Smalt

Artificial inorganic pigment

Pigments sorted by Colour | Type | First date of use | Elements | Pigment timelines

Composition and Properties of Smalt

Smalt is a finely ground glass containing cobalt. It is chemically stable, but it contains potassium and thus its stability is lower than glasses containing sodium. The pigment is known to have discolored in oil paintings but not in fresco. The discoloration is due to several factors and it has been thoroughly investigated (1 – 5).

Pigment

Painted swatch

Bottled Pigment

Video: 'A Study in Blue Pigments Part 4 - Smalt' by Kodner Galleries

Names

Alternative names

Starch blue

Color Index

PB 32, CI 77365

Word origin

From Anglo-French enamailler, from en– “in” + amailler “to enamel,” variant of Old French esmailler, from esmal “enamel,” from Frankish *smalt, from Proto-Germanic *smaltjan “to smelt”.

From Online Etymology Dictionary

Smalt

German

Smalt

French

Azzurro di smalto, smaltino

Italian

Esmalte

Spanish

Preparation of Smalt

Smalt can be prepared in the laboratory by heating quartz sand, potassium carbonate and a small amount of cobalt (II,III) oxide (Co₃O₄) for 30 minutes at a temperature of 1150° C.

Synthesis of Smalt in the Lab

Attention: all cobalt compounds are highly toxic and should only be handled by people trained to do so.

5 g of sand, 5 g of potassium carbonate and 0,5 g of the cobalt oxide are first homogenized in a mortar.

preparation-smalt-1

The mixture is then transferred into a crucible and put into an oven preheated to the required temperature.

preparation-smalt-2

After 30 minutes the still hot crucible is taken out by using forceps and protective gloves. The hot crucible is then thrown into a beaker filled with cold water. This leads to the disintegration of the prepared glass into small pieces.

preparation-smalt-5

After scratching the hard smalt out of the crucible the product is filtered, dried and homogenized in a mortar.

preparation-smalt-6

History of Use

Smalt has been found in European paintings as early as fifteenth century and its use continued until about nineteenth century. The following graph gives the frequency of its use in the paintings of the Schack Collection in the Bavarian State Art Collections in Munich (1).

smalt_history_of_use

An extensive collection of occurrences of this pigment in paintings from several historical periods can be found in the blog post ‘Pigment: Ground glass‘ by The Eclectic Light Company.

References

(1) Kühn, H., Die Pigmente in den Gemälden der Schack-Galerie, in: Bayerische Staatsgemäldesammlungen (Ed.) Schack-Galerie (Gemäldekataloge Bd. II), München 1969.

(2) Mira S. de Roo, The Trade in Blue During the 17th Century: An Examination of Western European Pigment Trade in Azurite, Indigo, Lapis Lazuli, and Smalt During the 17th Century Through Works in the National Gallery, London, Master of Philosophy, University of Glasgow, 2004.

(3) Heike Stege, Out of the blue? Considerations on the early use of smalt as blue pigment in European easel painting, Zeitschrift für Kunsttechnologie und Konservierung, vol. 18, no. 1, 2004, pp.121-42.

Examples of use

Antonis van Dyck, Equestrian Portrait of Charles I, about 1637-38

1 Blue sky: smalt mixed with white with a discontinuous layer of natural ultramarine and white painted on top in the bluest areas.

Diego Velázques, The Toilet of Venus (Rokeby Venus), 1648-51

Timeline for Blue Pigments

Identification

Fiber Optics Reflectance Spectrum (FORS)

FORS-spectrum-smalt

Spectra by A. Cosentino, Cultural Heritage Science Open Source (CHSOS)

Infrared Spectrum

Spectrum by S. Vahur, Database of ATR-IR spectra of materials related to paints and coatings, University of Tartu, Estonia

2. IR-Spectrum in the ATR-FT-IR spectra of different pure inorganic pigments, University of Tartu, Estonia

Raman Spectrum

Spectrum by Ian M. Bell, Robin J.H. Clark and Peter J. Gibbs, Raman Spectroscopic Library
University College of London

X-Ray Fluorescence Spektrum (XRF)

XRF Spectrum in the Free XRF Spectroscopy Database of Pigments Checker, CHSOS website.

References

(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)

(2) Jonynaite, D., Senvaitiene, J., Beganskiene, A., & Kareiva, A. Spectroscopic analysis of blue cobalt smalt pigment. Vibrational Spectroscopy, 52(2), 2010, 158–162. doi:10.1016/j.vibspec.2009.12.005

(3) Robinet, L., Spring, M., & Pagès-Camagna, S. (2013). Vibrational spectroscopy correlated with elemental analysis for the investigation of smalt pigment and its alteration in paintings. Analytical Methods, 5(18), 4628. doi:10.1039/c3ay40906f

(4) V. Kugler, S. Bean and M. Spring (2013). Quantitative EDX Analysis of Smalt Pigment in Sixteenth and Eighteenth-Century Paintings. Microscopy and Microanalysis, 19 (Suppl. 2) , pp 1428-1429. doi:10.1017/S1431927613009136.