Cyanotype is an iron-based photographic printing process that produces images in a characteristic Prussian blue by exposing paper coated with light-sensitive iron salts to ultraviolet light, usually through a negative or with objects placed directly on the paper. In the history of photography, it was introduced by the astronomer and chemist John Herschel in 1842, and it became the basis of the engineering blueprint for roughly a century.
Cyanotype is a contact printing process: the image is the same size as the negative or object, because the paper is far too slow for projection in an enlarger. Its simplicity, low cost and safe chemistry have kept it alive as one of the most widely practiced alternative processes.
How the cyanotype process works
The classic cyanotype uses two solutions mixed in equal parts: ferric ammonium citrate and potassium ferricyanide. Coated onto paper or fabric and dried in subdued light, the mixture is sensitive mainly to ultraviolet and violet light. Exposure reduces iron from the ferric to the ferrous state, and the ferrous iron reacts with the ferricyanide to form the insoluble blue pigment known as Prussian blue.
Processing is remarkably simple. The exposed print is washed in plain water, which removes the unexposed, still-soluble iron salts and leaves the blue image fixed in the paper fibers. As the print dries and oxidizes over a day or so, the blue deepens; a dilute bath of hydrogen peroxide is often used to accelerate this final darkening.
| Stage | What is done | Purpose |
|---|---|---|
| Sensitizing | Mix and brush the two iron solutions onto paper | Create the light-sensitive layer |
| Drying | Dry in dim light or darkness | Prevent fogging before exposure |
| Exposure | Contact print under sun or UV lamp | Form the latent and visible image |
| Washing | Rinse in running water for several minutes | Remove unexposed iron salts |
| Oxidation | Air dry or use dilute peroxide | Develop full blue density |
Cyanotype in the history of photography
Cyanotype appeared in the same pioneering decade as the daguerreotype and the calotype, but it followed a different path. Herschel valued it as a way to copy notes and diagrams, and in 1843 Anna Atkins began issuing a botanical work on British algae illustrated with cyanotype photograms, widely regarded as the first book illustrated with photographic images.
Pictorial photographers largely ignored it in the nineteenth century because the blue color was considered unsuitable for portraits and landscapes. Its real influence came from industry: from the 1870s onward, architects and engineers used cyanotype paper to copy technical drawings, and the word blueprint still survives long after the process itself disappeared from drafting offices in the mid-twentieth century.
| Process | Introduced | Image base | Reproducible |
|---|---|---|---|
| Daguerreotype | 1839 | Silver-mercury image on silvered copper | No, unique image |
| Calotype | 1841 | Silver image on paper from a paper negative | Yes, many prints |
| Cyanotype | 1842 | Prussian blue pigment in paper | Yes, by contact printing |
| Wet plate collodion | 1851 | Silver image on glass or metal | Yes from glass negatives |
| Tintype | 1850s | Collodion on lacquered iron | No, unique image |
Classic and modern cyanotype formulas
The classic two-part formula is forgiving but has drawbacks: it is slow, prone to bleeding of the image into the paper, and the mixed solution has a short working life. A modern variant, usually called the new cyanotype, uses ammonium iron oxalate and gives shorter exposures and finer shadow detail at the cost of slightly more demanding chemistry.
| Property | Classic two-part formula | New cyanotype formula |
|---|---|---|
| Main iron salt | Ferric ammonium citrate | Ammonium iron oxalate |
| Relative speed | Slower | Noticeably faster |
| Shadow density | Good with long exposures | Deeper and more even |
| Solution stability | Separate parts keep, mixed solution short-lived | Single solution keeps well in the dark |
| Ease of preparation | Very easy | More complex mixing |
Exposure times vary widely with light source, formula and negative density. Under strong summer sun a classic print may need 5 to 20 minutes, while a UV exposure unit gives more repeatable timings, which is essential when printing editions.
Contemporary printers rarely use camera negatives of the right size. Instead, they make digital negatives by printing an inverted, contrast-adjusted image onto transparent film with an inkjet printer. Because cyanotype has a long tonal scale and responds only to ultraviolet light, the negative is usually given a correction curve, determined by printing a step tablet, so that the full range of grays reproduces evenly. The same approach lets a photograph taken on any modern camera be printed as a cyanotype at the exact size of the film sheet.
Toning, permanence and common problems
Cyanotype images are stable in light but sensitive to alkaline conditions. Washing soda, alkaline buffered papers and some detergents can bleach the blue to pale yellow. Practitioners therefore choose unbuffered papers for printing and avoid buffered mounts in storage.
That same sensitivity is used deliberately in toning. A brief bleach in a weak alkaline bath followed by a bath of tannin from tea, coffee or oak gall can convert the blue to brown, purple or near-black tones, extending the expressive range of the process.
| Problem | Likely cause | Remedy |
|---|---|---|
| Weak, washed-out blue | Underexposure or alkaline water | Expose longer, add a little acid to the wash |
| Blue fog in the highlights | Coating exposed to light before printing | Coat and dry in dim light |
| Uneven tone | Patchy coating or sizing | Use a better-sized paper and even strokes |
| Image bleeding | Overwetting or weak sizing | Use a stiffer paper and a lighter coat |
| Fading over time | Contact with alkaline materials | Store with acid-free, unbuffered enclosures |
A distinctive property is partial recovery in darkness: a cyanotype faded by prolonged light exposure can regain some density when stored in the dark, as the iron compound reoxidizes.
The short version
A cyanotype is a blue-and-white photograph made with sunlight and iron salts. Paper is painted with a yellow-green chemical mix, something is placed on top, such as a leaf or a negative, and the paper is left in the sun.
Where light reaches the paper, it turns deep blue; where something blocked the light, it stays white. Rinsing in water stops the process. It is cheap, uses no darkroom and is often how people first try making photographs by hand.
Where it makes a difference
A secondary school art teacher plans a lesson on early photography for a class of 24 students. Instead of only showing reproductions of nineteenth-century photograms, the teacher prepares cyanotype paper the evening before by brushing the classic two-part solution onto watercolor paper and drying it in a dark cupboard.
During the lesson, students arrange ferns, lace and cut paper shapes on the sheets, cover them with glass and place them in the courtyard sun for about ten minutes. After rinsing in trays of tap water with a few drops of vinegar, the blue images emerge within seconds.
The students leave with finished prints made by hand, and the class discussion links their results directly to the botanical cyanotypes of the 1840s and to the blueprints once used in engineering offices.