Smalt is a deep blue pigment made from crushed glass that has been colored with cobalt. Widely used in European art from the 15th through the 19th century, it served as a more affordable alternative to natural ultramarine and azurite. If you’ve ever noticed a painting from this era where blue areas have faded to a dull grey or brownish tone, there’s a good chance smalt is the reason.
How Smalt Is Made
The production of smalt starts with cobalt ore, which is roasted to produce a substance historically called “zaffre.” This roasted cobalt oxide is then melted together with silica (sand) and potash, a flux derived from wood ash. The result is a vivid blue glass. To turn that glass into a usable pigment, manufacturers poured the molten material into cold water. The thermal shock caused the glass to shatter into small particles, which were then ground and sorted by size.
The grinding step mattered enormously. Coarser particles produced a richer, deeper blue, while finely ground smalt looked paler, sometimes almost grey. This is because the color comes from light interacting with relatively large glass fragments. Crush those fragments too small and you lose the intensity. Smalt was sold in different grades based on particle size and cobalt content, with cobalt oxide concentrations typically ranging up to about 4% by weight in the more intensely colored varieties.
Beyond cobalt, silicon, and potassium, smalt often contains trace amounts of arsenic. This isn’t an intentional addition. It comes from the original cobalt ore, which naturally contains arsenic as an impurity.
Where It Was Produced
The Ore Mountains, a region straddling the border of southeastern Germany (Saxony) and northwestern Czechia, became the dominant center of smalt production. Rich cobalt deposits on both the Czech and Saxon sides of the mountains gave this area a natural advantage, and smalt manufacturing thrived there from the late Middle Ages onward. The pigment was traded widely across Europe and eventually reached South America through Spanish dealers during the colonial period, as well as Japan, where it appeared in scroll and screen paintings.
How Artists Used It
Smalt found its way into oil paintings, wall paintings, and polychrome wooden sculptures across late medieval and post-medieval Europe. It was a practical choice: natural ultramarine, made from lapis lazuli, was extraordinarily expensive, and azurite had its own limitations. Smalt offered a vibrant blue at a fraction of the cost. Rembrandt’s studio used smalt frequently. It appears in works like his “Saul and David” (around 1652) and “Homer” (1663), where it was sometimes blended with other pigments or colorless glass to adjust tone and texture.
Smalt also worked well as a glaze for ceramics, where its near-transparency was an advantage rather than a drawback. In this context, it functioned similarly to other cobalt-based colorants used in porcelain and pottery. Sixteenth-century treatises by writers like Giambattista Della Porta and Ferrante Imperato discussed smalt alongside materials used in goldsmithing and ceramic production, reflecting its versatility beyond the painter’s palette.
Why Smalt Fades in Paintings
One of smalt’s most significant drawbacks is its instability in oil paint over time. In many old master paintings, areas that were once a rich blue have turned brownish-yellow or greenish grey. This isn’t just fading. It’s a chemical process involving the glass itself breaking down.
The potassium in smalt’s glass structure gradually leaches out when the pigment sits in an oil binder for decades or centuries. As the potassium migrates out of the glass particles, it reacts with fatty acids in the oil medium, forming potassium soaps. This does two things simultaneously: the glass particles lose the chemical environment that supports their blue color, and the surrounding oil yellows and degrades. The combination of colorless glass and discolored oil produces the muddy, washed-out appearance conservators regularly encounter. These changes can also disrupt the physical structure of the paint film itself, creating a rougher, more deteriorated surface.
This degradation is a major concern for art conservators. When you look at a 17th-century painting and see drab, brownish skies or dull drapery, you may be looking at what was originally a luminous blue. Understanding smalt’s chemistry helps conservators interpret the artist’s original intentions, even when the color is long gone.
Smalt Beyond Europe
While smalt is best known from European painting, its reach was broader than many people realize. In China, smalt has been identified on numerous antiques, though Chinese researchers have only recently begun studying it in depth. In Japan, it appeared in paintings from the 15th through 19th centuries. These findings suggest that cobalt glass pigments moved along trade routes more extensively than art historians once assumed, connecting European glass technology with artistic traditions across Asia.
By the 19th century, smalt fell out of favor as synthetic alternatives like synthetic ultramarine and cobalt blue became available. These newer pigments offered more consistent color, finer particle sizes, and far better stability in oil. Smalt is no longer used as a working artist’s pigment, but it remains a key subject in art conservation and materials science, helping researchers understand and preserve centuries of painted works.