Making Sap Green from Buckthorn Berries — The Medieval Manuscript Painter's Green

Collect the berries of common buckthorn (Rhamnus cathartica) or rock buckthorn (R. saxatilis) in late summer, while they are still unripe and green to yellow-green. Unripe berries produce a yellow-green pigment; fully ripe black berries produce a different colour (a dull brownish-purple, much less useful). You need approximately 500 g of berries for a small batch. The berries should be firm and bright green — not soft, dark, or wrinkled. Buckthorn is common in hedgerows and woodland edges across Europe and temperate Asia.

Crush the berries thoroughly in a stone mortar or by pressing in a cloth bag. Extract as much juice as possible — the juice is a bright yellow-green liquid. Strain through fine cheesecloth to remove seeds, pulp, and skin. You should obtain approximately 200-300 ml of bright green juice from 500 g of berries. The colour comes from a mixture of flavonoid glycosides, primarily rhamnetin and quercetin derivatives, combined with chlorophyll.

Dissolve 15-20 g of alum (potassium aluminium sulfate) in the strained juice. The alum acts as a mordant/fixative, brightening the colour and helping to precipitate the pigment onto an alumina substrate — similar to the lake pigment process. Stir until the alum is fully dissolved. The colour may shift slightly — becoming more vivid and yellower as the pH changes. If you want a bluer-green, add a small pinch of potassium carbonate (potash) to raise the pH; if you want a yellower green, the acid alum alone is sufficient.

Gently heat the alum-juice mixture in a glass beaker to evaporate excess water — reduce to approximately one-quarter of the original volume. Do not boil vigorously — gentle simmering preserves the colour. The concentrated liquid becomes thick and deeply coloured. Historically, this concentrate was poured into a cleaned pig bladder, tied off, and hung to dry — hence the name 'bladder green.' For our purposes, pour the concentrate into a shallow ceramic dish and let it dry at room temperature in the shade. The dried pigment forms a hard, glossy, dark green-brown cake.

To use, dissolve a small amount of the dried sap green cake in warm water with a few drops of gum arabic as a binder. The reconstituted pigment is a bright, transparent yellow-green — when diluted, it produces the most vivid, natural-looking foliage greens available from a single medieval pigment. For watercolour, dissolve with gum arabic. For egg tempera, dissolve in water first, then mix with egg yolk. Sap green mixes beautifully with blues (indigo, azurite) to produce deep forest greens, and with yellows (gamboge, ochre) for spring greens. Store the dried cake or liquid in sealed dark glass jars — light degrades the colour even in storage.