This is the fruit of the medicinal herb Chrozophora tinctorial and a lot of effort has been made to understand its make-up.
Medieval texts describe the practice of extracting a brilliant blue dye from the plant, which was stored as watercolour on cloth then applied as paint on many great works of European art.
Repeated efforts over two centuries to discover the molecular structure of the folium have been unsuccessful, but now a team led by Paula Nabais from Portugal’s Universidade NOVA has the answer.
And it was very much a team effort, with chemists joining forces with medieval art conservation scientists and a biologist with expertise in botany and Portuguese flora.
They consulted medieval texts to learn how best to collect C. tinctoria, gathered fruits over three summers, extracted the compound that produces the pigment, then isolated and purified it.
Using a combination of liquid and gas chromatography, mass spectrometry and nuclear magnetic resonance they determined that it represents a newly described class of dyes: it is neither an anthocyanin dye, a class found in many blue flowers and fruits, nor an indigo dye, the most stable natural blue variety of pigment.
And that’s useful knowledge if you want to preserve objects as precious as 1000-year-old manuscript illuminations.
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