how it works

What happens on the paper

A cyanotype is paper brushed with iron salts and left in the sun under a negative or a pressed plant. Where light gets through, the iron turns Prussian blue; the rest washes away. The simulator follows each of those steps, and below you can see how its prints compare with real ones.

a freshly coated sheet, pale yellow-green with brush marks

the sheet in the contact frame with a pressed fern under the glass

1 Light turns iron blue

The coating is a mix of two iron salts, ferric ammonium citrate and potassium ferricyanide. When ultraviolet light hits the citrate, it turns Fe³⁺ iron into Fe²⁺. The Fe²⁺ then reacts with the ferricyanide around it to form Prussian blue, a pigment that stays locked in the paper fibers.

The simulator follows the light down through the coating. The yellow ferricyanide and the new blue both absorb some of it on the way, so the darkest parts of a print fill in more slowly than the rest.

2 How much light makes how much blue

Printers test their process with a step tablet: a strip of film in 21 gray steps, each letting through about 30% less light than the one before. Mike Ware, the chemist behind much of the modern work on cyanotype, printed one in sunlight and measured every step with a densitometer.

fig. 1 Each orange dot is one step of Ware’s print: how much light reached the paper through that step of film, and how blue it came out. The blue line is the simulator printing the same tablet. On average they differ by 0.017, about a tenth of the gap between neighboring steps. Source: Mike Ware, Cyanomicon, fig. 9.1.

The simulator isn’t told how long a print should take. It works out the exposure from the chemistry and the brightness of each light, and the times it arrives at land where printers’ own times do.

fig. 2 Gray bars: the exposures printers use for a brush-coated sheet, from published test strips and exposure surveys (alternativephotography.com, Christina Z. Anderson, Sandy King). Blue dots: the simulator’s own standard time for each.

3 The color of the blue

The blue on screen isn’t picked from a palette. It comes from how strongly Prussian blue absorbs each color of light, how light scatters inside paper, and how the eye sees color. Here it is step by step next to a real print.

fig. 3 Top: ten steps of a real cyanotype tablet, scanned with color calibration by Mike Ware. Bottom: the simulator’s blue at the same depth. A difference (ΔE) under about 2 is hard to see side by side.

4 Water and air

The wash carries away the yellow salts that never saw light, along with a little loosely held blue; alkaline water takes more. Out of the wash, a print is paler than it will end up. Part of its blue is in a colorless form, Prussian white, which takes oxygen from the air and turns blue again over the next day.

fig. 4 How much of the blue each wash removes. Gray: measured on real prints, from Mike Ware, “A Blueprint for Conserving Cyanotypes” (2003, table 6) and Moor & Moor. Blue: the simulator.

fig. 5 The blue returning as a print dries. Ware (2003) reports at least 95% back within a day; the simulator reaches 96% at 24 hours.

5 Real prints, side by side

Three 19th-century cyanotypes from the Metropolitan Museum of Art’s open-access collection, printed again in the simulator. Anna Atkins laid her algae and leaves directly on the paper, so their outlines, and how thin each part was, are traced from her prints. Everything else comes from the simulator: the brushed coat, the paper, the exposure, the wash and the color of the blue. Henry Bosse printed from a glass negative, so his print was turned back into a positive and printed the way a photo is on the bench.

Drag across each pair to compare.

fig. 6 Anna Atkins, Ptilota plumosa, from Photographs of British Algae, about 1853. Re-printed on smooth cotton paper in summer sun.

fig. 7 Anna Atkins, Spiraea aruncus (Tyrol), 1851–54. Her leaves let some light through, so they print pale cyan with their veins showing. Traced, they come out whiter, since a tracing can only guess how thick a leaf was.

fig. 8 Henry P. Bosse, U.S. Government Bridge at Rock Island, Illinois (High Water), 1888. Bosse’s print is paler and flatter than a fresh one, from a shorter exposure or more than a century of light; the re-print uses the full range of a calibrated negative.

6 Up close

Up close, a cyanotype is mostly paper. The blue sits in the fibers and hollows of the sheet, and the edge of the coat is wherever the brush ran dry. The papers in the simulator are scans of real cold-pressed and smooth paper, and both the coating and the light follow their surface.

a close-up of Atkins' Ptilota plumosa: fine white fronds on a grainy blue ground — fig. 9 The same few centimeters of the same frond, real and simulated, at the museum scan’s resolution.

the same part of the frond printed by the simulator — fig. 9 The same few centimeters of the same frond, real and simulated, at the museum scan’s resolution.

the edge of a brushed coat on cold-pressed cotton, magnified — fig. 10 The edge of the coat on three papers, zoomed in on the bench. Each is about 2 cm across.

the edge of a brushed coat on smooth cotton paper, magnified — fig. 10 The edge of the coat on three papers, zoomed in on the bench. Each is about 2 cm across.

7 Methods and sources

Coating. A hake brush laid down as hundreds of small dabs. The liquid levels out, soaks in and dries with a faint tide line.
Paper. Six papers, each with its own surface, fibers, sizing and color.
Exposure. Ultraviolet light absorbed layer by layer through the coating; heavy overexposure turns the shadows gray-bronze until the wash.
Wash and drying. The yellow salts clearing, loose blue lost to the water, the blue returning in air or peroxide, and tea or coffee toning.
Color. Light scattering in paper (Kubelka–Munk), the eye’s color response (CIE 1931) and sRGB.
Testing. 24 automated checks against published measurements, plus browser tests of the whole process.

Sources: Mike Ware, Cyanomicon, and “A Blueprint for Conserving Cyanotypes” (AIC, 2003); Christina Z. Anderson, Cyanotype (2019); Sandy King’s UV light-source tests; the alternativephotography.com exposure survey; Moor & Moor on washing. Images: Julia Margaret Cameron, Sir John Herschel, 1867; Anna Atkins, Ptilota plumosa and Spiraea aruncus; Henry P. Bosse, Rock Island bridge; all the Metropolitan Museum of Art, open access. Pressed plants: Naturalis Biodiversity Center, Leiden. Paper, wood, felt and brass surfaces: ambientCG and Poly Haven.