Sap Green: The Buckthorn Berry Pigment That Faded from the Palette

Open a modern watercolor box and you will find a pan labeled "Sap Green." The color is a warm, yellowish green, perfect for spring foliage and sunlit meadows. But squeeze out the paint and compare it to a genuine sap green made from buckthorn berries, and you are looking at two different materials. The modern version is a mixture of synthetic pigments, usually a phthalo green and a yellow, blended to imitate the historical color. The original sap green was a lake pigment made from the juice of ripe buckthorn berries, mixed with alum and gum arabic, and sold as a sticky syrup inside a dried pig's bladder. It was also called "bladder green" for this reason. And it had one serious problem: it faded in sunlight.

Sap green is a lake pigment produced from the juice of buckthorn berries, primarily from Rhamnus catharticus (common buckthorn) and related species. The berries produce a dark, yellowish-green juice that is mixed with an alkali (soda or lime) and then combined with alum and gum to form a thick, syrupy pigment. Sap green was used in watercolor painting and manuscript illumination from the medieval period through the eighteenth century. The primary colorant is kaempferol, though other buckthorn species produce rhamnetin or quercetin. The pigment is now obsolete in its genuine form due to its poor lightfastness, and the name survives as a color designation for modern pigment mixtures.

This entry covers what sap green is, how it was made, its history from Roman dyes to eighteenth-century watercolors, why it disappeared, and how the name lives on in modern paint ranges.

What Is Sap Green and How Is It Made?

Sap green is a lake pigment, meaning a soluble dye that has been precipitated onto an inert substrate to make it insoluble. The process begins with ripe buckthorn berries, which are crushed to release a dark purple-red juice. An alkali, either soda or lime, is added to the juice, shifting the color toward a dark, yellowish green. This green liquid is then mixed with alum (aluminum potassium sulfate) and gum arabic to form a thick syrup. The syrup was traditionally stored in dried pig bladders, which gave the pigment its alternative name "bladder green."

The color varies depending on the berry species and the ripeness of the fruit. Unripe buckthorn berries produce green shades, while ripe berries produce yellow. The primary colorant in Rhamnus catharticus is kaempferol, a yellow flavonoid compound. Other buckthorn species, such as Rhamnus infectorius (Persian berries), produce rhamnetin or quercetin as the main colorant. The different chemical compositions produce slightly different shades, ranging from a golden yellow to a deep olive green.

The Museum of Fine Arts Boston's CAMEO database describes sap green as an obsolete lake pigment, noting its use in watercolor paints in the seventeenth and eighteenth centuries. Read more at CAMEO.

Origins and History

Roman Dyes and Medieval Manuscripts

The use of buckthorn berries as a dye dates to the ancient Roman period, roughly 27 BCE to the fifth century CE. Buckthorn plants are native to east Asia and north Africa and were cultivated in Europe since Roman times. The berries were used to dye textiles, producing colors ranging from brownish-yellow to green depending on the mordant and the ripeness of the berries.

In medieval manuscript illumination, buckthorn berry pigments were used for green passages and for mixing with blue pigments to create deeper greens. The fourteenth-century Italian treatise De Arte Illuminandi recommended mixing buckthorn green with azurite to produce a rich, deep green for foliage and backgrounds. Notable examples of sap green in medieval illumination include the ninth-century Book of Kells and the fifteenth-century Book of Hours of Isabella Stuart, where the pigment appears in decorative borders and foliage.

The Seventeenth and Eighteenth Centuries

Sap green became particularly popular among European landscape watercolorists in the seventeenth and eighteenth centuries. The pigment was first recorded under the name "Sap Green" in 1758, though the material itself had been in use for centuries before the name was applied. The name may refer to the sap-like consistency of the syrup, or to the fact that the pigment was sold in a viscous form rather than as a dry powder.

The yellow variety of buckthorn lake, made from unripe berries, was known as "Stil de Grain" or "Dutch Pink." Before the seventeenth century, the word "pink" commonly referred to a yellow-brown color rather than a pale red, which is why this yellow lake pigment carries a name that now seems misleading. Both the yellow and green variants of buckthorn lake were used in house paints in England during the eighteenth century, where their low cost made them attractive despite their poor durability.

Decline and Replacement

Sap green's fatal weakness was its lightfastness. Like most plant-based lake pigments, genuine sap green fades rapidly under exposure to ultraviolet light. Watercolors made with sap green would lose their green passages within months of display, leaving landscapes with bleached, empty areas where foliage had been painted. By the nineteenth century, with the advent of commercially produced paints, manufacturers began offering more permanent alternatives. The first sap green "hues" were mixtures of Prussian blue and yellow pigments, which were more lightfast but still not fully permanent.

Today, sap green survives as a color name rather than a pigment. Modern sap green paints are mixtures of various synthetic pigments, chosen to imitate the warm, yellowish-green tone of the original. According to Jackson's Art, modern sap green varies wildly between brands because each manufacturer uses a different mixture of pigments. Some are muted and mossy, others are bright and saturated. Read more at Jackson's Art.

Key Artists and Works

Medieval Illuminators

The anonymous illuminators of the Book of Kells (ninth century, Trinity College Dublin) used buckthorn-based green pigments in the manuscript's intricate decorative borders and foliage patterns. The pigment was mixed with blue minerals to create the deep greens seen in the vine scrolls and interlace patterns. The Book of Hours of Isabella Stuart (c. 1431, Fitzwilliam Museum, Cambridge) also contains sap green in its marginal decoration, where the pigment's warm, natural green tone suited the botanical motifs.

Seventeenth-Century Landscape Watercolorists

Sap green was a staple of the seventeenth-century landscape watercolor tradition. Dutch and English topographical artists used it for foliage, meadows, and distant hills. The pigment's transparency made it ideal for layering over pen-and-wash drawings. Unfortunately, the fading of sap green in many of these works means that the landscapes we see today in museums are often paler than the artists intended, with blank areas where green foliage once stood.

Alexander Cozens (c. 1717-1786)

The English landscape painter Alexander Cozens, known for his ink-wash landscape compositions, used sap green in his watercolor studies. Cozens developed a system for generating landscape compositions from ink blots, and his watercolor additions relied on the transparent, naturalistic greens that sap green provided. The fading of these greens has altered the appearance of many of his surviving works.

How to Spot Sap Green in Historical Works

Identifying genuine sap green in historical paintings is difficult because the pigment has often faded, leaving little original material to analyze. In medieval manuscripts, areas that now appear blank or pale yellow where green foliage should be may have once contained sap green. Chemical analysis using HPLC (high-performance liquid chromatography) can identify the flavonoid colorants kaempferol, rhamnetin, or quercetin, which are specific to buckthorn berry pigments.

In well-preserved manuscripts, sap green has a warm, slightly olive tone that is distinct from the cooler, more saturated greens produced by mineral pigments like verdigris or malachite. If you see a soft, naturalistic green in a medieval illumination that looks like it could have come from a plant rather than a mineral, it may be sap green or a related buckthorn lake.

Related Pigments and Terms

Sap green belongs to the family of lake pigments, which are made by precipitating soluble dyes onto inert substrates. It is closely related to other plant-based yellows and greens, including Dutch Pink (Stil de Grain), which is made from the same buckthorn berries using a different process. The pigment connects to the broader history of gesso grounds used in manuscript illumination, and to the wet-on-wet techniques used in watercolor painting. The fading of sap green in historical works relates to broader questions of pigment stability discussed in our entry on underpainting. For more on how pigments create the effects you see in art, read our post on how to read a painting.

See Sap Green in Person

Genuine sap green is rare in museum collections because of its tendency to fade. The best places to see it are in well-preserved medieval manuscripts kept in dark storage and displayed under low light. The Book of Kells at Trinity College Dublin and the illuminated manuscripts at the Fitzwilliam Museum in Cambridge may contain surviving examples of buckthorn green pigment.

If you want to handle the pigment yourself, A. Gallo produces a genuine Buckthorn Berry Green watercolor, one of the few commercially available versions of the original pigment. For modern sap green, any major paint brand offers a version, though the mixtures vary significantly. For more on pigments and their history, read our entries on underpainting and gesso, or explore our guide on how to read a painting.