The Evidence for Turnsole
Report of H H Bland, B.Sc., C.Chem., FRSC, for UK Forensic Science Services Ltd, dated 5 September 1988:/p>
Blue paint samples were submitted to a chemical test in dilute acetic acid, with the result that the blue colour changed to an orange/red colour. It had the character of an indicator dye like litmus, changing from blue in alkali to red in acid. The most commonly used dye of this nature in late medieval times was Turnsole (folium), particularly by the manuscript illuminators of southern Europe (it being a Mediterranean species of plant).
Letter from H H Bland to T J Benoy dated 17 January 1989:
Letter reports discussion between H H Bland and Miss J Kirby, Organic Chemist at the National Gallery, London (member of the international symposium DHA, Dyes in History and Archaeology). Miss Kirby confirmed that she had not previously encountered the use of such dyes in oil paintings, only for manuscript illumination and medieval water colours.
Email letter from Dr Jan Wouters, former head of the Laboratory for Materials and Techniques, Royal Institute for Cultural Heritage, Brussels (member of the international symposium DHA) to T J Benoy dated 12 February 2003:
Dr Wouters, an expert on the analysis of organic dyes and pigments, confirms his opinion from the description of H H Bland's analysis that the blue paint is made from Turnsole.
Letter from Dr Caroline Villers, former Director of the Department of Conservation & Technology, Courtauld Institute of Art, to T J Benoy dated 8 May 2003:
Dr Villers recommends publication of the Turnsole finding and aspects of further investigation.
Email letter from Dr Arie Wallert, curator/scientific examination, Department of Paintings, Rijksmuseum, Amsterdam (member of the international symposium DHA) to T J Benoy dated 5 June 2003:
Dr Wallert, an expert on organic dyes, confirms that for Turnsole to exist it would have to be bound in a carbohydrate or proteinaceous medium and cannot exist in an oil medium. He also confirms that Turnsole has never previously been recorded in an easel painting, only for manuscript illumination.
Email letter from H H Bland to T J Benoy dated 25 March 2004:
Letter reports discussion between H H Bland and Dr Jan Wouters following Dr Wouters' analysis by HPLC of blue paint samples from the Tondo. Dr Wouters considers the HPLC analysis reinforces the existence of the Turnsole blue dye.
Statement of Howell G M Edwards, M.A., D.Phil., B.Sc., C.Chem., FRSC, Emeritus Professor of Molecular Spectroscopy, University of Bradford, dated 29 June 2006:
Following his Raman spectroscopic analysis of paint samples from the Tondo painting, Professor Edwards confirms that the medium is most likely to be a vegetable-derived, starch based glue. It therefore has the necessary carbohydrate characteristics for the existence of Turnsole.
Paper dated March 2003 (with addendum dated April 2003) by T J Benoy entitled 'The Illuminator's Art, Turnsole and Raphael':
Paper was produced in response to the query from Patricia Rubin, former Deputy Director and Professor of Renaissance Studies, Courtauld Institute of Art, "why would Raphael have any interest in the use of the Turnsole pigment?"
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.