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Cobalt glass is a deep blue colored glass prepared by adding cobalt salts of alumina to the molten glass. It is appreciated for its attractive color. It is also used as an optical filter in flame tests to filter out the yellow flame caused by the contamination of sodium, and expand the ability to see violet and blue hues.
For pigment use
Powdered cobalt glass, called smalt (or also simply cobalt glass), has been historically important as a pigment in painting, pottery, for surface decoration of other types of glass and ceramics, and other media. It is colored to a deep powder blue hue using cobalt ions derived from cobalt oxide. Cobalt aluminate can be used in a similar way, and is now more common, often known as "cobalt blue". Cobalt glass and smalt are made from silica and are not the same as cobalt blue, which is colored by cobalt ion in a similar way, but using a melting process with alumina.
The earliest known example of cobalt aluminate glass dates to a lump from about 2000BC in ancient Mesopotamia, very possibly intended for use as a pigment; it is then rare until the modern era. About five centuries later cobalt oxide smalt appears as a pigment in Egyptian pottery, and soon after in the Aegean region, and this is the pigment normally known as smalt. In paintings, smalt has a tendency to lose its colour over a long period and is little used today. However used in ceramics for underglaze decoration it keeps its colour well, and is the main blue used in blue and white pottery from a wide range of dates and areas, including Chinese blue and white porcelain from the Yuan and Ming dynasties, Renaissance Italian maiolica and Delftware.
Chinese porcelain used smalt glazes from the Tang Dynasty onwards, though Chinese cobalt glass is found from the Chou dynasty (1122-221 BC). Cobalt was used as a pigment in Central Asia from the 13th century. A fragment of a mud painting in the ancient Tangut city of Khara-Khoto has been found to contain smalt, judged to be dated between 11th to 13th century. A large quantity of smalt was purchased for the decoration of the gallery of François Ier at Fontainebleau in 1536. Smalt, normally now discoloured, is common in European paintings from the 15th to 17th centuries. for example, it was found in Hans Holbein the Younger's portrait of Sir William Butts (ca. 1540), in Michael Pacher's painting "The Early Fathers' Altar" (ca. 1483), and in the frescos of Domenico Ghirlandaio (1449–1494).
The invention of a European smalt process is sometimes credited to a Bohemian glassmaker named Christoph Schurer, around 1540-1560 The process used for producing cobalt smalt glass at the Blaafarveværket industrial manufacturing center in Norway in the 19th century has been documented as smelting cobalt oxide together with quartz and potassium carbonate. The result was an intensive blue glass-like substance that was ground and sold to producers of glassware and porcelain.
- Encyclopedia Iranica, "Cobalt"
- Smalt (conservation science), Tate Gallery
- Encyclopedia Iranica, "Cobalt"
- Watt, J. C .Y. (1979). "Notes on the Use of Cobalt in Later Chinese Ceramics". Ars Orientalis (Freer Gallery of Art, The Smithsonian Institution and Department of the History of Art, University of Michigan) 11: 63–85. ISSN 0571-1371. JSTOR 4629297.
- Jules Marie J. Guiffrey; Léon Emmanuel S.J. Laborde (marq. de) (1877). J.M.J. Guiffrey, ed. Les comptes des bâtiments du roi, 1528-1571. Suivis de documents inéd. sur les châteaux royaux et les beaux-arts au XVIE siècle, recueillis par le marq. L. de Laborde (in French). Paris. p. 96. Retrieved 6 November 2010.
- See: (1) Mauro Bacci and Marcello Picollo, "Non-Destructive Spectroscopic Detection of Cobalt(II) in Paintings and Glass," Studies in Conservation, vol. 41, no. 3, pages 136-144 (1996); especially page 136 ; and (2) C. Altavilla and E. Ciliberto, "Decay characteristics of glassy pigments: an XPS investigation of smalt paint layers," Applied Physics A, vol. 79, pages 309-314 (2004); especially page 309.
- See also: Bruno Mühlethaler and Jean Thissen, "Smalt," pages 113-130 in Ashok Roy, ed., Artists' Pigments: A Handbook of Their History and Characteristics, vol. 2 (Oxford, England: Oxford University Press, 1997).
- Rutherford John Gettens; George Leslie Stout (1966). Painting materials: a short encyclopaedia. Courier Dover Publications. p. 158. ISBN 978-0-486-21597-6. Retrieved 6 November 2010.
- Tone Sinding Steinsvik (2000). Koboltgruvene og blaafarveværket: en del af den store verden (in Norwegian). Modums blaafarveværket. ISBN 978-82-90734-20-1. Retrieved 6 November 2010.
Smalt, Bruno Mühlethaler and Jean Thissen, Studies in Conservation, Vol. 14, No. 2 (May, 1969), pp. 47–61, JSTOR
- Smalt Pigments through the Ages