Understanding Smalt Pigment: Its History, Degradation, and Preservation in Art

Smalt, a vibrant blue pigment, has a storied history in the art world, revered for its use by old masters and yet notorious for its tendency to degrade over time. Originating from cobalt glass, smalt’s extensive use by European masters, notably Rembrandt, speaks volumes about its significance in art history. However, this pigment’s susceptibility to discoloration presents both a challenge and a fascinating area of study for artists and conservators today. This article delves into the historical use of smalt, its deterioration mechanisms, and contemporary insights into maintaining the integrity of artworks featuring this pigment.

Smalt pigment is found in many areas of Rembrandt’s Night Watch. Follow this link for a close-up view of Rembrandt’s Night Watch.

Historical Significance of Smalt Pigment

The history of smalt dates back to the 15th century, with its use predominantly in European easel paintings. This pigment, composed of potassium glass infused with cobalt, was a cheaper alternative to the more expensive ultramarine. Evidence of its use, from wall paintings in Mongolia to frescoes in Istanbul, dates back to the 12th century before becoming a staple in European artworks. Smalt’s vibrant blue hue was highly valued, but its propensity to degrade over time, shifting from blue to grey-yellowish, posed significant conservation challenges.

Origin and Method of Producing Smalt Pigment

The term smalt originated from the Italian smaltare, which means “to melt. “However, this term did not always refer to blue cobalt glass pigments. Various terms have been ambiguously used to indicate the blue color. Pliny the Elder used the word sappirus for lapis lazuli but also vitreous materials, such as cyanus for Egyptian blue and caeruleus for the color and the pigment used in wall paintings.

The terms smalto and smaltino were used in early Italian literature and were not always associated with smalt. These terms might refer to a vitrified pigment composed of sand, copper, and potassium nitrate, as described by Vitruvius.

Microphotographs of smalt pigment.

Zaffre is an impure cobalt oxide obtained after the sulfur and arsenic are burned off in the roasting process of cobalt-bearing minerals. Recent studies have reported that this results when roasting cobalt ores at temperatures up to 700°C and mixing with siliceous sands. Blue smalt is produced when zaffre is melted with potash and sand; the violet variety may be obtained by adding soda instead of potash. Based on these references, zaffre may be considered the primary product for the manufacturing of smalt pigment.

Rembrandt’s Innovative Use of Smalt

Rembrandt, the Dutch master, is renowned for his innovative use of materials, including smalt. His late works, such as Homer (1663), exemplify his experimental approach to painting techniques. Rembrandt often mixed smalt with other pigments, not just for its color but also for its drying properties and textural effects. The cobalt in smalt served as a drier, enhancing the drying time of oil paints significantly. Additionally, smalt’s low refractive index allowed it to be used in creating translucent layers, adding depth and complexity to his works​.

Rembrandt’s use of smalt in Homer exemplifies his experimental approach to painting.

Sample locations of the seven paint cross-sections from various smalt paints in the painting and a chart showing proportions by volume of the pigments identified in the cross-sections. The composition of each of these smalt paints was used to make the paint reconstructions by the authors of the paper, “The role of smalt in complex pigment mixtures in Rembrandt’s Homer 1663: combining MA-XRF imaging, microanalysis, paint reconstructions.’

Rembrandt used smalt not only for its blue color but also mixed it with red or yellow pigments to make purple or green and to enliven brown tones. This gave his paints more texture and made them dry faster. Unfortunately, smalt mixed with an oil-binding medium tends to lose color, changing bright blue areas into murky brown.

Rembrandt used smalt in the background architecture of the Night Watch, added it to brown earth pigments, and brushed it on with broad textured strokes. He also applied it in the shadows of the blue details in the costume of Van Ruytenburch. However, very little blue smalt is intact in this painting.

Causes of Smalt Degradation

The primary cause of smalt’s discoloration is the leaching of potassium ions from the glass matrix. This leaching process is accelerated by the presence of moisture and the interaction with the oil medium used in paintings. The loss of potassium leads to a structural change in the cobalt ions from a tetrahedral (blue) to an octahedral (colorless) coordination state. Environmental factors, such as humidity and cleaning agents, further exacerbate this degradation, forming crusts on the paint surface due to the migration of potassium and other cations​​​.

Insights from Experimental Studies

Recent studies have employed advanced techniques such as SEM-EDX, spectro-colorimetry, and X-ray absorption spectroscopy to simulate and analyze the deterioration of smalt. These investigations have shown that artificially aged smalt closely mimics the natural degradation observed in historical paintings. The data reveal a significant shift in color coordinates, with the dominant wavelength moving from deep blue (479 nm) to orange-yellow (578 nm), reflecting the pigment’s color transition over time​.

Preventive Measures and Preservation of Smalt Pigment

Understanding the mechanisms behind smalt degradation has led to developing strategies to mitigate these effects. One approach is using smalt with higher potassium content, which has been shown to resist degradation better. Also, maintaining paintings under controlled microclimatic conditions—specifically low-humidity environments—can significantly slow the degradation process​.

The Value of Smalt Pigment for Artists Today

Smalt remains a pigment of great historical and artistic value. Its use by masters like Rembrandt highlights its importance in art history, while contemporary studies on its degradation provide valuable insights for preservation. By understanding the causes of smalt’s discoloration and implementing preventive measures, artists and conservators can better preserve the vibrant blue hues that have captivated audiences for centuries.

Understanding Smalt Pigment: The History and Chemistry Behind the Pigment

Dive into the fascinating world of Smalt, a vibrant blue pigment with a rich history and unique chemical properties. In this video, we explore the origins of Smalt, its historical uses in art, and the chemistry that gives it its distinctive hue. Join us for an in-depth look at how this pigment has been utilized from the Renaissance to modern times, and learn about its impact on art and science.

Smalt Pigments and Smalt Watercolor

Bibliography

Cavallo, G., Riccardi, M.P. “Glass-based pigments in painting: smalt blue and lead–tin yellow type II.” Archaeol Anthropol Sci 13, 199 (2021). https://doi.org/10.1007/s12520-021-01453-7

Cianchetta, I., Colantoni, I., Talarico, F., D’Acapito, F., Trapananti, A., Maurizio, C., Fantacci, S., & Davoli, I. (2012). “Discoloration of the smalt pigment: Experimental studies and ab initio calculations.” Journal of Analytical Atomic Spectrometry, 27(11), 1941-1948. DOI: 10.1039/c2ja30132f. Link.

Novle, P. and A. van Loon, "New insights into Rembrandts Susanna: changes of format - smalt discoloration - identification of vivianite - fading of yellow and red lakes - lead white paint," ArtMatters 2, 76-97 (2005). Link.

Paschinger, H., & Richard, H. (1995). "Blaupigmente der Renaissance und Barockzeit in Österreich." Naturwissenschaften in der Kunst, 63-66.

Spring, Marika, Higgitt, C., & Saunders, D. (2005). “Investigation of Pigment-Medium Interaction Processes in Oil Paint Containing Degraded Smalt.” National Gallery Technical Bulletin, 26, 56-70.

Spring, M., Penny, N., White, R., & Wyld, M. (2001). “Colour Change in The Conversion of the Magdalen attributed to Pedro Campaña.” National Gallery Technical Bulletin, 22 (2001): 54–63. http://www.jstor.org/stable/42616151.

van der Weerd, J., Keune, K., Geldof, M., & Van Asperen de Boer, J. (2001). “Imaging microspectroscopic secondary ion mass spectrometric and electron microscopic studies on discoloured and partially discoloured smalt in cross-sections of 16th century paintings.” Chimia, 55(10), 952-960.

van Loon, A., Noble, P., de Man, D. et al. “The role of smalt in complex pigment mixtures in Rembrandt’s Homer 1663: combining MA-XRF imaging, microanalysis, paint reconstructions, and OCT” Herit Sci 8, 90 (2020). https://doi.org/10.1186/s40494-020-00429-5 “Insight into the chemistry of deteriorated smalt pigment through analysis of samples from paintings in the National Gallery London” National Gallery Technical Bulletin, 26, 2005, pp. 56-70.