Alongside Indigo, dayflower blue is one of the prevalent blue colourants used in historical Japanese woodblock prints. It was largely replaced by Prussian blue, a synthetic pigment which was introduced to the Japanese art market in the 1820s.
The petals of the delicate dayflower produce a blue dye. Tsuyukusa (Commelina communis L.), the traditional wild Japanese variety of the flower or aobana (Commelina communis L. var. hortensis Makino), a cultivated variety of dayflower which produces larger petals, are both harvested in summer. The metallo-anthocyanin-containing dye  is released from the flower by submerging the petals in water. Traditionally, the dye is extracted by hand by gently squeezing the petals.
The blue dye could be used fresh by traditional textile dyers to draw their preparatory designs , or brushed onto Japanese paper several times, to obtain a deep saturated colour. Once dry, the paper acts as a carrier for the concentrated dye (like a clothlet) which can be stored for future use. The process is called aobanagami (blue flower paper). When ready to use, a small amount of the blue paper is wetted to release the dye.
The dye extraction process leads to differences in saturation of colour and different tones are achieved by using fresh or aged dye. The more recent the manufacture of the aobanagami, the brighter the blue dye obtained. The longer the dye is stored on the paper carrier before use, the greyer it will become as the dye will oxidise and lose its blue hue over time . The final colourant obtained, dayflower blue or aigami (literally, blue paper), ranges from a bright blue to a grey colour with hints of beige.
As well as being used on its own in hand-coloured and full-colour prints , dayflower blue was also used in combination with a red, such as safflower, to create purple tones. It has also been identified with yellows to create a green, primarily on the early benizuri-e prints (1740s-1760s) .
It was initially believed that dayflower was extremely light sensitive because of the grey tones observed on prints which were thought to be faded blues. However, more recent research shows that the variation in tones from blue to grey were achieved by using fresh or aged dye, as discussed above. Studies have shown that dayflower blue is not actually as light sensitive as previously believed  and that it falls in to an intermediate category of light sensitivity. Like many dyes, it will fade if subjected to unnecessary UV exposure.
Rather than light sensitivity, dayflower blue’s main identifying characteristic is its extreme sensitivity to water, which is not observed in any other blue used in Japanese woodblock prints (i.e., indigo or Prussian blue). Tidelines and uneven colour shifts to beige are indicators of this kind of damage. The fresh blue dye is more sensitive than the aged colour, but both are extremely water sensitive.
Through poor storage or display the moisture content of a print can go up. This can trigger the dayflower blue colour to strike-through to the verso of a print, while in areas having suffered only minor water damage, dayflower blue can react by changing colour to a pale tan colour in an extreme reaction to even a minimal increase in atmospheric moisture content.
Because of this extreme solubility, dayflower was frequently the last colour to be printed by the print maker. Its solubility presented challenges in printing uniformly without diminishing the colour or marking it . Despite these printing challenges and its moisture sensitivity, dayflower blue appears to have been used more often than indigo on Japanese woodblock prints.
It was the primary blue colorant used on early prints before it was replaced by Prussian blue, but a recent study  shows that the purple colour mixture of dayflower blue and safflower red remained the same even after this introduction. As such, this proves that despite the evolution of materials and techniques happening in Japan in the late Edo period, some traditional colourants and techniques continued to be used.
Julia Poirier, Book and Paper Conservator
Edo in Colour: Prints from Japan’s Metropolis opens at the Chester Beatty on Friday 28th May. We do hope you’ll be able to visit!
Inspired by the seasonality of these artworks, this exhibition will be shown in two parts: Part 1: 28 May – 29 August 2021; Part 2: 4 September – 5 December 2021. The exhibition will close for changeover from 30 August to 4 September.
 M. Shiono, N. Matsugaki, K. Takeda, ‘Structure of commelinin, a blue complex pigment from the blue flowers of Commelina communis,’ Proceedings of the Japan Academy, Series B, Physical and Biological Sciences, 84(10), 2008, pp. 452–456.
 Riyo Kikuchi, Records of Dyeing and Weaving Techniques at Tokyo National Research Institute for Cultural Properties, Department of Intangible Cultural Heritage, Tokyo National Research Institute for Cultural Properties, accessed 18th May 2021.
 Shiho Sasaki and Elizabeth Coombs, ‘Dayflower Blue: Its Appearance and Lightfastness in Traditional Japanese Prints”, in: Scientific Research on the Pictorial Arts of Asia: Proceedings of the Second Forbes Symposium at the Freer Gallery of Art, Paul Jett, John Einter, Blythe McCarthy (eds.), London: Archetype Publications, 2005, pp. 48-57.
 Shiho Sasaki and P. Webber, ‘A Study of Dayflower Blue Used in Ukiyo-e Prints,’ in: Works of Art on Paper—Books, Documents, and Photographs: Techniques and Conservation, V. Daniels, A. Donnithorne, P. Smith (eds.), London: International Institution for Conservation of Historic and Artistic Works, 2002, pp. 185-88.
 Michele Derrick, Richard Newman & Joan Wright, ‘Characterization of Yellow and Red Natural Organic Colorants on Japanese Woodblock Prints by EEM Fluorescence Spectroscopy,’ Journal of the American Institute for Conservation, 56:3-4, 2017, pp. 171-193.
[6, 7] Shiho Sasaki, ‘Dayflower Blue,’ Scientific Research.
 J. Pérez-Arantegui, David Rupérez, D. Almazán and Nerea Díez-de-Pinos, ‘Colours and pigments in late ukiyo-e art works: A preliminary non-invasive study of Japanese woodblock prints to interpret hyperspectral images using in-situ point-by-point diffuse reflectance spectroscopy,’ Microchemical Journal 139, 2018, pp. 94-109.
For more information on the production of aobanagami check out this wonderful resource.