Turnsole or folium was a dyestuff prepared from the annual plant Chrozophora tinctoria.
Turnsole became a mainstay of medieval manuscript illuminators starting with the development of the technique for extracting it in the thirteenth century, when it joined the vegetable-based woad and indigo in the illuminator's repertory. However, the queen of blue colorants was always the expensive lapis lazuli or its substitute azurite, ground to the finest powders. According to its method of preparation, turnsole produced a range of translucent colors from blue, through purple to red, according to its reaction to the acidity or alkalinity of its environment, in the chemical reaction, not understood in the Middle Ages, that is most familiar in the Litmus test.
Folium ("leaf"), was actually derived from the three-lobed fruit (illustration), not the leaves. In the early fifteenth century, Cennino Cennini, in his Libro dell' Arte gives a recipe "XVIII: How you should tint paper turnsole color" and "LXXVI To paint a purple or turnsole drapery in fresco." (though neither of these recipes use or describe turnsole). Textiles soaked in the dye vat would be left in a close damp cellar in an atmosphere produced by pans of urine. It was not realized that the oxidizing urine was producing ammonia, but the technique reminds us how foul-smelling was the dyer's art.
The colorant was downgraded to a shading glaze and fell out of use in the illuminator's palette by the turn of the seventeenth century, with the easier availability of less fugitive mineral-derived blue pigments.
Turnsole was used as a food colorant, mentioned in Du Fait de Cuisine which suggests steeping it in milk. The French Cook by François Pierre La Varenne (London 1653) mentions turnsole grated in water with a little powder of Iris.
Herbals indicated that the plant grows on sunny, well-drained Mediterranean slopes and called it solsequium ("sun-follower") from its habit of turning its flowers to face the sun; alternatively it might be called "Greater Verucaria"; early botanical works gave it synonyms of Morella, Heliotropium tricoccum and Croton tinctorium.
- Thompson and Hamilton 1933:41
- So named in a recipe for producing the colorant, Pro tornasolio faciendo, British Library, Sloane Mss 1754, folio 235 verso, quoted in Daniel V. Thompson, Jr., "Medieval Color-Making: Tractatus Qualiter Quilibet Artificialis Color Fieri Possit from Paris, B. N., MS. latin 6749", Isis 22.2 (February 1935, pp. 456-468) p 458 note.
- Daniel V. Thompson, Jr and G.H. Hamilton, 1933. De Arte Illuminandi: The Technique of Manuscript Illumination (New Haven: Yale University Press) pp 41–43.