
- Leithner, Joseph. In: Biographisches Lexikon des Kaiserthums Oesterreich.
- Gérard Emptoz: Le Binome Gay-Lussac et Thenard: une longue amitié . In: Société des amis de la Bibliothèque de l'Ecole polytechnique SABIX (ed.): Bulletin de la SABIX . tape 50 , 2012, p. 31-44 (French, openedition.org ).
- The year 1799 is also mentioned very often for Thénard's discovery, e.g. B. in the biography of Louis-Jacques Thenard. on britannica.com If Chaptal commissioned Thénards to work after he became Minister of the Interior, it could not have happened before 1801 at the earliest.
- Louis Jacques Thénard: Sur les Couleurs, suives d'un procédé pour preper une couleur bleue aussi belle que l'outremer. In: conseil des mines (ed.): Journal des mines . tape 15 , no. 1 , 1804, p. 128-136 ( annales.ensmp.fr [PDF]).
- Notice des products de l'industrie française précedée d'une historique des exposìtìons anterieures . 1834, p. 157 ( limited preview in Google Book search).
- Minori Taguchi, Takayuki Nakane, Kenjiro Hashi, Shinobu Ohki, Tadashi Shimizu: Reaction temperature variations on the crystallographic state of spinel cobalt aluminate . In: Dalton Transactions . tape 42 , no. April 19 , 2013, doi : 10.1039 / c3dt32828g .
- U. Lavrenčič Štangar, B. Orel, M. Krajnc: Preparation and Spectroscopic Characterization of Blue CoAl 2 O 4 Coatings . In: Journal of Sol-Gel Science and Technology . tape 26 , no. 1-3 , January 1, 2003, pp. 771-775 , doi : 10.1023 / a: 1020770810027 .
- Hermann Schmalzried: Ion and electron disorder in CoCr 2 O 4 and CoAl 2 O 4 . In: Reports of the Bunsen Society for Physical Chemistry . tape 67 , no. 1 , January 1, 1963, p. 93-96 , doi : 10.1002 / bbpc.19630670117 .
- color sample cobalt blue (pigment) : according to CIECAM ab plane . In: Bruce MacEvoy: Handprint. Watercolors . The picture at seilnacht.com matches the hue, but does not reach the luminosity of the pigment, accessed May 16, 2006.
- A. Roy: The Palettes of three Impressionist Paintings. In: National Gallery Technical Bulletin. Volume 9, 1985, pp. 12-20.
- Pierre-Auguste Renoir, 'La Yole' Colourlex.
- 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: Scandinavian Journal of Work, Environment & Health . tape 18 , no. 2 , April 1992, pp. 101-104 , PMID 1604269 .