The dyestuff Turnsole was in regular use in the late Middle Ages by manuscript illuminators, who valued the range of glowing, transparent colours from blue, through purple to red which it could produce. Then known as folium, it was subsequently downgraded to a shading glaze and fell out of use in the illuminator's palette by the turn of the seventeenth century, with the advent of synthetic blue pigments. The existence of the Turnsole dye has never previously been recorded in a painting, as distinct from manuscript illumination.
Traditions of secrecy have attended descriptions of the Turnsole dye's techniques of manufacture and there has also been confusion about the plant's horticulture. Herbal and botanical treatises indicate that the plant grows on sunny hillsides (often amongst vines) and is found in countries bordering the Mediterranean and in the Middle East, where some societies have used it for cosmetics. Its botanical name is Crozophora tinctoria, but herbalists have over the centuries variously titled it as Solsequium, Morella, Heliotropium tricoccum and Croton tinctorium. To manuscript illuminators of the Middle Ages it was known simply as folium. Part of the plant's mystery is that it is easily confused with other herbs and weeds, which may have similarities but not the characteristic tri-lobed fruit which yield the dye. For example, its habit of turning towards the sun (from which its name is derived) has confused it with the common Sunflower, Helianthus annus, which behaves in the same way; and Heliotrope itself, Heliotropium europeum, has been known as Greater Turnsole but is in fact a member of the Borage Family. Having harvested the seeds and soaked cloths with their juice, the main technical problem was how to induce the shades of blue and purple by submitting the cloths to the correct alkaline environment with the appropriate PH factor. There is a fourteenth-century recipe explaining how this was done by exposing the cloths to a vaporous environment of ammonia induce by urine in a dark, damp cellar.
To the manuscript illuminators working in monastery scriptoria, the virtues of Turnsole would have been that it was economical and readily available, where the plant grew in favourable conditions. Moreover the use of clothlets was an effective form of colour production and the range of blue, purple and red colours themselves were rich and transparent and much admired by late medieval patrons and book collectors.