Sap green
A green made from berries, stored in a pig's bladder, and guaranteed to fade. Sap green is the most beautiful transparent green of the medieval palette and the least permanent thing in this atlas.
Origin
Made from the ripe berries of buckthorn — Rhamnus species, particularly Rhamnus cathartica — crushed, the juice evaporated to a thick syrup and mixed with alum. The syrup never fully dried, so it was traditionally sold in small bladders that the painter pierced when needed, which gives the pigment its German and Spanish names, Saftgrün and verde vejiga. Different berries and different ripeness gave everything from yellow-green to a deep olive; unripe buckthorn berries were separately used to make a yellow lake.
In art history
Sap green belongs to manuscript illumination and to watercolour, and it appears in this atlas in the eras where those traditions dominate. Medieval illuminators used it for foliage and landscape backgrounds, and it survived into the English watercolour tradition of the eighteenth and nineteenth centuries, where its transparency and its natural, slightly yellow-leaning green made it the obvious colour for trees. That it faded was well known; it was used anyway, because mixing a green from blue and yellow in watercolour gives something flat, and sap green glows.
In the studio
It is a dye rather than a true pigment — soluble colour rather than suspended particles — which explains both its transparency and its impermanence. It is fugitive in strong light, shifting first towards brown and then vanishing, and it is affected by damp and by alkalis. Many medieval green landscapes have simply gone, leaving pale brown foliage that reads as a stylistic choice and is in fact an absence. In oil it was rarely used, being unstable and slow.
Afterlife
Nothing labelled sap green today contains buckthorn. Modern sap green is a mixture, usually phthalo green with an earth or a quinacridone, chosen to match the traditional hue while being fully lightfast. It is one of the clearest cases in the pigment world of a name that has survived as a colour description long after the material behind it was abandoned.
Where it appears
Every era and artist in this atlas whose palette includes this pigment.