Smalt

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Smalte, historical dye collection of the TU Dresden

The smalt (also Schmalte ) with a cobalt (II) oxide blue colored potassium - silicate glass ; it is powdered cobalt glass . It was mainly produced in blue paint works and used as a pigment from the middle of the 16th to the 19th century . Depending on the cobalt content , their color is transparent, pale blue, blue, deep blue or dark blue and more or less translucent; finely ground powder is paler and lighter than coarser powder. It is fireproof and was therefore used in particular to decorate ceramics , but was also often used in painting. Since Schmalte was much cheaper than ultramarine , it was used on a large scale, especially in the 17th century. Today it is mainly used by restorers.

history

Madonna of the Rocks (Louvre)

Smalte is considered the oldest known cobalt pigment. For its production you need cobalt oxide, which is contained in a mixture called Zaffer or Safflor, which is created by roasting cobalt-containing ores. The use of cobalt in blue pigments in ancient times is well documented. Blue glasses and pottery colored with cobalt can be found back to 2000 BC. In ancient Egypt , Babylonia and Persia . Cobalt blue was also used in ancient Greece and the Roman Empire . However, many ancient Roman and Egyptian blue glasses or ceramics contain little or no cobalt, but instead Egyptian blue , a copper pigment. This can be explained by the fact that cobalt is less common and therefore significantly less common as a mineral than copper. It is unclear whether blue glass production was established in Venice in the Middle Ages; one with cobalt d. H. Glass colored with Zaffer is said to have been made in Venice in 1443.

The discovery of the blue cobalt glass smalt in Europe was generally attributed to the glassmaker Christoph Schürer , who is said to have made it in his glassworks, the Eulenhütte , since around 1540 . This information is uncertain because it was recorded later, based on the tradition of the chronicler Christian Lehmann .

Portrait of Francesco Mazzola by Parmigianino, around 1523

Examples of an earlier use of smalt are the pictures The Birth (around 1524) by Girolamo Romanino and Portrait of Francesco Mazzola (1523) by Parmigianino . There is also evidence that Leonardo da Vinci used smalt as early as the end of the 15th century for his paintings and frescoes, such as the first Madonna in the Rock (1483–6) and later the Mona Lisa (1503–6). In the second half of the 15th century, smalt was increasingly used in painting, e.g. B. by Dieric Bouts († 1475). In the painting "Portrait of Sir William Butts", which was begun between 1540 and 1543, Smalte was detected. The painting was attributed to Hans Holbein the Younger (1497–1543), and this was previously interpreted as one of the oldest evidence of Smalte in painting. However, the original picture had been painted over and the picture was also attributed to the painter John Bettes the Elder. In the 16th century it gained importance in European panel painting . In Baroque painting it was important for depictions of the sky. In the 17th and 18th centuries it was an important blue pigment: after the advancing Turks had occupied the Hungarian azurite mines at the beginning of the 17th century, the export of azurite came to a standstill and the importance of smalt increased. From the 17th century onwards, the most important production facilities for smalt were in Albernau , Niederpfannenstiel and Oberschlema in Saxony, so that the Saxon electors controlled a large part of Central European smalt production for a long time.

In the past, smalt was used as a whitener in papermaking .

From around the beginning of the 19th century, smalt was used less and less as a paint. Instead, the stronger Thénards blue was used, and the Berlin blue , known since the beginning of the 18th century , was used more frequently. Synthetic ultramarine was available in significant quantities from around the middle of the 19th century - Carl Leverkus had opened the first manufacturing factory in 1834, and Wilhelm Büchner had built a factory in 1845 that used an improved process. So the use of smalt continued to decline.

In 1969 it was documented that smalt can be discolored in oil paints over time.

Manufacturing

To make smalt, the glass base is mixed from quartz sand and potash . For the blue color, about 5% to 7% safflower must be added. These powders are mixed well. The mixture is then melted at a temperature of 1150 ° C. for 30 to 45 minutes. The resulting glass body is quenched in cold water. As a result, it disintegrates into granules and is then ground into pigment powder. Smalt was produced in specialized blue paint factories.

properties

Smalte has good lightfastness, is acid and alkali resistant and compatible with all pigments . It is used ground as a blue color, whereby the following types differ according to the fineness:

  • Scattered blue (also scattered sand): coarser, angular powder
  • Coleur : medium-fine powder
  • Eschel (also Aeschel): the finest powder

The intensity of the color depends on the cobalt content. Cobalt-rich smalts are referred to as royal blue , the darkest smalts as azure blue .

Use and evidence

Wertheimer house with smalt paint

The most common use of smalt is in the glass and ceramic industry, where it is a pigment for further processing. Smalt can be found in all binders and all techniques, with it being mainly used in oil and lime techniques. Occasional discoloration occurs on the pigment surface due to an alternating reaction with oil-based binders .

In Wertheim in Baden-Württemberg , the blue version of the half-timbered 1999 with smalt was restored according to the original findings on a half-timbered house (next to the county museum).

Smalt can easily be detected in paintings by a microscopic examination: It is easily recognizable due to its transparency and the shell-like breakage of the glass particles, which often contain small air bubbles, even at small enlargements.

Composition of the pigment

Smalt is a glass , i. H. a mixture of the amorphous solid type with a very variable composition. Smalte therefore does not have a precisely defined chemical formula, but is a solid solution of cobalt (II) oxide CoO in potassium silicate glass. Usually it contains 3 to 7 wt% CoO. But there were also pale variants that z. B. were used for the tinting of paper and which contained only 1 to 2 wt% CoO, as well as strong blue variants, for example for painting, with up to about 15 or 18 wt% CoO. Smalte was not made from pure cobalt oxide, but from safflower obtained by roasting cobalt ores. Since ores containing arsenic were mostly used, smalts usually also contain arsenic ( arsenic (III) oxide As 2 O 3 ), mostly in proportions similar to CoO, as well as smaller amounts of nickel oxide, bismuth oxide and iron oxides. The SiO 2 content is between 57 and 73%, that of K 2 O between 4 and 16%. A mixture of, for example, 9% As 2 O 3 , 10% CoO, 67% SiO 2 and 15% K 2 O can be described by the ratio formula 2 As 2 O 3 · 6 CoO · 50 SiO 2 · 7 K 2 O.

In contrast to the aluminum-rich Thénards Blue , which is also based on cobalt, the aluminum oxide content in Smalte is very low.

Aging of the pigment

The smalt in the now brown background of this picture painted by John Betts the Elder has discolored.

The pigment embedded in the oil colors of the painting can age with them, whereby alternating reactions occur with the oil and possibly acidic binders. It fades or changes color to a matt gray or gray-green. As a result, the appearance of paintings from the original image has changed significantly over the course of decades or centuries. Affected are z. B. Works by the Dutch baroque painter Hendrick ter Brugghen (1588–1629) or the Abbey in the Eichwald by Caspar David Friedrich . As the paint ages, potassium is released from the smalt glass. The coloring cobalt is mostly retained in the pigment, but its color fades because the environment around the cobalt changes: instead of a tetrahedral arrangement of four oxygen ions around the Co 2+ , which is responsible for the blue color, it is then six octahedral arranged oxygen ions surrounded.

literature

  • Josef Riederer: The smalt. In: German color magazine. 22 (9), 1968, ISSN  0341-065X , pp. 386-395.
  • Heike Binger: The blue pigment smalt. A brief historical overview and the use of smalt as a scattering material in the sources. In: Restauro . 102 (1), 1996, pp. 36-39.

Web links

Wiktionary: Smalte  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ^ Entry "Schmalte" on Zeno.org (from Brockhaus Conversations-Lexikon . Vol. 5. Amsterdam 1809, pp. 118–119).
  2. ↑ Entry “Blaufarbenwerke” on Zeno.org (from Brockhaus Bilder-Conversations-Lexikon . Volume 1. Leipzig 1837, pp. 260–261).
  3. ↑ a b c d e Joris Dik, M. den Leeuw, W. Verbakel, R. Peschar, R. Schillemans, Henk Schenk: The Digital Reconstruction Of A Smalt Discolored Painting By Hendrick Ter Brugghen . In: Journal of Art Technology and Conservation . tape 16 , 2002, ISBN 90-90-16479-0 , pp. 130–146 (English, online in the Digital academic repository of the University of Amsterdam [PDF; 5.2 MB ; Retrieved January 11, 2018] Chapter 6. In: Joris Dik: Scientific analysis of historical paint and the implications for art history and art conservation. The case studies of naples yellow and discolored smalt. Dik, Amsterdam 2003 [Zugl .: Diss., Univ. Amsterdam, 2003]).
  4. ↑ Smalte. In: seilnacht.com (Thomas Seilnacht: Didaktik der Naturwissenschaften. Bern).
  5. ^ Julian Henderson: The raw materials of early glass production . In: Oxford Journal of Archeology . tape 4 , no. 3 , November 1985, pp. 267-291 , doi : 10.1111 / j.1468-0092.1985.tb00248.x (English, researchgate.net ).
  6. ↑ Walter Noll: Chemistry before our time: Ancient pigments. In: Chemistry in Our Time . 14, 1980, p. 37, doi: 10.1002 / ciuz.19800140202 .
  7. ^ A b John Dallas Donaldson, Detmar Beyersmann: Cobalt and Cobalt Compounds. In: Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH Verlag, Weinheim 2005, doi: 10.1002 / 14356007.a07_281.pub2 (English; access via subscribed institutions).
  8. ↑ Heinz Berke: Chemistry in antiquity: the invention of blue and purple color pigments. In: Angewandte Chemie . Volume 114, Ausg. 14, 2002, p. 2595, doi : 10.1002 / 1521-3757 (20020715) 114: 14 <2595 :: AID-ANGE2595> 3.0.CO; 2-C .
  9. ↑ Harry Garner: An Early Piece of Glass from Eridu . In: Iraq . tape 18 , no. 2 , 1956, p. 147 , doi : 10.2307 / 4199608 (English).
  10. ↑ Center D'Information du Cobalt (Ed.): Cobalt Monograph . Prepared in collaboration with the staff of Battelle Memorial Institute, Columbus, Ohio. M. Weissenbruch, Brussels 1960, OCLC 921191777 (English, 515 pages).
  11. ^ Edward V. Sayre: Some Ancient Glass Specimens With Compositions of Particular Archaeological Significance . Ed .: Brookhaven National Laboratory. Upton, NY July 1964 (English, digital.library.unt.edu [PDF; 1.6 MB ; accessed on September 7, 2018]).
  12. ↑ Stuart J. Fleming, Leigh-Ann Bedal, Charles P. Swann: Glassmaking at Geoy Tepe (Azerbaijan) during the eariy 2nd millennium BC: a study of blue colors using PIXE spectrometry . In: Computer Applications & Quantitative Methods in Archeology (Ed.): Proceedings CAA Conference . 1993, chap. 28 , p. 199–204 (English, online [PDF; 1.6 MB ; accessed on September 9, 2018]).
  13. ^ Filomena Gallo: Glass in Northern Adriatic area from Roman to Medieval period: a geochemical approach for provenance and production technologies. Universita degli Studi di Padova, 2012 (Dissertation; English; PDF; 13.6 MB )
  14. ↑ Sarah Maltoni, Tania Chinni, Mariangela Vandini, Enrico Cirelli, Alberta Silvestri, Gianmario Molin: Archaeological and archaeometric study of the glass finds from the ancient harbor of Classe (Ravenna-Italy) . New evidence. In: Heritage Science . tape 3 , no. 1 , December 2015, p. 1-19 , doi : 10.1186 / s40494-015-0034-5 (English).
  15. ↑ E. Palamara, N. Zacharias, L. Papakosta, D. Palles, EI Kamitsos, J. Pérez-Arantegui: Studying a Funerary Roman Vessel Glass Collection from Patras, Greece . An Interdisciplinary Characterization and Use Study. In: STAR: Science & Technology of Archaeological Research . tape 2 , no. 2 , October 3, 2016, ISSN  2054-8923 , p. 203-216 , doi : 10.1080 / 20548923.2016.1239868 (English).
  16. ↑ R. Arletti, MC Dalconi, p Quartieri, M. Triscari, G. Vezzalini: Roman colored and opaque glass . A chemical and spectroscopic study. In: Applied Physics A . tape 83 , no. 2 , May 2006, p. 239–245 , doi : 10.1007 / s00339-006-3515-2 (English).
  17. ↑ a b Bernhard Neumann, Gertrud Kotyga: Antique glasses, their composition and color . In: Angewandte Chemie . tape 38 , no. 38 . Wiley-VCH, September 17, 1925, ISSN  1521-3757 , p. 857-864 , doi : 10.1002 / anie.19250383803 .
  18. ↑ a b c d e f Rutherford John Gettens, George Leslie Stout: Painting Materials: A Short Encyclopaedia . Dover Publications Inc., New York 1966, ISBN 0-486-21597-0 , Pigments and Inert Materials - Smalt, pp. 157–159 (English, preview in Google Book Search - first edition: van Nostrand, New York 1942, Dover, New York 2015, ISBN 0-486-21597-0 ): “Smalt was the earliest of the cobalt pigments. It is artificial, in the nature of glass ... "
  19. ↑ a b Collection of mixed news on Saxon history. Fourth volume. Johann Christoph Stößel, Chemnitz 1770, news of the emergence of the blue color works, in the Obererzgebürge, p.  363–367 ( slub-dresden.de ): “Christoph Schürer, a glassmaker from the Platten, moved to Neudeck, to the Eulen Hütte, and made glass there. When he was at Schneeberg for the first time, and saw beautifully colored cobalt lying around, he took some nasty pieces home with him, tried it in the glass oven, and saw that it was melting, he put ashes and other things in the glass, and made a beautiful blue glass from it. "
  20. ^ A b Johann Beckmann, William Johnston, William Francis, John William Griffith: A history of inventions, discoveries, and origins . Henry G. Bohn, London 1846, Cobalt, Zaffer, Smalt, p. 483 (English, archive.org ): “the invention seems to fall about 1540 or 1560. […] Christopher Schurer, a glass-maker at Platten, a place which still belongs to Bohemia, retired to Neudeck, where he established his business . ”
  21. ^ The National Gallery, London: Girolamo Romanino - The Nativity - NG297.1 - National Gallery, London. In: nationalgallery.org.uk. Retrieved September 8, 2018 .
  22. ^ The National Gallery, London: Parmigianino - Portrait of a Collector - NG6441 - National Gallery, London. In: nationalgallery.org.uk. Retrieved September 8, 2018 .
  23. ↑ a b c d e Marika Spring, Catherine Higgitt, David Saunders: Investigation of Pigment-Medium Interaction Processes in Oil Paint containing Degraded Smalt . In: National Gallery Company Limited, Ashok Roy (Ed.): National Gallery Technical Bulletin . tape 26 . Yale University Press, 2005, ISSN  0140-7430 , pp. 56–70 (English, nationalgallery.org.uk [PDF; 4.7 MB ; accessed on January 11, 2018]).
  24. ↑ Analysis of the Materials used in the Earlier Mona Lisa. In: monalisa.org. The Mona Lisa Foundation, August 9, 2018, accessed September 2, 2018 .
  25. ↑ Smalt. In: webexhibits.org, accessed January 19, 2018.
  26. ↑ Erika Michael: Hans Holbein the Younger: A Guide to Research. Garland, New York / London 1997, ISBN 0-8153-0389-0 ( preview in Google Book Search).
  27. ↑ History of Smalt. In: webexhibits.org, accessed January 19, 2018.
  28. ↑ a b Joyce Plesters: A Preliminary Note on the Incidence of discolouration of Smalt in Oil Media . In: International Institute for Conservation of Historic and Artistic Works (Ed.): Studies in Conservation . tape 14 , no. 2 . Taylor & Francis, May 1, 1969, ISSN  0039-3630 , pp. 62-74 , doi : 10.1179 / sic.1969.006 .
  29. ↑ a b How Smalt is made. In: webexhibits.org, accessed January 19, 2018.
  30. ↑ a b c d 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 .
  31. ↑ Jaap J. Boon, Joyce Townsend: The changing properties of smalt over time. Project January 2007 – January 2007. In: tate.org.uk. Tate gallery, accessed on January 11, 2018 (English): "Being a glass, smalt is transparent and the color is never strong."
  32. ↑ In new splendor: Caspar David Friedrich returns to the Alte Nationalgalerie. In: smb.museum, accessed January 19, 2018.