Tsuyakusa
Blue

Tsuyukusa (露草), “dewflower,” is the Japanese vernacular name for the Asiatic dayflower Commelina communis, and the source plant of two related but functionally distinct Edo-period blue pigments. The first, aigami, is the more pigment-saturated form used directly as a paint or print colour for blue passages of an image. The second (recorded here as tsuyakusa-blue) is a more diluted, more fugitive form of the same dye that conservation sources associate with under-drawing work in ukiyo-e production, where a water-soluble guide mark could be applied and would later vanish from the finished print.

The technical context is the multi-block colour-printing system of mature Edo-period ukiyo-e. A nishiki-e (“brocade picture”) full-colour print required multiple separate woodblocks: one keyblock cut with the master line drawing, and additional blocks for each colour, with complex prints sometimes demanding ten or more. The artist supplied a brush drawing in ink (the hanshita-e) for the keyblock, and that drawing was destroyed in the carving process. For the colour blocks, a different approach was needed: registration marks (kentō) had to be carved into each colour block to match the keyblock impression precisely, but the carver also needed a guide drawing showing exactly which areas of the block were to remain raised (the printing areas) and which were to be cut away. The standard solution was to take a fresh impression of the keyblock onto thin paper, hand-tint that impression in dilute tsuyukusa-blue water-colour over the areas to be printed in that block’s colour, and paste the marked-up impression onto the new colour-block as a cutting guide.

The reason tsuyukusa-blue specifically was chosen for this role is its known fugitiveness. The dye is essentially water-soluble (commelinin, the active anthocyanin-flavone metalloanthocyanin complex, washes out of paper on contact with water), and so when the finished print was sized and washed during the final paper-finishing stage, any residual tsuyukusa-blue marks on the printed sheet vanished completely, leaving only the intended printed colours. The same water-sensitivity that made aigami impractical as a durable finished-print blue was, by this reading, the precise quality that made a more diluted form useful as a temporary alignment marker.

Analytical studies of Edo-period prints, including Raman spectroscopy and LC-MS surveys, have confirmed dayflower blue among the colorants present, with the Cesaratto et al. (2018) timeline work showing that traditional colorants including dayflower blue remained in use through the late Edo period even as synthetic dyes arrived. Whether the technique extended into the early-twentieth-century shin-hanga and sosaku-hanga revivals is not firmly established in the available conservation literature. The introduction of permanent printing inks and modern proofing techniques after the Second World War rendered the traditional water-soluble under-drawing approach obsolete.