In 1843, Anna Atkins began making a book whose pages were photographs—but she did not point a camera at the seaweed.

She laid actual specimens on light-sensitive paper and let sunlight draw their shapes. The pages emerged Prussian blue, with the plants preserved as pale silhouettes.

The same chemical process would become familiar in a very different place: architects’ and engineers’ blueprints.

How can sunlight, iron salts and a rinse in water turn seaweed into both scientific evidence and art?

A cyanotype begins with paper coated in iron-based, light-sensitive chemicals. Put a flat object on the dry paper and expose it to sunlight: the uncovered areas react, while the object shields the paper beneath it. Washing removes the unreacted material and leaves a deep Prussian-blue image around the pale silhouette.

So the specimen is not photographed through a lens. It is placed directly against the photographic surface. Botanists call the result a useful record; photographers call it a photogram.

Sir John Herschel developed the cyanotype process in 1842. Atkins, a botanist and accomplished scientific illustrator, quickly recognized what it could do for difficult specimens. Fine algae can be tedious to draw and easy to distort. Direct contact with sensitized paper could preserve branching structures and delicate edges with remarkable fidelity.

Beginning in 1843, she privately issued Photographs of British Algae: Cyanotype Impressions in installments. Museum historians describe it as the first book printed and illustrated photographically. Atkins collected or obtained specimens, arranged them by hand, exposed every plate and labeled the species.

Cyanotype was inexpensive, stable and simple to process: after exposure, water did much of the finishing work. Those same qualities later made it practical for reproducing line drawings. Architects and engineers could place a translucent drawing over sensitized paper and expose it, producing the white lines on blue ground that gave us the word blueprint.

Atkins did not invent the chemistry, and her algae book did not itself create architectural blueprinting. The surprising connection is that one process served three different needs: a botanist’s demand for accuracy, an artist’s eye for composition and an engineer’s need to copy a drawing.

The blueprint began as a way for light to trace what was placed in front of it—and one of its first great uses was to let seaweed draw its own portrait.

The Metropolitan Museum of Art — Photographs of British Algae

The Metropolitan Museum of Art — Spiraea aruncus (Tyrol)

Natural History Museum — Anna Atkins’s cyanotypes and process

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